<PAGE>
 
                                                                 Exhibit 10.14


                                LEASE AGREEMENT

                                    between

                             INFOMART-Dallas, L.P.

                                      and

                   FOCAL COMMUNICATIONS CORPORATION OF TEXAS


                                  INFOMART(R)
                            The Technology Community
                             1950 Stemmons Freeway
                              Dallas, Texas 75207
                                  214-800-8000
<PAGE>
 
                                 LEASE AGREEMENT

                                    INFOMART
                            THE TECHNOLOGY COMMUNITY

--------------------------------------------------------------------------------

THIS LEASE AGREEMENT (the "Lease") is made and entered into as of the 15th day
of December, 1998, by and between INFOMART-Dallas, L.P., a Texas limited
partnership ("Landlord"), whose address is 1950 Stemmons Freeway, Dallas, Texas
75207 and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, a Delaware corporation
("Tenant"), whose address is 200 LaSalle Street, Suite 800, Chicago, Illinois
60601. If there shall be more than one party executing this Lease as Tenant,
their obligations shall be joint and several. As used in this Lease, the terms
set forth in Article I of this Lease shall have the respective meanings
indicated in such Article.

Subject to all of the terms and conditions of this Lease, and in consideration
of the mutual covenants and obligations contained in this Lease, Landlord and
Tenant agree as follows:


                             ARTICLE 1 - DEFINITIONS
                             -----------------------

SECTION 1.1. BASE RENTAL shall mean Thirty-eight Thousand One Hundred Fifty-nine
and 11/100ths Dollars ($38,159.11) per month from the Commencement Date hereof
until June 30, 2009.

SECTION 1.2.  BASE YEAR shall mean 1999.

SECTION 1.3. THE "BUILDING" shall mean the information processing market center
located upon the real property (the "Property") described in Exhibit "A"
attached hereto and incorporated herein.

SECTION 1.4. BUILDING RULES shall mean rules and regulations adopted and altered
by Landlord from time to time for the safety, care and cleanliness of the Leased
Premises and the Building and for the preservation of good order therein, all of
which will be sent by Landlord to Tenant in writing and shall thereafter be
carried out and observed by Tenant. The initial Building Rules "Rules and
Regulations" are attached hereto as Exhibit "B". In the event of a conflict
between this Lease and such Rules and Regulations or the "INFOMART Policy
Statement" (as defined below), this Lease shall control for all purposes.
Landlord shall provide Tenant with a copy of all changes to the Rules and
Regulations and INFOMART Policy Statement which are approved, adopted, amended
or modified after the date this Lease is entered into. Landlord shall not
discriminate among the tenants of the Buildings regarding the enforcement of
Rules and Regulations and INFOMART Policy Statement.
<PAGE>
 
SECTION 1.5. COMMENCEMENT DATE shall mean the date on which Tenant commences
occupancy of the Leased Premises which shall be the date Tenant takes possession
of the Leased Premises for the purpose of equipping, furnishing, and improving
the Leased Premises. The term "Rental Commencement Date" (as defined below)
shall mean as defined in Section 1.21 hereof.

SECTION 1.6. COMMON AREAS shall mean those areas devoted to corridors, elevator
foyers, restrooms, mechanical rooms, janitorial closets, electrical and
telephone closets, vending areas, lobby areas, meeting rooms, auditoriums,
exhibit halls and other similar facilities provided for the common use or
benefit of tenants generally.

SECTION 1.7. INFOMART shall mean "INFOMART - The Technology Community" and shall
include that certain Building and Property as the same currently exists or as it
may from time to time hereafter be expanded or modified.

SECTION 1.8. INSURANCE COSTS shall mean all costs incurred by Landlord in
providing insurance, including but not limited to, property, liability and
casualty insurance, on the Building and Property, but excluding all insurance
costs which Tenant is required to provide under Section 5.3 hereof.

SECTION 1.9. LEASE TERM shall mean a term commencing on the Rental Commencement
Date and continuing for one hundred twenty (120) months.

SECTION 1.10. LEASED PREMISES shall mean Suite No. 6060 in the Building, as
outlined or marked in red on the floor plan of the Building attached to this
Lease as Exhibit "C".

SECTION 1.11. PERMITTED USE shall mean use for professional offices, and for the
installations, operation and maintenance of equipment and facilities in
connection with Tenant's telecommunications business and use for the display and
marketing of information processing and communications products and services and
for offices, storage and service areas incidental and related to such use.

SECTION 1.12.  RELOCATION SPACE Intentionally omitted.

SECTION 1.13. RENTABLE SQUARE FEET shall mean the Usable Square Feet of the
Leased Premises, together with an additional amount representing a portion of
the Common Areas, Service Areas and other non-tenant space on floors two (2)
through six (6) in the Building. For purposes of this Lease, the parties have
agreed that the Leased Premises shall be deemed to consist of 19,249 Rentable
Square Feet on floors two (2) through six (6) of the Building which shall be
deemed to consist of 1,056,200 Rentable Square Feet. However, both Landlord and
Tenant acknowledge that neither of these figures was calculated by measuring the
areas of actual Common Areas, Service Areas and other non-tenant spaces in the
Building and neither Landlord nor Tenant shall have a right to demand
remeasurement or recalculation of the Rentable Square Feet amounts for Floors
two (2) through six (6) or the Leased Premises, and regardless of the actual
size of the Building of Leased Premises, such areas shall be deemed to be the
measurements set forth in this Section 1.13.

                                       4
<PAGE>
 
SECTION 1.14. SECURITY DEPOSIT as used in Section 6.9 hereof shall mean
Thirty-six Thousand One Hundred Fifty-nine and 11/100ths Dollars ($38,159.11).

SECTION 1.15. SERVICE AREAS shall mean those areas within the outside walls used
for elevator mechanical rooms, building stairs, elevator shafts, flues, vents,
stacks, pipe shafts and vertical penetrations (but shall not include any such
areas for the exclusive use of a particular tenant).

SECTION 1.16. TAXES shall mean all taxes and assessments and governmental
charges, whether federal, state, county or municipal, and whether they be by
taxing districts or authorities presently taxing the Leased Premises or the
Property or any part thereof, or by others, subsequently created or otherwise,
and any other taxes and assessments attributable to the Property or its
operation but shall not include penalties for late or non-payment thereof.
Should taxes become payable over a number of years, only the portion of taxes
attributable to the Lease Term shall be included herein..

SECTION 1.17. TENANT'S PROPORTIONATE SHARE shall mean a fraction, the numerator
of which is the number of Rentable Square Feet comprising the Leased Premises,
and the denominator of which is the number of Rentable Square Feet comprising
floors two (2) through six (6) of the Building. Accordingly, the parties
acknowledge and agree that Tenant's Proportionate Share under this Lease is 1.18
percent.

SECTION 1.18. TRADE FIXTURES shall mean any and all signs placed by Tenant
within the Leased Premises pursuant to provisions hereof and any and all items
of property used by Tenant in the Leased Premises, including but not limited to
furniture and equipment; provided, however, that the term Trade Fixtures shall
not include any permanent leasehold improvements, including but not limited to
any floor, wall or ceiling coverings, any interior walls or partitions, any
lighting fixtures, track lights or any property which is a part of or associated
with any electrical, plumbing, or mechanical system, notwithstanding that the
same may have been installed within the Leased Premises. Not withstanding the
foregoing, Tenant's generator and telecommunications equipment, including, but
not limited to, Tenant's telephone switches, shall be deemed Trade Fixtures.

SECTION 1.19. USABLE SQUARE FEET shall mean the gross number of square feet
enclosed by the surface of the exterior glass walls, the midpoint of any walls
separating portions of the Leased Premises from those of adjacent tenants, the
slab penetration line of all walls separating the Leased Premises from Service
Areas and the corridor side of walls separating the Leased Premises from Common
Areas.

SECTION 1.20. UTILITY COSTS shall mean all costs incurred by Landlord in
providing electricity, gas, water and sewage disposal facilities to the
Building, including, without limitation, electricity used for heating, air
conditioning, operation of office machines and other equipment used on or about
the Building, and elevator and escalator service and lighting, but excluding all
such costs which Tenant may, from time to time, be obligated under the
provisions of Section 2.5 hereof to pay on a separately metered basis and
excluding such costs which other tenants of the Building may, from time to time,
be obligated to pay under the provisions of their leases.

                                       5
<PAGE>
 
SECTION 1.21. RENTAL COMMENCEMENT DATE shall mean the earlier of the date Tenant
receives a Certificate of Occupancy for the Leased Premises or September 1, 1999
provided that if the Commencement Date does not occur on or before February 1,
1999, the Rental Commencement Date shall be delayed one day for each day after
February 1, 1999 on which the Commencement Date occurs.


                                    ARTICLE 2
                                    ---------

SECTION 2.1. LEASED PREMISES AND TERM. Landlord does hereby lease, demise and
let to Tenant and Tenant does hereby lease and take from Landlord the Leased
Premises for a term beginning on the Commencement Date and continuing in full
force and effect for the Lease Term, unless this Lease is terminated earlier
pursuant to the provisions hereof. The Leased Premises are demised hereby
subject to all easements, restrictions, agreements of record, mortgages and
deeds of trust, and zoning and building laws. If Landlord is unable to deliver
possession of the Leased Premises to Tenant as of the Commencement Date
specified in Article 1 for any reason, including, without limitation, the
holding over of any tenant or occupant of the Leased Premises, then the term
"Commencement Date" shall mean such subsequent date upon which the Landlord is
able to deliver possession of the Leased Premises to Tenant, and such failure to
deliver possession of the Leased Premises on the Commencement Date specified in
Article 1 hereof shall not constitute a default by Landlord hereunder or render
Landlord liable for any loss or damage that may be incurred as a result of such
failure. If the Leased Premises are delivered to Tenant for occupancy on a date
prior to the Commencement Date specified in Article 1 hereof, Tenant agrees to
accept and occupy the Leased Premises on such date and the term "Commencement
Date" shall mean such date. Tenant shall commence to furnish, equip, and improve
the Leased Premises, in accordance with Section 4.2(a) hereof, on the
Commencement Date. Landlord shall have no obligation to furnish, equip or
improve the Leased Premises and Tenant has been afforded the right to inspect
the Leased Premises prior to the Commencement Date. By occupying the Leased
Premises, Tenant shall be deemed to have accepted the same and to have
acknowledged that the same comply fully with Landlord's covenants and
obligations hereunder. Notwithstanding the foregoing, to the extent that the
Leased Premises are not delivered to Tenant on or before April 1, 1999, Tenant
shall have the right to terminate this Lease by providing notice to Landlord
after such date.

Landlord acknowledges that Tenant's business to be conducted in the Lease
Premises requires the installation in the Leased Premises of certain
communications equipment by telecommunications customers of Tenant ("Customers")
in order for such Customers to interconnect with Tenant's terminal facilities.
Not withstanding anything contained in Section 4.7 of the Lease, Landlord agrees
that no consent shall be required for a co-location agreement between Tenant and
any such Customer for the purposes of permitting such a telecommunications
connection, so long as (i) such Customer agrees in writing to comply with all
obligations of Tenant under this Lease to the extent relating to the portion of
the Leased Premises in question, and (ii) each co-location agreement is in
writing and is consistent with the provisions of this Lease. Co-location shall
not be deemed under any circumstances to be a sublet or assignment of the Leased
Premises.

                                       6
<PAGE>
 
SECTION 2.2. USE. The Leased Premises may be used and occupied by Tenant solely
for the Permitted Use and for no other purpose. Warehousing and on-site delivery
to customers is prohibited in the Building or any part thereof. Payment for
products or services that are of a retail sales nature are prohibited (provided,
however, that payment or partial payment for orders taken at the Leased Premises
for future delivery to a buyer will be allowed if it is within the Tenant's
normal business practice and is not of a retail sales nature, it being the
intention hereof to permit payments or partial payments intended to bind an
order for future delivery without in any way qualifying or circumventing the
prohibition within the Building against retail sales). Tenant warrants and
represents to Landlord that it is a producer of hardware, software or services
utilizing information processing equipment (and, if the Tenant's business
includes the resale of products or services, Tenant warrants and represents that
it adds to or enhances the value of such products or services). Tenant shall not
use or allow the Leased Premises to be used in any manner which obstructs or
interferes with the rights of other tenants of the Building or injures or annoys
such tenants, and Tenant shall not cause, maintain or permit any nuisance in, on
or about the Leased Premises or the Building, or permit or suffer to be
committed any defacement, injury or waste to, in, on, or about the Leased
Premises or the Building.

         SECTION 2.3. BASE RENTAL. Tenant agrees to pay the Base Rental to
Landlord for each month during the Lease Term as herein provided. Base Rental
for the first month of the Lease Term shall be due and payable in advance on the
Rental Commencement Date, and Base Rental for each and every month thereafter
during the Lease Term shall be due and payable in advance on the first day of
the month. If the Rental Commencement Date is a day other than the first day of
a calendar month or in the event this Lease terminates on other than the last
day of a calendar month, then Base Rental for such month or months shall be
prorated and the installment or installments so prorated shall be paid in
advance. In the event that Tenant fails to make any payment of Base Rental or
any other sums due hereunder on or before the date any such payment becomes due
and payable, the Tenant shall also be obligated to pay interest on such past due
amounts at a rate equal to the lesser of the prime rate plus four percent (4%)
per annum or the highest rate permitted by law, such interest being in addition
to and cumulative of any other rights and remedies which Landlord may have
hereunder with regard to the failure of Tenant to make any payment of Base
Rental or any other sum due hereunder. As used herein, the phrase "prime rate"
means, on any day, the rate of interest per annum then most recently established
by NationsBank of Texas, N.A. as its general reference rate of interest, taking
into account such factors as such bank may deem appropriate, it being understood
that such rate is not necessarily the lowest or best rate actually charged to
any customer of such bank or a favored rate.

SECTION 2.4. TENANT'S PROPORTIONATE SHARE OF TAXES, INSURANCE COSTS AND UTILITY
COSTS. In addition to the payment of Base Rental, Tenant shall pay to Landlord
Tenant's Proportionate Share of Utility Costs, Insurance Costs and Taxes, in
accordance with the following provisions:

(a)      Tenant shall pay to Landlord, either in the form of a lump sum payment
         due and payable upon demand by Landlord or on a monthly basis
         contemporaneously with the payment of Base Rental, as Landlord may
         elect, (i) an amount reasonably estimated by Landlord to be Tenant's
         Proportionate Share of all Utility Costs for each calendar year or
         portion thereof during the Lease Term, (ii) an amount reasonably
         estimated by Landlord to be

                                       7
<PAGE>
 
         Tenant's Proportionate Share of all Insurance Costs for each calendar
         year or portion thereof during the Lease Term and (iii) an amount
         reasonably estimated by Landlord to be Tenant's Proportionate Share of
         the amount, if any, by which Taxes for each calendar year or portion
         thereof during the term of this Lease exceed Taxes for the Base Year.

(b)      If at any time Landlord shall have reasonable grounds to believe that
         actual Utility Costs, Insurance Costs or Taxes incurred will vary from
         such estimates, then Landlord reserves the right to revise such
         estimates accordingly. Upon any such revision, Landlord shall notify
         Tenant and Landlord may, at Landlord's election, either (i) require
         Tenant to make a lump sum payment to Landlord reflecting such revised
         estimate or (ii) require that the monthly payments due and payable to
         Landlord by Tenant under this Section be revised to an amount which
         will amortize such revised estimate over the remainder of the calendar
         year in which any such revision is made by Landlord.

(c)      As soon as reasonably practical following the end of any calendar year
         during which such payments were made by Tenant, Landlord shall deliver
         to Tenant a statement reconciling the amounts paid by Tenant for such
         year against the actual charges for such items and a lump sum payment
         (or credit against the next succeeding installments of Base Rental, if
         any, in case of amounts owed by Landlord to Tenant) shall be made from
         Tenant to Landlord or from Landlord to Tenant, as the case may be, so
         that Tenant shall have paid to Landlord only Tenant's Proportionate
         Share of (i) Utility Costs for the previous calendar year, (ii),
         Insurance Costs for the previous calendar year, and (iii) the amount,
         if any, by which Taxes for the previous calendar year exceed Taxes for
         the Base Year and no more, which obligation to make such reconciliation
         payment shall survive the termination of the Lease. Tenant shall have
         the right to audit copies of paid invoices for Common Area Utility
         Costs, Taxes, and Insurance for the then previous calendar year within
         the Lease Term; but Tenant shall not have the right to audit Landlord's
         records for any previous calendar year within the Lease Term or for any
         year prior to the Commencement Date of this Lease. If Tenant's audit
         reveals Landlord's statement for such expenses have been overstated by
         more than five percent (5%), then Tenant shall submit a copy of its
         audit to Landlord. If Landlord disagrees with the results of Tenant's
         audit, Landlord shall submit such audit, and Landlord's records
         relative to such expenses, to Landlord's independent auditors for their
         review. If Landlord or Landlord's auditors agree that such expenses
         have been overstated by more than five percent (5%), then Landlord
         shall pay all reasonable costs and expenses of such audit, and, in any
         event, shall, within thirty (30) days after the completion of
         Landlord's review of such audit, reimburse Tenant for any and all
         overages. If the audit conducted by Landlord or Tenant reveals that
         Landlord's statement for such expenses was correct, then Tenant shall
         pay all reasonable costs and expenses incurred by Landlord relative to
         such audit. Further, if either audit reveals that Landlord undercharged
         Tenant relative to these expenses, then Tenant shall, within thirty
         (30) days after the completion of such audit, reimburse Landlord for
         any undercharges. Tenant shall not have the right to audit Landlord's
         records relative to such expenses more than once in any calendar year.

(d)      If the Commencement Date is a day other than the first day of a
         calendar month or if this Lease terminates on other than the last day
         of a calendar month, then the amounts due and owing by Tenant to
         Landlord under this Section shall be prorated accordingly.

                                       8
<PAGE>
 
SECTION 2.5. SEPARATELY METERED UTILITIES AND UTILITY USAGE. Tenant shall pay
upon demand or receipt of an invoice all amounts due and owing with respect to
utilities furnished to the Leased Premises which may, from time to time, be
separately measured and charged to the Tenant by Landlord or any public utility
as may furnish such utilities to the Leased Premises. In the event that any
electrical services required or used in the Leased Premises shall exceed seven
(7) watts per square foot of Usable Square Feet within the Leased Premises.
Tenant shall cause the installation of all facilities necessary to separately
meter electrical usage within the Leased Premises and/or cause the installation
of such riser or risers, wiring, transformer, or electrical panels as are
required to meet Tenant's excess electrical requirements and Tenant shall pay to
Landlord or, at the election of Landlord, to the applicable public utility,
promptly upon receiving any invoice, all charges for electrical usage within the
Leased Premises in excess of seven (7) watts per square foot of Usable Square
Feet within the Leased Premises; which payment, if any, shall be in addition to
sums required to be paid by Tenant pursuant to Section 2.4 above. Landlord
covenants that (a) there are currently 7 watts of electricity per square foot of
Usable Square Feet serving the floor upon which the Leased Premises are located
and (b) there are currently at least 28 watts of electricity per square foot of
Usable Square Feet available to Tenant's use in the Leased Premises. To the
extent that Tenant requests the ability to upgrade the electrical service to the
Leased Premises to an amount in excess of 28 watts of electricity per square
foot of Usable Square Feet of the Leased Premises, Tenant shall have the right,
at its sole cost, to upgrade the Building's electrical system for the floor on
which the Leased Premises are located in order to meet such excess capacity
request. In such event, Landlord shall have the right to review Tenant's plans
and specifications for such upgrade and Landlord shall have the right to
supervise Tenant's installations necessary to upgrade the electrical service to
the Leased Premises. Tenant shall promptly reimburse Landlord for Landlord's
documented actual out-of-pocket costs for such supervision. Notwithstanding the
foregoing, Landlord may refuse to install, and may withhold consent for Tenant's
installation of, any riser, wiring, transformer, or electrical panel if, in
Landlord's sole reasonable judgment (exercised in good faith), the same would
cause permanent damage or injury to the Building or the Leased Premises or cause
or create a dangerous or hazardous condition or entail excessive or unreasonable
alterations, repairs, or expense or interfere with or disturb other tenants or
occupants of the Building. In no event shall Landlord incur any liability or
obligation with respect to Landlord's refusal to install, or withholding consent
for Tenant's installation of, any such additional electrical facilities or
equipment.

SECTION 2.6. ADDITIONAL RENT; PAYMENTS. All sums of money due and payable by
Tenant to Landlord under the term of this Lease in addition to the Base Rental
shall constitute additional rent hereunder. Landlord shall have the same
remedies for default in the payment of additional rent as are available to
Landlord in the case of a default in the payment of Base Rental. All rent shall
be payable at Landlord's address as provided herein (or at such other address as
may be designated by Landlord from time to time). Tenant agrees to pay all rent
under this Lease at the times and in the manner herein provided, without demand,
counterclaim or set-off, except as set forth herein.


                                       9
<PAGE>
 
                                    ARTICLE 3
                                    ---------

SECTION 3.1. UTILITIES. Landlord shall use reasonable efforts to cause public
utilities to furnish electricity to the Leased Premises and water to the
Building to the extent and in such manner as is reasonably deemed by Landlord to
be standard for the Building.

SECTION 3.2. SERVICES TO BE FURNISHED BY LANDLORD TO TENANT. Landlord shall
furnish or cause to be furnished during the Lease Term:

(a)      Central heating and air conditioning to the Leased Premises and
         enclosed public areas of the Building in season;

(b)      Non-exclusive passenger escalator and elevator service and
         non-exclusive freight elevator service;

(c)      Electric lighting service for all public areas of the Building;

(d)      Janitorial service for the corridors and other public areas of the
         Building; and

(e)      Public toilets and restrooms and public drinking fountains;

Such services shall be provided during normal business hours, reasonably
established by Landlord, at such locations, in such manner and to the extent
deemed reasonable by Landlord, to be adequate for the use and occupancy of the
Building, with due regard for the prudent control of energy.

SECTION 3.3. LANDLORD'S FAILURE TO PROVIDE UTILITIES OR SERVICES. Failure by
Landlord to any extent to furnish or cause to be furnished the utilities or
services described in Section 3.1 and 3.2, or any cessation or interruption
thereof, resulting from any cause, including without limitation, mechanical
breakdown, overhaul or repair of equipment, strikes, riots, acts of God,
shortages of labor or material, compliance by Landlord with any voluntary or
similar governmental or business guidelines, governmental laws, regulations or
restrictions, or any other similar causes shall not render the Landlord liable
in any respect for damages to either person or property, for any economic loss
or other consequential damages incurred by Tenant as a result thereof, be
construed as an eviction of Tenant, result in an abatement of rent, or relieve
Tenant from its obligation to perform or observe any covenant or agreement
contained in this Lease. Notwithstanding the foregoing, in the event that the
Leased Premises are rendered untenantable due to the failure or interruption of
the utilities or services described in Sections 3.1 and 3.2 hereof (for any
reason other than Tenant's fault or neglect) for ten (10) consecutive days and
Tenant does not, in fact, use the applicable portion of the Leased Premises
during such period of such untenantability, Base Rental hereunder shall abate
with respect to that portion of the Leased Premises so rendered untenantable
from the business day Tenant first ceases to use that portion of the Leased
Premises to the earlier to occur of (i) the date Tenant again commences to use
that portion of the Leased Premises or (ii) the date on which that portion of
the Leased Premises is again rendered tenantable.

                                       10
<PAGE>
 
SECTION 3.4. PEACEFUL ENJOYMENT. Subject to the other terms of this Lease,
Landlord covenants that Tenant shall, and may peacefully have, hold and enjoy
the Leased Premises for the Lease Term free of any claims by any party claiming
by, through or under Landlord, provided that Tenant pays the rent to be paid by
Tenant under this Lease and performs all of Tenant's covenants and agreements
herein provided after notice and cure periods, if any.

                                    ARTICLE 4
                                    ---------

SECTION 4.1. OPERATION. If the Leased Premises front on the atrium within the
Building, at all other times during all normal business hours of the Building,
Tenant shall not abandon the Lease Premises and shall ensure that the lights
remain on in the Leased Premises during normal business hours. If the Leased
Premises front on the atrium of the Building, the failure of Tenant to
substantially complete its improvements at the Leased Premises by December 31,
1999, with adequate staff, shall, at the option of Landlord, be an event of
default hereunder.

SECTION 4.2.  ALTERATIONS, IMPROVEMENTS AND ADDITIONS.

(a)      Tenant shall furnish, equip and improve the Leased Premises with
         partitions, lighting fixtures, wall and floor coverings, paintings and
         other interior decoration suitable for a trade mart and of a quality
         and design consistent with the standards generally observed by
         Landlord. Prior to the commencement of any such work, Tenant shall
         submit to Landlord for its written approval, detailed plans and
         specifications providing for the initial furnishing, equipping and
         improving of the Leased Premises. Two (2) complete sets of final
         working drawings and  specifications  of materials  relating to all
         improvements ("Improvements") that Tenant desires to be installed in
         the Leased Premises shall be submitted to Landlord no later than thirty
         (30) days prior to the date specified in Section 1.5 hereof. Such
         drawings and the specifications of materials shall be subject to
         approval by Landlord which approval shall not be unreasonably withheld.
         Any delay occasioned as a result of Landlord's disapproval of Tenant's
         plans and specifications shall not delay the Commencement Date under
         this Lease except as set forth herein. Upon the approval of the plans
         and specifications by Landlord, Tenant shall commence to equip,
         furnish, and improve the Leased Premises, and shall diligently and
         continuously prosecute such work to substantial completion on or before
         December 31, 1999. The failure of Tenant to substantially complete such
         work on or before the date specified in the preceding sentence shall,
         at the option of Landlord, be an event of default hereunder. Any
         further alterations, improvements or additions to the Leased Premises
         (including constructing partitions, installing light fixtures or
         painting or changing the color of any painted surface or the color type
         of any wall, floor or ceiling covering) shall likewise require
         Landlord's prior written approval. In the event that Landlord and
         Tenant cannot agree on Tenant's plans and specifications within sixty
         (60) days after submission thereof to Landlord, Tenant shall have the
         right to terminate this Lease by providing written notice to Landlord
         within sixty-five (65) days after submission of plans to Landlord.

         In addition, but provided no event of default has occurred and is
         continuing under the Lease, Tenant shall have the right at any time
         during the Lease Term to upgrade the Building's and/or Tenant's
         HVAC/chilled water capacity, or install its own HVAC system

                                       11
<PAGE>
 
         at its sole expense with Landlord's prior written approval.

(b)      Any and all furnishing, equipping and improving of or other alteration
         or addition to the Leased Premises shall be:

         (i)      made at Tenant's sole cost, risk and expense;

         (ii)     performed in a prompt, good and workmanlike manner with labor
                  and materials of such quality as Landlord may reasonably
                  require;

         (iii)    constructed in accordance with all plans and specifications
                  approved in writing by Landlord prior to the commencement of
                  any such work, provided, however, that Landlord shall have no
                  responsibility with respect to, nor any liability as a result
                  of, defects or deficiencies therein;

         (iv)     prosecuted diligently and continuously to completion and in
                  such manner so as to minimize interference with the normal
                  business operations of other tenants in the Building, the
                  performance of Landlord's obligations under this Lease or any
                  mortgage or ground lease covering or affecting all or any part
                  of the Building or the Property, and any work being done by
                  contractors engaged by Landlord with respect to or in
                  connection with the Building; and

         (v)      performed by contractors approved in writing by Landlord, and
                  if requested by Landlord any such contractor and all work to
                  be performed by such contractor shall be fully bonded with
                  companies and in amounts acceptable to Landlord in its sole
                  discretion.

(c)      Any and all alterations, improvements and additions to the Leased
         Premises (except for Trade Fixtures as specified in Section 4.4 hereof)
         shall constitute a part of the Leased Premises, and shall be owned by
         and become the property of Landlord effective as of the termination of
         this Lease. Tenant shall have no (and hereby waives all) rights to
         payment or compensation for any such alteration, improvement or
         addition to the Leased Premises.

SECTION 4.3. MAINTENANCE AND REPAIRS. Tenant shall maintain the Leased Premises,
all plate glass and all Trade Fixtures and other improvements situated therein
in first class, clean, and safe condition. Tenant shall repair or replace any
damage to the Building, or any part thereof, caused by Tenant or Tenant's
agents, employees, customers or invitees. All such repair or replacement shall
be performed in accordance with the conditions set forth in Section 4.2.(b) (i),
(ii), (iii), (iv) and (v).

SECTION 4.4. TRADE FIXTURES. Landlord and Tenant agree that all Trade Fixtures
installed in the Leased Premises shall be and remain the property of Tenant and,
so long as Tenant is not in default hereunder, may be removed by Tenant prior to
or upon the expiration of the Lease Term. Tenant shall repair any damage caused
by such removal and restore the Leased Premises to such condition as existed
prior to the installation of such Trade Fixtures. Any such repair and
restoration shall be performed in accordance with the conditions set forth in
Section

                                       12
<PAGE>
 
4.2(b) (i), (ii), (iii), (iv) or (v). Any Trade Fixtures which are not removed
from the Leased Premises upon cessation of occupancy by Tenant and upon prior
written notice to Tenant's last known address shall become the property of
Landlord. Tenant shall have no (and hereby waives all) rights to payment or
compensation for any such item.

SECTION 4.5.  LAWS AND REGULATIONS; BUILDING RULES; INFOMART POLICY STATEMENT.

(a)      Tenant shall comply with all laws, ordinances, rules and regulations of
         any governmental authority relating to the use, condition or occupancy
         of the Leased Premises or the Building, including the furnishing,
         equipping and improving thereof.

(b)      Tenant shall, and shall cause its employees, agents, customers and
         invitees to comply with the Building Rules adopted and altered by
         Landlord from time to time. All changes in such rules will be sent by
         Landlord to Tenant in writing.

(c)      Landlord has prepared a policy  statement  with respect to the
         operation of the Building  attached  hereto as  Exhibit  "D" which may
         from time to time be  amended,  revised  or  supplemental  at
         Landlord's  sole discretion (the "INFOMART  Policy  Statement"). Tenant
         shall be responsible for conducting its operations within the Leased
         Premises  and the  Building in  compliance  with the  INFOMART  Policy
         Statement.  The failure of the Landlord to successfully  enforce any
         provisions of the INFOMART Policy  Statement  against Tenant,  or
         against any other  tenant or occupant of the  Building,  shall not be
         deemed to be a waiver of the  requirements  of the INFOMART  Policy
         Statement.  Landlord  shall not be  responsible  to Tenant for
         nonperformance  by any  other  tenant  or  occupant  of the  Building
         of any of the  requirements  of the Building Rules or the INFOMART
         Policy  Statement;  and Tenant shall be liable for all injuries or
         damages sustained by Landlord or Landlord's  agents or by other
         tenants,  occupants,  or invitees of the Building arising  by  reason
         of any  breach of the  requirements  of the  Building  Rules or the
         INFOMART  Policy Statement by Tenant or Tenant's agents, employees or
         invitees.

SECTION 4.6. LANDLORD'S ACCESS. Landlord and its representatives, agents,
officers and contractors shall have the right to enter upon the Leased Premises
at any reasonable time for any reasonable purpose, at any time for any
emergency, and if a default by Tenant exists hereunder, at any time to show the
Leased Premises to prospective tenants. Landlord agrees that to the extent
possible it will not unreasonably interfere with the conduct of Tenant's
business in the exercise of its rights hereunder.

SECTION 4.7. ASSIGNMENT AND SUBLETTING BY TENANT.

(a)      Tenant shall not, by operation of law or otherwise, (i) assign,
         transfer, mortgage, pledge, hypothecate or otherwise encumber this
         Lease, the Leased Premises or any interest therein, (ii) grant any
         concession or license within the Leased Premises, (iii) grant or
         transfer any management privileges or rights with respect to the Leased
         Premises, (iv) sublet all or any part of the Leased Premises or any
         right or privilege appurtenant to the

                                       13
<PAGE>
 
         Leased Premises, or (v) permit any other party to occupy or use all or
         any part of the Leased Premises except for as provided in Section 2.2.
         If Tenant is other than an individual person, any conveyance,
         assignment or transfer of any interest in Tenant shall be deemed to
         constitute a transfer or assignment prohibited by the immediately
         preceding sentence. No consent granted by Landlord to any transfer,
         assignment or other transaction prohibited by this Section shall
         release Tenant from any of Tenant's obligations under this Lease or be
         deemed to constitute a consent to any subsequent assignment,
         subletting, occupancy or use of the Leased Premises by another person.
         Subject to the foregoing, the rights and obligations of the parties to
         this Lease shall inure to the benefit of and be binding upon their
         respective successors, assigns, heirs and legal representatives. Any
         attempted assignment or sublease by Tenant in violation of the terms
         and covenants of this paragraph shall be void and constitute a default
         by Tenant.

(b)      One half of all net cash or other net proceeds of any assignment, sale
         or sublease of Tenant's interest in this Lease and/or the Leased
         Premises (not including Trade Fixtures), whether consented to by
         Landlord or not, shall be paid to Landlord notwithstanding the fact
         that such proceeds exceed the rents called for hereunder, unless
         Landlord agrees to the contrary in writing, and Tenant hereby assigns
         all rights it might have or ever acquire in any such proceeds to
         Landlord.  This covenant and assignment shall benefit Landlord and its
         successors in ownership of the Building and shall bind Tenant, Tenant's
         heirs, executors, administrators, personal representatives, successors
         and assigns.  Any assignee, sublessee, or purchaser of Tenant's
         interest in this Lease (all such assignees, sublessees or purchasers
         being hereinafter referred to as "Successors"), by occupying the Leased
         Premises and/or assuming Tenant's obligations hereunder, shall be
         deemed to have assumed liability to Landlord for all amounts paid to
         persons other than Landlord by such Successor in consideration of any
         such sale, assignment or subletting, in violation of the provision
         hereof.  The acceptance by Landlord of any rent from any sublessee or
         assignee of Tenant shall not constitute Landlord's consent to such
         assignment or sublease. Notwithstanding the foregoing, Tenant may, upon
         written notice to Landlord of the identity of the Affiliate, assign or
         sublet its interest under this Lease in the Leased Premises to an
         "Affiliate" of Tenant, provided (i) that such assignee or subtenant
         assumes in full, the obligations of Tenant under this Lease, (ii) such
         Affiliate's business operations are consistent with the Permitted Use,
         and (iii) such sublease or assignment shall not operate to release
         Tenant from its obligations under this Lease. As used herein, the term
         "Affiliate" shall be a corporation which controls, is controlled by, or
         is under common control with Tenant, control to be determined, for
         purposes hereof, by the ownership of in excess of fifty percent (50%)
         of the issued and outstanding voting stock of such entity.  In
         addition, the term "Affiliate" shall include any entity (a) resulting
         from a merger or consolidation with Tenant, (b) any entity succeeding
         to substantially all the business and assets of Tenant at the Leased
         Premises, (c) any subsidiary, spin-off, affiliate or parent of Tenant,
         or (d) any entity resulting from the reorganization of Tenant outside
         of a bankruptcy organization.  In the case of sublease or assignment to
         an Affiliate, Landlord shall not have any right to share in any excess
         rents.

                                       14
<PAGE>
 
SECTION 4.8. LIGHT, AIR AND VIEW. Neither the diminution nor the shutting off of
any natural light, air, or view nor any other effect on the Leased Premises by
any structure or condition now or hereafter existing on property adjacent to the
Building shall affect this Lease, abate rent, or otherwise impose any liability
on Landlord.

SECTION 4.9. TAXES. Tenant shall pay all ad valorem and similar taxes or
assessments levied upon or applicable to any of Tenant's Trade Fixtures or any
other improvements, equipment, fixtures, furniture or other property situated in
the Leased Premises and all license and other fees or charge imposed on the
business conducted by Tenant on the Leased Premises. Upon request by Landlord,
Tenant will furnish Landlord annually with official tax receipts and other
official receipts showing payment of such taxes, assessments, fees and charges.
If Landlord shall be required to pay a higher ad valorem tax as a result of
Tenant's leasehold improvements, then Tenant shall pay to Landlord, upon demand,
the amount of such increase in ad valorem taxes.

SECTION 4.10. LIENS. Tenant shall not place or permit to be placed any lien,
affidavit, charge or order upon INFOMART, the Building or the Leased Premises or
any part thereof or any interest therein. In the event that any such lien,
affidavit, charge or order attaches, regardless of the validity or
enforceability thereof, Tenant shall promptly cause the same to be discharged of
record by payment or by the filing of a statutory bond in lieu of payment within
ten (10) days of Tenant's receipt of notice of such lien. In the event any such
lien is attached to the INFOMART, the Leased Premises or the Building, then in
addition to any other right or remedy of Landlord, Landlord may but shall not be
obligated to discharge the same. Any amount paid by Landlord for any of the
aforesaid purposes shall be paid by the Tenant to Landlord on demand as
additional rent.

SECTION 4.11. SUBORDINATION TO MORTGAGES AND LEASES. This Lease shall be subject
and subordinate at all times to (a) all ground or underlying leases now existing
or which may hereinafter be executed affecting the Building, the Leased Premises
and/or the Property (b) the lien or liens of all mortgages and deeds of trust in
any amount or amounts whatsoever now or hereafter placed on the Building, the
Leased Premises and/or the Property or Landlord's interest or estate therein or
on or against such ground or underlying leases and (c) all renewals,
modifications, consolidations, replacements and extensions thereof. The
subordinations set forth herein shall be self-operative and effective without
the necessity of execution of any further instruments by any party; provided,
however, Tenant shall execute and deliver upon demand by Landlord any
instruments, releases or other documents requested by any lessor or mortgager
for the purpose of confirming the provisions hereof or further subjecting and
subordinating this Lease to any such ground lease, mortgage or deed of trust. In
the event of the enforcement by the trustee or the beneficiary under any such
mortgage or deed of trust, of the remedies provided for by law or by such
mortgage or deed of trust, upon request of any person or party succeeding to the
interest of Landlord as a result of such enforcement, Tenant will automatically
become the Tenant of such successor in interest without change in the terms or
provisions of this Lease; provided, however, that such successor in interest
shall not be bound by (i) any payment of rent or additional rent for more than
one month in advance except prepayments actually delivered to such successor in
the nature of security for the performance by Tenant of its obligations under
this Lease, (ii) any payment of the security deposit or any other

                                       15
<PAGE>
 
deposit unless such security deposit or other deposit has actually been
delivered to such successor or (iii) any amendment or modification of this Lease
made without the written consent of such trustee or such beneficiary or such
successor in interest, and Tenant shall execute and deliver an instrument or
instruments confirming the attornment and other agreements provided for herein.
Further, notwithstanding anything contained in this Lease to the contrary, in
the event of any default by Landlord in the performance of its covenants or
obligations hereunder which would give Tenant the right to terminate this Lease,
Tenant shall not exercise such right unless and until (i) Tenant gives written
notice of such default (which notice shall specify the exact nature of said
default and the steps necessary to cure same) to the holder of any mortgage or
deed of trust encumbering the Building, the Leased Premises and/or the Property
who has theretofore notified Tenant in writing of its interest and the address
to which notices are to be sent, and (ii) such holder, upon becoming entitled to
do so, through foreclosure of its lien, or accepting a deed in lieu of
foreclosure, or otherwise, fails to cure or cause to be cured such default
within thirty (30) days from the later of the receipt of such notice from Tenant
or its becoming entitled to do so, or, if such default relates to a condition
which cannot reasonably be cured within such period, such holder commences to
cure within such period and thereafter diligently prosecutes the completion of
such cure. Landlord shall, in the event that a mortgage or deed of trust lien is
placed upon the Property and/or the Building, utilize reasonable efforts to
obtain a non-disturbance agreement from the holder thereof with respect to
Tenant's leasehold interests under the Lease, which non-disturbance agreement
shall be deemed to be sufficient if in form and content identical to the form of
Subordination, Non-Disturbance and Attornment Agreement attached hereto as
Exhibit "L" and incorporated herein by reference for all purposes.

SECTION 4.12. CERTIFICATES. At any time and from time to time during the Lease
Term, within ten (10) days after written request by Landlord, Tenant will
execute, acknowledge and deliver to Landlord and any other persons specified by
Landlord a certificate certifying (to the extent of being true and accurate) (i)
that this Lease is in full force and effect, (ii) the date and nature of each
modification to this Lease , (iii) the date to which rental and other sums
payable to this Lease have been paid, (iv) that Tenant is not aware of any
default under this Lease which has not been cured, except such defaults as may
be specified in said certificate, and (v) such other matters as may be
reasonably requested by Landlord. Any such certificate may be relied upon by
Landlord and by any other person to whom it is delivered for such purpose.

SECTION 4.13. LIMITATION ON WEIGHT. Tenant shall not permit upon the floor of
the Leased Premises any weight exceeding seventy-five (75) pounds per square
foot of floor area.

                                    ARTICLE 5
                                    ---------

SECTION 5.1. CONDEMNATION. If all of the Building, or the whole or substantially
the whole of the Property (including surface and covered parking associated with
the Building) or the Leased Premises should be taken for any public or
quasi-public use, by right of eminent domain or otherwise or should be sold in
lieu of condemnation, then this Lease shall terminate as of the date when
physical possession of the Building, or the Leased Premises, or the Property is
taken by the condemning authority. If less than the whole of the Building or
less than the whole

                                       16
<PAGE>
 
 or substantially the whole of the Property (including surface and covered
parking associated therewith) or the Leased Premises is thus taken or sold,
Landlord (whether or not the Leased Premises are affected thereby) may terminate
this Lease by giving written notice thereof to Tenant; in which event this Lease
shall terminate as of the date when physical possession of such portion of the
Building, Property, or Leased Premises is taken by the condemning authority. If
this Lease is not so terminated upon any such taking or sale, the Base Rental
payable hereunder shall be diminished by a prorata amount representing that
portion of the Base Rental allocable to the portion, if any, of the Leased
Premises subject to such taking, and Landlord shall, to the extent Landlord
deems feasible, restore the Building shell to substantially their former
condition, but such work shall not exceed the scope of the work done by landlord
in originally constructing the Building, nor shall Landlord in any event be
required to spend for such work an amount in excess of the amount received by
Landlord as compensation for such taking. All amounts awarded upon a taking of
any part or all of the Property, the Building or the Leased Premises shall
belong to Landlord, and Tenant shall not be entitled to and expressly waives all
claim to any such compensation.

SECTION 5.2. CASUALTY DAMAGE. If the Leased Premises or any part thereof shall
be damaged by fire or other casualty, Tenant shall give prompt written notice
thereof to Landlord. In case the Building shall be so damaged that substantial
alteration or reconstruction of the Building shall, in Landlord's sole
discretion, be required (whether or not the Leased Premises shall have been
damaged by such casualty) or in the event any mortgagee of Landlord's should
require that the insurance proceeds payable as a result of a casualty be applied
to the payment of the mortgage debt or in the event of any material uninsured
loss to the Building, Landlord may, at its option, terminate this Lease by
notifying Tenant in writing of such termination within ninety (90) days after
the date of such casualty. If Landlord does not thus elect to terminate this
Lease, Landlord shall commence and proceed with reasonable diligence to restore
the Building shell; except that Landlord's obligation to restore shall not
require Landlord to spend for such work an amount in excess of the insurance
proceeds actually received by Landlord as a result of the casualty. When the
repairs described in the preceding sentence have been completed by Landlord,
Tenant shall restore all improvement necessary to permit Tenant's re-occupancy
of the Leased Premises, and the restoration of Tenant furniture and equipment.
All cost and expense of reconstructing the Leased Premises shall be borne by
Tenant. Landlord shall not be liable for any inconvenience or annoyance to
Tenant or injury to the business of Tenant resulting in any way from such damage
or the repair thereof, except that, subject to the provisions of the next
sentence, Landlord shall allow Tenant a fair diminution of rent during the time
and to the extent the Leased Premises are unfit for occupancy and are
unoccupied. If the Leased Premises or any other portion of the Building be
damaged by fire or other casualty resulting from the fault or negligence of
Tenant or any Tenant's agents, employees, or invitees, the rent hereunder shall
not be diminished during the repair and restoration of the Building and Tenant
shall be liable to Landlord for rent and for the cost of repair and restoration
of the Building caused thereby to the extent such cost and expense is not
covered by insurance proceeds actually received by Landlord. Notwithstanding the
foregoing, in the event that it is estimated that Landlord will need more than
one hundred eighty (180) days to repair the damage caused by such casualty,
Tenant shall have the right to terminate this Leased by providing written notice
to Landlord within five (5) days of Landlord's notice to Tenant.

                                       17
<PAGE>
 
SECTION 5.3. INSURANCE.

(a)      Landlord shall not be obligated to insure any of Tenant's goods, Trade
         Fixtures, furniture or any other property placed in or incorporated in
         the Leased Premises or the Building. Landlord shall maintain insurance
         during the Lease Term as required by then current lender.

(b)      Tenant shall, at its sole cost and expense, procure and maintain during
         the Lease Term, commercial general liability insurance (such insurance
         to afford minimum protection of not less than $5,000,000.00 combined
         single limit coverage of bodily injury, property damage or combination
         thereof), property insurance with respect to Tenant's personal
         property, inventory and leasehold improvements written on an all "All
         Risk" basis for full replacement cost, worker's compensation and
         employer's liability insurance, comprehensive catastrophe liability
         insurance and such other insurance as Landlord may, from time to time,
         reasonably require. In addition, Tenant agrees to obtain a fire legal
         liability endorsement or other coverage satisfactory to Landlord which
         removes the "owned, rented or occupied" property exclusion from
         Tenant's liability policy. All such insurance shall be maintained by
         companies on forms and in amounts approved by Landlord.

(c)      In the event that Tenant fails to take out or maintain any policy
         required by this Article to be maintained by Tenant, such failure shall
         be a defense to any claim asserted by Tenant against Landlord by reason
         of any loss sustained by Tenant that would have been covered by such
         policy.

(d)      All policies of insurance required to be maintained by Tenant shall
         provide that the Landlord shall be given at least thirty (30) days
         prior written notice of any cancellation or non-renewal of any such
         policy. A duplicate original of each such policy or a duly executed
         certificate of insurance with respect to each such policy shall be
         deposited with Landlord by Tenant on or before the Commencement Date,
         and a duplicate original of each subsequent policy or a duly executed
         certificate of insurance with respect to each subsequent policy shall
         be deposited with Landlord at least fifteen (15) days prior to the
         expiration of the policy then in force.

(e)      Tenant shall not do or permit anything to be done in the Building or
         about the Leased Premises nor bring nor keep nor permit anything to be
         brought to or kept therein, which will in any way increase the existing
         rate of or affect any fire or other insurance which Landlord carries
         upon any part of the Building or any of its contents, or cause a
         cancellation or invalidation of any such insurance. If the annual
         premiums to be paid by Landlord with respect to any insurance obtained
         by Landlord covering any part of the Building or any of its contents
         shall exceed the standard rates because of Tenant's operations, or
         contents of the Leased Premises or because improvements with respect to
         the Leased Premises result in extra-hazardous exposure, Landlord shall
         have the further right, exercisable in Landlord's sole discretion, to
         terminate this Lease by giving written notice of such election to
         Tenant.

                                       18
<PAGE>
 
(f)      Subject to the conditions hereinafter specified in this Subsection (f)
         and only to the extent that and so long as the same is permitted under
         the laws and regulations governing the writing of insurance within the
         State of Texas with respect to the respective insurance that is to be
         carried by either Landlord or Tenant covering losses arising out of the
         destruction or damage to the Leased Premises or its contents or to
         other portions of the Building or to Tenant's occupancy and operation
         of the Leased Premises without invalidating or nullifying any such
         policy, or providing a defense to the applicable insurance carrier with
         respect to the coverage of any such policy, all such insurance carried
         by either Landlord or Tenant shall provide for a waiver of rights of
         subrogation against Landlord and Tenant on the part of the insurance
         carrier. Notwithstanding the foregoing, nothing contained herein shall
         require either party to obtain the inclusion of such a waiver of rights
         of subrogation in the event that, because of the cost or premium
         attributable to such waiver, the obtaining of such waiver is not
         feasible and reasonable. Except as otherwise provided in Section 5.2
         hereof or in the event that such waivers contemplated by this sentence
         will invalidate, nullify, or provide a defense to coverage under any
         such insurance policy or are not obtainable for the reasons described
         in this Subsection (f), Landlord and Tenant each hereby waive any and
         all rights of recovery, claims, actions or causes of action against the
         other, its agents, officers, or employees, or any loss or damage that
         may occur to the Leased Premises or the Building, or any improvements
         thereto, which loss or damage is covered by valid and collectible
         insurance policies, to the extent that such loss and damage is
         recoverable under such insurance policy. The waivers set forth in the
         immediately preceding sentence shall be in addition, and not
         substitution for, any other waivers, indemnities, or exclusions of
         liabilities as set forth in this Lease, including, without limitation,
         Sections 5.5 and 5.6 of the Lease.

SECTION 5.4. SURRENDER OF LEASED PREMISES. Upon termination of this Lease or
Tenant's right to possession of the Leased Premises, Tenant shall peaceably and
quietly surrender the Leased Premises to Landlord, broom-clean and in a good
state of repair and condition, excepting only ordinary wear and tear. Upon
request of Landlord, Tenant shall demolish or remove all or any portion of any
Trade Fixtures and other property or the making of any such alteration,
improvement, addition or change. Notwithstanding the foregoing, Tenant shall be
required to remove only those items which Landlord specified are to be removed
at the time that Landlord consents to the installation thereof. All such
demolition, removal and restoration shall be performed in accordance with the
conditions set forth in Section 4.2(b). Upon termination of this Lease, Tenant
will also surrender to Landlord all keys to the Leased Premises and inform
Landlord of all combinations on locks, safe, and vaults, if any, at the Leased
Premises.

SECTION 5.5. DAMAGES FROM CERTAIN CAUSES. Landlord and Landlord's agents and
employees shall not be liable or responsible to Tenant or any person claiming
through Tenant for any loss or damage or injury to business or to any property
or person in, upon or about the Leased Premises or any other portion of the
Building arising at any time from any cause, negligent or otherwise, other than
solely by reason of the gross negligence or willful misconduct of Landlord or of
Landlord's employees or agents acting within the scope of their employment or
authority.

                                       19
<PAGE>
 
SECTION 5.6. HOLD HARMLESS. Landlord shall not be liable to Tenant, or to
Tenant's agents, employees, contractors, customers or invitees or to any other
person whomsoever for any injury or damage to person or property caused by or
arising out of an act, omission or neglect of Tenant, its agents, contractors,
subtenants, employees, customers, licensees, concessionaires or invitees or any
other person entering the Building under express or implied invitation of Tenant
or other tenants of the Building, and Tenant agrees to indemnify and hold
Landlord harmless from all liability and claims for any such damage and from all
claims, costs, damages or liabilities arising out of the foregoing, including
without limitation attorneys' fees and all other out-of-pocket expenses incurred
in connection therewith. Likewise, Landlord shall indemnify and hold Tenant
harmless from all liability and claims for any damage incurred by Tenant as a
result of and from all claims for any damage incurred by Tenant as a result of
and from all claims, costs, damages, or liabilities arising out of the gross
negligence or willful misconduct of Landlord, its agents or employees,
including, without limitation, attorneys' fees and all other out-of-pocket
expenses incurred in connection therewith. In any case in which Landlord or
Tenant has agreed to indemnify the other, such indemnity shall be deemed to
include an obligation on the part of the indemnifying party to appear on behalf
of the indemnified party in any and all proceedings involving a claim or cause
of action covered by such indemnity and to defend the indemnified party against
such claim or cause of action, all at the indemnifying party's cost; provided,
however, at the option of any party indemnified hereunder, such party shall have
the right to appear on its own behalf, employ its own legal counsel and defend
any claim or cause of action indemnified in this Section, all at the cost of the
indemnifying party's.

                                    ARTICLE 6
                                    ---------

SECTION 6.1. DEFAULT BY TENANT. The occurrence of any one or more of the
following events shall constitute a default by Tenant under this Lease:

(a)      Failure of the Tenant to pay rent or any other amount due under this
         Lease within ten (10) days after Tenant's receipt of Landlord's written
         notice of such failure to pay provided Landlord shall be required to
         give such notice only once in any twelve (12) month period and
         thereafter Tenant shall be in default if any such payment is not
         received when due and without notice;

(b)      Failure of the Tenant to perform, observe, or comply with or default
         under any of the terms, covenants, conditions or provisions contained
         in Section 4.1 of this Lease within twenty-four (24) hours after
         written notice to Tenant with respect thereto, if such condition cannot
         reasonably be cured within such twenty-four (24) period, Tenant shall
         thereafter diligently and continuously prosecute such cure to
         completion within five (5) days from the date of Landlord's notice of
         such default provided that Landlord shall be required to give such
         notice only once in any twelve (12) month period and the second
         violation of the provisions contained in Section 4.1 hereof by Tenant
         shall constitute a default by Tenant hereunder whether or not Landlord
         shall provide Tenant with notice thereof and;

                                       20
<PAGE>
 
(c)      Failure of the Tenant to perform, observe, or comply with or default
         under the negative covenant or agreement set forth in Sections 4.7 and
         4.10 of the Lease and all other covenants and agreements set forth in
         this Lease which prohibit or restrict Tenant from taking or omitting to
         take any action without the consent of the Landlord or which requires
         the Tenant to take action upon the request of the Landlord;

(d)      Failure of the Tenant to perform, observe, or comply with or default
         under any of the terms, covenants, conditions or provisions contained
         in this Lease (other than covenants to pay rent, the covenants set
         forth in Sections 4.1 and 8.3 of this Lease, negative covenants and
         agreements set forth in this Lease and all other covenants and
         agreements set forth in this Lease which prohibit or restrict the
         Tenant from taking or omitting to take any action upon request of the
         Landlord) and such failure or default is not cured to Landlord's
         satisfaction within thirty (30) calendar days after the Landlord has
         given Tenant written notice thereof or, if such condition cannot
         reasonably be cured within such thirty (30) day period, Tenant shall
         thereafter diligently and continuously prosecute such cure to
         completion within eighty (80) days from the date of Landlord's notice
         of such default, except as such eighty (80) day period otherwise be
         delayed by the provisions of Section 8.1 hereunder;

(e)      The interest of Tenant under this Lease shall be levied on under
         execution or other legal process;

(f)      Any petition in bankruptcy or other insolvency proceedings shall be
         filed by or against Tenant, or any petition shall be filed or other
         action taken to declare Tenant a bankrupt or to delay, reduce or modify
         Tenant's debts or obligations or to reorganize or modify Tenant's
         capital structure or indebtedness or to appoint a trustee, receiver or
         liquidator of Tenant or of any property of Tenant, or any proceeding or
         other action shall be commenced or taken by any governmental authority
         for the dissolution or liquidation of Tenant to the extent not cured or
         removed within sixty (60) days notice from Landlord;

(g)      Tenant shall become insolvent, or Tenant shall make an assignment for
         the benefit of creditors, or Tenant shall make a transfer in fraud of
         creditors, or a receiver or trustee shall be appointed for Tenant or
         any of its properties;

(h)      Tenant shall abandon the Leased Premises or any substantial portion
         thereof;

(i)      Tenant shall do or permit to be done anything which creates or causes
         to be filed a lien, security interest or other encumbrance (whether
         consensual or created by operation of law or otherwise) against all or
         any part of the Leased Premises, the Building or any property situated
         therein or Tenant's interest in this Lease; or

(j)      The death or legal incapacity of Tenant if Tenant is an individual
         person or the termination, dissolution or liquidation of Tenant, if
         Tenant is a corporation, partnership, or other entity.

                                       21
<PAGE>
 
(k)      Failure of the Tenant to perform, observe, or comply with or default
         under any of the terms, covenants, conditions or provisions contained
         in Section 8.3 hereof and default is not cured within fifteen (15) days
         of Landlord's notice to Tenant of such default.

SECTION 6.2. LANDLORD'S REMEDIES. Upon the occurrence of any default by Tenant
under this Lease, Landlord may, at its sole option, do any one or more of the
following, without any notice or demand for possession whatsoever, and Tenant
hereby waives any and all notice and demand requirements imposed by applicable
law:

(a)      Terminate this Lease, whereupon Landlord shall have the remedies set
         forth in Section 6.3 below;

(b)      Without having terminated this Lease, enter upon and take possession of
         the Leased Premises, whereupon Landlord shall have the remedies set
         forth in Section 6.4 below; or

(c)      Upon Tenant's failure to perform, observe or comply with the covenants
         set forth in Sections 2.2, 4.1, or 4.7 of this Lease, Landlord may,
         without terminating this Lease and without taking possession of the
         Leased Premises, collect from Tenant, in addition to any rent payable
         by Tenant to Landlord under this Lease, as liquidated damages, a sum
         equal to twice the Base Rental (computed on a daily basis) for each day
         or any portion thereof that such default by Tenant continues, Landlord
         and Tenant agreeing that actual damages which might be sustained by
         Landlord by reason of such failure are uncertain and difficult to
         ascertain and that said sum would be reasonable and just compensation
         for such failure.

SECTION 6.3. TERMINATION OF LEASE. Upon termination of this Lease by Landlord,
pursuant to Section 6.2(a), Landlord may forthwith repossess the Leased Premises
and be entitled to recover as damages a sum of money equal to the total of (i)
the cost of recovering the Leased Premises, (ii) the cost of removing and
storing Tenant's or any other occupant's property, (iii) the unpaid rent accrued
at the date of termination, and (iv) any other sum of money or damages that may
be owed to Landlord as the result of the exercise of Landlord's rights at law or
in equity.

SECTION 6.4. TERMINATION OF POSSESSION. Upon termination of Tenant's right of
possession to the Leased Premises pursuant to Section 6.2(b), Landlord may
repossess the Leased Premises by forcible entry or detainer suit or otherwise,
without demand or notice of any kind to Tenant and without terminating this
Lease, in which event Landlord may (but shall not be obligated to) relet the
same for the account of Tenant for such rent and upon such terms as shall be
satisfactory to Landlord. In such event, Tenant shall be liable for and shall
pay to Landlord all rent payable by Tenant under this Lease plus an amount equal
to (i) the cost of recovering possession, (ii) the cost of decorations, repairs,
changes, alterations and additions to the Leased Premises, (iii) the cost of
collection of the rent accruing from such reletting, and (iv) any other costs
incurred by Landlord in connection with such reletting, reduced by any sums
received by Landlord through reletting the Leased Premises; provided, however,
that in no event shall Tenant be entitled to any excess of any sums obtained by
reletting over and above rent provided in this Lease to be paid by Tenant to
Landlord. For the purpose of such reletting, Landlord is

                                       22
<PAGE>
 
authorized to decorate or to make any repairs, changes, alterations or additions
in or to the Leased Premises that Landlord may deem necessary or advisable.
Landlord may file suit to recover any sums falling due under the terms of this
Section from time to time, and no delivery to or recovery by Landlord of any
portion due Landlord hereunder shall be any defense in any action to recover any
amount not theretofore reduced to judgment in favor of Landlord. No reletting
shall be construed as an election on the part of Landlord to terminate this
Lease unless a written notice of such intention is given to Tenant by Landlord.
Notwithstanding any such reletting without termination, Landlord may at any time
thereafter elect to terminate this Lease for such previous default.

SECTION 6.5. LANDLORD'S RIGHT TO PERFORM TENANT'S OBLIGATIONS., Should Tenant
fail to perform any of its obligations hereunder, Landlord may (but shall not be
obligated to), after written notice to Tenant, (except in an emergency, when no
notice shall be required) enter upon the Leased Premises and perform all or any
part of such obligations. Upon demand, Tenant shall reimburse Landlord for the
cost to Landlord of performing such obligations plus profit and overhead in an
amount equal to fifteen percent (15%) of such cost. No action taken by Landlord
under this Section shall relieve Tenant from any of its obligations under this
Lease or from any consequences or liabilities arising from the failure to
perform such obligations.

SECTION 6.6. CUMULATIVE REMEDIES. The rights and remedies of Landlord under this
Article shall be non-exclusive and shall be in addition to and cumulative of all
other remedies available to Landlord under this Lease or at law or in equity.

SECTION 6.7.  LANDLORD'S LIEN. Intentionally omitted.

SECTION 6.8. HOLDING OVER. In the event Tenant remains in possession of the
Leased Premises after the expiration or termination of this Lease without the
execution of a new lease, then Tenant shall be deemed to be occupying the Leased
Premises as a tenant from month to month at a rental equal to 150% of the Base
Rental for the first forty-five (45) days and 200% of the Base Rental thereafter
and shall otherwise remain subject to all the conditions, provisions and
obligations of this Lease insofar as the same are applicable to a month to month
tenancy. No holding over by Tenant after the expiration or termination of this
Lease shall be construed to extend the Lease Term or in any other manner be
construed as permission by Landlord to hold over.

SECTION 6.9. SECURITY DEPOSIT. Tenant shall pay the Security Deposit to Landlord
on the date this Lease is executed by Tenant. Landlord may commingle the
Security Deposit with its other funds and shall receive and hold the Security
Deposit without liability for interest. Upon default by Tenant, Landlord may,
from time to time, without prejudice to any other remedy, apply such Security
Deposit to the extent necessary to make good any arrears of rental or any other
damage, injury, expense or liability caused by Landlord by reason of default by
the Tenant. After any such application of Security Deposit, Tenant shall, upon
request of Landlord, pay to Landlord the amount so applied so as to restore the
Security Deposit to its original amount. Any remaining balance of the Security
Deposit shall be returned by Landlord to Tenant within a reasonable period of
time after the termination of this Lease. If Landlord transfers its

                                       23
<PAGE>
 
interest in the Leased Premises during the term of this Lease, Landlord may
assign the Security Deposit to the transferee and thereafter shall have no
further liability for the return of such Security Deposit.

                                    ARTICLE 7
                                    ---------

SECTION 7.1. ATTORNEY'S FEES AND OTHER EXPENSES. In the event of the default by
either party hereto in the performance or observance of any of the terms,
agreements or conditions contained in this Lease, the defaulting party shall be
liable for and shall pay the prevailing party all expenses incurred by the
prevailing party in enforcing any of the prevailing party's remedies for any
such default, including, without limitation, the prevailing party's reasonable
attorney's fees.

SECTION 7.2. AMENDMENTS, BINDING EFFECT. This Lease may not be altered, changed
or amended, except by instrument in writing signed by both parties hereto. No
provision of this Lease shall be deemed to have been waived by Landlord unless
such waiver be in writing signed by Landlord and addressed to Tenant, nor shall
any custom or practice which may evolve between the parties in the
administration of the terms hereof be construed to waiver or lessen the right of
Landlord to insist upon the performance by Tenant in strict accordance with the
terms hereof. The terms and conditions contained in this Lease shall apply to,
inure to the benefit of, and be binding upon the parties hereto, and upon their
respective successors in interest and legal representatives, except as otherwise
herein expressly provided.

SECTION 7.3. NON-WAIVER. No course of dealing between Landlord and Tenant or any
other person, nor any delay on the part of Landlord in exercising any rights
under this Lease, nor any failure to enforce any provision of this Lease, nor
the acceptance of rental by Landlord shall operate as a waiver of any rights of
Landlord, except to the extent, if any, expressly waived in writing by Landlord.
The waiver by Landlord of any agreement, condition or provision herein contained
shall not be deemed a waiver of any subsequent breach of the same or any other
agreement, condition or provision herein contained.

SECTION 7.4. NOTICES. Any notice or other communications to Landlord or Tenant
required or permitted to be given under this Lease must be in writing and shall
be effectively given if hand delivered or transmitted by facsimile to the
addresses for Landlord and Tenant stated above or if sent by United States Mail,
certified or registered, return receipt requested, to said addresses. Any notice
mailed shall be deemed to have been given three (3) calendar days following the
date of deposit of such item in a depository of the United States Postal
Service. Any notice by facsimile shall be deemed delivered upon confirmation of
receipt on the day of receipt or, if after 5:00 p. m. the next business day. Any
other form of notice effected other than by mail shall be deemed to have been
given at the time of actual delivery. Either party shall have the right to
change its address to which notices shall thereafter be sent by giving the other
written notice thereof.

SECTION 7.5. INTEREST. All amounts of money payable by Tenant to Landlord under
this Lease shall bear interest from the date due until paid at the rate of
interest set forth in Section 2.3.

                                       24
<PAGE>
 
SECTION 7.6. MERGER OF ESTATES. The voluntary or other surrender of this Lease
by Tenant or a mutual cancellation thereof, shall not constitute a merger; and
upon such surrender or cancellation of this Lease, Landlord shall have the
option, in Landlord's sole discretion, to (i) either terminate all or any
existing subleases or subtenancies, or (ii) assume Tenant's interest in any or
all subleases or subtenancies.

SECTION 7.7. OTHER TENANTS OF BUILDING. Neither this Lease nor Tenant's
continued occupancy of the Leased Premises is conditioned upon the opening of
any store or business in the Building, nor upon the continued operation of any
such store or business.

SECTION 7.8. CONSENT BY LANDLORD. In all circumstances under this Lease where
the prior consent or permission of Landlord is required before Tenant is
authorized to take any particular type of action, such consent must be in
writing and the matter of whether to grant such consent or permission shall be
within the sole and exclusive judgment and discretion of Landlord, except as set
forth herein, and it shall not constitute any nature of breach by Landlord under
this Lease or any defense to the performance of any covenant, duty or obligation
of Tenant under this Lease that Landlord delayed or withheld the granting of
such consent or permission.

SECTION 7.9. LEGAL INTERPRETATION. This lease and the rights and obligations of
the parties hereto shall be interpreted construed and enforced in accordance
with the laws of the State of Texas and the United States. All obligations of
the parties hereto shall be performable in, and all legal actions to enforce or
construe this Lease shall be instituted in the courts of Dallas County, Texas.
The determination that one or more provisions of this Lease is invalid, void,
illegal or unenforceable shall not affect or invalidate the remainder. All
obligations of either party requiring any performance after the expiration of
the Lease Term shall survive the expiration of the Lease Term and shall be fully
enforceable in accordance with those provisions pertaining thereto. Section
titles appearing in this Lease are for convenient reference only and shall not
be used to interpret or limit the meaning of any provision of this Lease.

SECTION 7.10. ENTIRE AGREEMENT. Tenant agrees that this Lease supersedes and
cancels any and all previous statements, negotiations, arrangements, brochures,
agreements and understandings, if any, between Landlord and Tenant or displayed
by Landlord to Tenant with respect to the subject matter of this Lease, the
Leased Premises or the Building, and that there are no representations,
agreements or warranties (express or implied, oral or written) between Landlord
and Tenant with respect to the subject matter of this Lease, the Leased Premises
or the Building other than contained in this Lease.

SECTION 7.11. ASSIGNMENT BY LANDLORD. Landlord shall have the right at any time
to transfer and assign in whole or in part, by operation of law or otherwise,
its rights, benefits, privileges, duties and obligations hereunder or in the
Property.

SECTION 7.12. TENANT'S AUTHORITY. Tenant represents and warrants that it has the
full right, power and authority to enter into this Lease and to perform its
obligations hereunder, and that upon execution of this Lease by Tenant, this
Lease shall constitute a valid and legally binding obligation of Tenant. If
Tenant signs as a corporation, each of the persons executing this

                                       25
<PAGE>
 
Lease on behalf of Tenant does hereby covenant and warrant that Tenant is a duly
and validly existing corporation, that the execution of this Lease by such
persons on behalf of Tenant has been duly authorized by all necessary corporate
action and that Tenant is qualified to do business in the State of Texas.

SECTION 7.13. LANDLORD'S LIABILITY. Any provisions of this Lease to the contrary
notwithstanding, Tenant hereby agrees that no personal, partnership or corporate
liability of any kind or character whatsoever now attaches or at any time
hereafter under any condition shall attach to Landlord or its partners or
venturers for payment of any amounts payable under this Lease or for the
performance of any obligation under this Lease. The exclusive remedy of Tenant
for the failure of Landlord to perform any of its obligations under this Lease
shall be to proceed against the interest of Landlord in and to the Building. In
no event shall Landlord incur any liability or obligation to Tenant or other
person or entity with respect to any action, omission, or inaction, negligent or
otherwise, of Landlord, except as such may be due to the gross negligence or
willful misconduct of Landlord.

SECTION 7.14. TIME OF ESSENCE. In all instances where Tenant is required under
this Lease to pay any sum or do any act at a particular time or within a
particular period, it is understood that time is of the essence.

SECTION 7.15. INSTRUMENTS AND EVIDENCE REQUIRED TO BE SUBMITTED TO LANDLORD.
Each written instrument and all evidence of the existence or non-existence of
any circumstances or condition which is required by this Lease to be furnished
to Landlord shall in all respects be in form and substance satisfactory to
Landlord, and the duty to furnish such written instrument or evidence shall not
be considered satisfied until Landlord shall have acknowledged that it is
satisfied therewith.

SECTION 7.16. COUNTERPARTS. This Lease may be executed in any number of
counterparts, each of which when so executed and delivered shall be an original,
but such counterparts shall together constitute one and the same instrument.

SECTION 7.17. GENDER AND NUMBER. The pronouns of any gender shall include the
other gender and either the singular or the plural shall include the other.

SECTION 7.18. RECORDATION. Tenant agrees not to record this Lease or any
instrument to which this Lease may now or hereafter be attached.

                                    ARTICLE 8
                                    ---------

SECTION 8.1. FORCE MAJEURE. Except for the payment of Base Rental or any
additional sums payable by Tenant hereunder, whenever a period of time is herein
prescribed for the taking of any action by Landlord or Tenant, Landlord or
Tenant shall not be liable or responsible for, and there shall be excluded from
the computation of such period of time, any delays due to strikes, riots, acts
of God, shortages of labor or materials, war, governmental laws, regulations or
restrictions, or any other cause whatsoever beyond the control of such party.

                                       26
<PAGE>
 
SECTION 8.2. COMMISSIONS. Landlord has agreed to pay commission to Grubb & Ellis
with respect to the negotiation and execution of this Lease. Tenant hereby
indemnifies and holds Landlord harmless against any loss, claim, expense or
liability with respect to any other commissions or brokerage fees claimed on
account of the execution and/or renewal of this Lease or the expansion of the
Leased Premises hereunder, if applicable, due to any action by Tenant. The
provisions of this paragraph shall survive the termination of this Lease.

SECTION 8.3. USE OF THE TERM "INFOMART". Tenant may use the term "INFOMART" in
any of its activities, provided that (a) such usage by Tenant is in compliance
with such agreement relative to Landlord's use of the term "INFOMART" ("License
Agreement") and (b) copies of all proposed written materials and advertising
containing reference to the term "INFOMART" shall be furnished to Landlord in
advance for its review and written approval. Any permitted use of the term
"INFOMART" by Tenant shall additionally include the phrase "The Technology
Community" immediately after the first (1st) usage of such term in any material.
Landlord shall notify Tenant in writing after such proposed usage has been
verified, and Tenant may thereafter commence such requested usage. Tenant shall
not permit any third party to use the term "INFOMART" in any of its activities
and shall report to Landlord any unauthorized uses of such term which comes to
its attention. The breach by Tenant of any provisions of this Section 8.3 shall
constitute an event of default under this Lease and Tenant shall cease and
desist immediately from any unauthorized use within fifteen (15) days of
Landlord's notice of such breach. Tenant shall indemnify and hold Landlord
harmless from against any loss, cost, claim, liability, cause of action, or
expense whatsoever (including, without limitation, attorney's fees and other
costs and expenses of defending against any such claim) arising or alleged to
arise from any unauthorized use by Tenant, or its agents or employees of the
term "INFOMART".

SECTION 8.4. EFFECT OF DELIVERY OF THIS LEASE. Landlord has delivered a copy of
this Lease to Tenant for Tenant's review only, and the delivery hereof does not
constitute an offer or option to Tenant. This Lease shall not be effective until
a copy executed by both Landlord and Tenant is delivered to and accepted by
Landlord, and this Lease has been approved by Landlords' mortgagee.

SECTION 8.5. EXHIBITS, SCHEDULES AND RIDERS. The Exhibits and Riders attached to
this Lease are hereby incorporated herein and hereby made a part of this Lease.

                                       27
<PAGE>
 
IN TESTIMONY HEREOF, the parties have executed this Lease as of the day and year
first above written.

                             L A N D L O R D
                             ---------------

                             INFOMART-Dallas, L.P., a Texas limited partnership

                             By:INFOMART USA, L.P., a Texas limited partnership

                             By: /s/ Thomas E. Jones
                                --------------------------------
                             Name: Thomas E. Jones

                             Title:  Authorized Agent

                             T E N A N T
                             -----------

                             FOCAL COMMUNICATIONS CORPORATION OF
                             TEXAS, a Delaware corporation

                             By: /s/ Brian F. Addy
                                --------------------------------
                             Name:   Brian F. Addy
                                  ------------------------------
                             Title:  Executive Vice President
                                   -----------------------------

Exhibit "A" - Property Description
Exhibit "B" - Building Rules
Exhibit "C" - Designation of Leased Premises
Exhibit "D" - INFOMART Policy Statement
Exhibit "E" -  Parking
Exhibit "F" - Telecommunications Equipment License
Exhibit "G" - Satellite/Antenna License
Exhibit "H" - Generator
Exhibit "I" - Renewal Option
Exhibit "J" - Work Letter (Allowance)
Exhibit "K" - Rooftop Cooling Provisions
Exhibit "L" - Subordination, Non-Disturbance and Attornment Agreement


                                       28
<PAGE>
 
                                   EXHIBIT "A"
                                   -----------
      To Lease Agreement By and Between INFOMART-Dallas, L.P., as Landlord
            and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                              PROPERTY DESCRIPTION
                              --------------------

BEING a 25.454 acre tract of land situated in the City of Dallas, Dallas County,
Texas and out of the James A. Sylvester Survey, Abstract No. 1383 and being a
part of City of Dallas Block No. 6053, also being the same tract of land
conveyed to Dallas Market Center Company by a Special Warranty Deed recorded in
Volume 82113, Page 3240 of the Deed Records of Dallas County, Texas, said 25.454
acre tract of land being more particularly described as follows:

BEGINNING at a 1/2 inch iron rod found for the point of intersection of the
southwesterly right-of-way line of the Chicago Rock Island and Pacific Railroad
with the northwesterly right-of-way line of Oak Lawn Avenue;

THENCE with the northwesterly right-of-way line of Oak Lawn Avenue the
following:

         South 31 31'40" West a distance of 366.74 feet to an "X" chiseled in
         concrete found for corner in a curve to the right, the radius point of
         said curve bearing North 50 08'58" West a distance of 241.00 feet from
         said "X";

         Southwesterly with said curve to the right through a central angle of
         03 09'20" an arc distance of 13.27 feet to an "X" chiseled in concrete
         set for the point of reverse curvature of a curve to the left having a
         radius of 259.00 feet;

         Southwesterly with said curve to the left through a central angle of 11
         28'43" an arc distance of 51.89 feet to a 1/2 inch iron rod found for
         the point of reverse curvature of a curve to the right having radius of
         129.00 feet;

         Southwesterly with said curve to the right through a central angle of
         24 06'22" an arc distance of 138.22 feet to a 1/2 inch iron rod set for
         the point of compound curvature of a curve to the right having a radius
         of 50.00 feet;

         Northwesterly with said curve to the right through a central angle of
         24 06'22" an arc distance of 21.04 feet to a 1/2 inch iron rod found in
         the northeasterly right-of-way line of Stemmons Freeway for the point
         of compound curvature of a curve to the right having a radius of
         1130.92 feet;

         THENCE with the northeasterly right-of-way line of Stemmons Freeway the
         following:

         Northwesterly with said curve to the right through a central angle of
         07 24'40" an arc distance of 146.28 feet to a 1/2 inch iron rod found
         for the point of tangency of said curve;

                                       29
<PAGE>
 
         North 55 33'45" West a distance of 816.18 feet to a 1/2 inch iron rod
         found for point of curvature of a curve to the left having a radius of
         3289.04 feet;

         Northwesterly with said curve to the left through a central angle of 01
         23'21" an arc distance of 79.74 feet to a bolt in concrete found for
         the most southerly corner of a tract of land leased to Southwestern
         Furniture Mart Co. from Industrial Properties Corporation as recorded
         in Volume 67076, Page 0690 of the Deed Records of Dallas County, Texas;

THENCE departing the northerly right-of-way line of Stemmons Freeway with the
easterly line of the Southwestern Furniture Mart Company tract, North 09 21'30"
East a distance of 1064.46 feet to a 1/2 inch iron rod found for corner in the
curving southwesterly right-of-way line of the Chicago, Rock Island and Pacific
Railroad, the radius point of said curve being situated South 33 11'48" West a
distance of 1599.88 feet;

THENCE with the southerly right-of-way lien of the Chicago, Rock Island and
Pacific Railroad the following:

         Southeasterly with said curve to the right through a central angle of
         02 41'48" an arc distance of 75.30 feet to a 1/2 inch iron rod found
         for corner;

         North 52 07'00" East a distance of 30.11 feet to a 1/2 inch iron rod
         found for corner in a curve to the right, the radius point of said
         curve being situated South 32 19'18" West a distance of 1553.95 feet;

         Northwesterly with said curve to the right through a central angle of
         21 26'39" an arc distance of 581.59 feet to a 1/2 inch iron rod set for
         corner;

         North 45 16'10" East a distance of 53.07 feet to 1/2 inch iron rod set
         for corner;

         South 31 48'40" East a distance of 976.20 feet to the POINT OF
         BEGINNING;

CONTAINING an area of 25.454 acres of land.

                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------

                                       30
<PAGE>
 
                                   EXHIBIT "B"
                                   -----------

      To Lease Agreement By and Between INFOMART-Dallas, L.P., as Landlord
            and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

--------------------------------------------------------------------------------

                              RULES AND REGULATIONS
                              ---------------------

1.       No additional locks shall be placed on the doors of the Leased Premises
         by Tenant, nor shall any existing locks be changed unless Landlord is
         immediately furnished with two keys thereto. Landlord will without
         charge furnish Tenant with two keys for each lock existing upon the
         entrance doors when Tenant assumes possession with the understanding
         that at the termination of the lease these keys shall be returned or
         paid for at five dollars ($5.00) each. A deposit of one dollar ($1.00)
         each shall be required for additional keys.

2.       Tenant shall not at any time display a "For Rent" sign upon the
         Building or the Leased Premises, or advertise the Leased Premises for
         rent.

3.       Safes and other unusually heavy objects shall be placed by Tenant only
         in such places as may be approved by Landlord. Any damage caused by
         overloading the floor or by taking in or removing any object from the
         Leased Premises or the Building shall be paid by Tenant.

4.       Windows facing on corridors shall at all times be wholly clear and
         uncovered (except for such signs as Landlord may approve) so that a
         full unobstructed view of the interior of the Leased Premises may be
         had from the corridors, unless otherwise approved in writing by
         Landlord.

5.       No vehicles or animals shall be brought into the Building, other than
         as required by handicapped persons.

6.       Tenant shall not make any changes in the pipes, ducts, or wiring
         serving the Leased Premises or add any additional pipes, ducts, or
         wiring without the prior written consent of Landlord, and any such
         changes or additions shall be made in such manner as Landlord may
         direct.

7.       No sign, tag, label, picture, advertisement, or notice (other than
         price tags of customary size used in marking samples) shall be
         displayed, distributed, inscribed, painted or affixed by Tenant on any
         part of the outside of the Building or of the Leased Premises without
         the prior written consent of the Landlord.

8.       In the event Landlord should advance upon the request, or for the
         account of the Tenant, any amount for labor, material, packing,
         shipping, postage, freight or express upon articles delivered to the
         Leased Premises or for the safety, care, and cleanliness of the Leased
         Premises, the amount so paid shall be regarded as additional rent and
         shall be due and payable forthwith to the Landlord from the Tenant.

                                       31
<PAGE>
 
9.       The corridors and hallways of the Building shall not be used by Tenant
         for any purpose other than ingress to or egress from the Leased
         Premises.

10.      Tenant shall not do or permit to be done within the Leased Premises
         anything which would unreasonably annoy or interfere with the rights of
         other tenants in the Building, or which might constitute a potential
         hazard to other tenants or visitors.

11.      During the thirty (30) days prior to the expiration of this Lease,
         Landlord may show the Leased Premises to prospective tenants.

12.      Tenant shall not put or operate any steam engine, boiler, industrial
         machinery or stove in the Building or upon the Leased Premises or do
         any cooking thereon or use or allow to be kept in the Building or upon
         the Leased Premises any explosives or any kerosene, camphene, bottled
         gas, oil or other highly flammable materials, except gas supplied
         through metal pipes for heating purposes and normal and customary
         cleaning and janitorial supplies to the extent permitted under
         applicable laws.

13.      Landlord reserves the right to prescribe reasonable qualifications for
         admission into the Building.

14.      Models, salespersons or other employees or representatives of Tenant,
         shall not model, demonstrate display, or show in any manner any
         merchandise outside of the Leased Premises in the Building or on the
         Property without Landlord's prior written consent.

15.      As a courtesy, but not as an obligation, Landlord may, at Landlord's
         option, upon request by Tenant, receive and store articles or
         merchandise delivered to Tenant at the Building; provided, however that
         such articles of merchandise are properly addressed and identified and
         all postage, handling and delivery charges are prepaid by Tenant.
         Landlord assumes no responsibility whatsoever for the loss, damage or
         destruction of such articles of merchandise received at the Building by
         Landlord on behalf of Tenant, and Tenant hereby waives all claims
         against Landlord for any damage or loss arising at any time from the
         loss, damage or destruction of such articles of merchandise. Tenant
         agrees to pay to Landlord as additional rent the amount of all storage,
         delivery, handling and other expenses incurred by Landlord as a result
         of the receipt and storage of such articles of merchandise.

16.      Canvassing, peddling, soliciting and distribution of handbills or any
         other written material in the Building or in the Building's parking
         areas are prohibited, and each tenant shall cooperate to prevent the
         same.

17.      If the Leased Premises front on the atrium within the Building, Tenant
         shall cause the Leased Premises to be kept open for business and
         occupied by Tenant's personnel during all normal business hours of the
         Building.

18.      These Rules and Regulations are in addition to, and shall not be
         construed to in any way

                                       32
<PAGE>
 
         modify or amend, in whole or in part, the terms, covenants, agreements
         and conditions of any lease of space in the Building.

19.      Landlord reserves the right to make such other and reasonable rules and
         regulations as in its judgment may from time to time be needed for the
         safety, care and cleanliness of the Building, and for the preservation
         of good order therein.

20.      Smoking is not permitted within the Building. Smoking within the Leased
         Premises is at the discretion of Tenant, provided, however, that such
         smoke does not migrate into the Building's common areas, hallways, etc.
         or into another tenant's premises. Tenant hereby indemnifies Landlord
         from any and all claims resulting from Tenant's permitting of smoking
         within the Leased Premises.

21.      Tenant shall comply with the INFOMART Policy Statement. Tenant shall be
         liable for all injuries and damages sustained by Landlord or Landlord's
         agents or by other tenants, occupants, or invitees of the Building by
         reason of any breach of the requirements of the INFOMART Policy
         Statement by Tenant or Tenant's agents, employees or invitees.

22.      Landlord may amend these Rules and Regulations from time to time and
         such changes shall be binding upon Tenant.



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                    Landlord                 Tenant
                            -----------            -----------


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                                       34
<PAGE>
 
                                   EXHIBIT "C"
                                   -----------
            To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

--------------------------------------------------------------------------------

                         DESIGNATION OF LEASED PREMISES
                         ------------------------------
                        (For illustrative purposes only)

                            [Floorplan Appears here]




                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------

                                       35
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                                       36
<PAGE>
 
                                  EXHIBIT "D"
                                  -----------

    To Lease Agreement By and Between INFOMART-Dallas, L.P., as Landlord and
              FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

--------------------------------------------------------------------------------

                            INFOMART POLICY STATEMENT
                            -------------------------
           GENERAL POLICIES AND PROCEDURES REGARDING INFOMART, DALLAS
           ----------------------------------------------------------


1.       PERMANENT TENANT QUALIFICATIONS. Permanent tenants must be producers of
         hardware, software or services utilizing information processing
         equipment unless otherwise specifically approved by Landlord. If the
         tenant's business includes the resale of products or services, the
         tenant must add to or enhance the value of such products or services.

2.       TEMPORARY EXHIBITOR REGULATIONS.

         (a)      During designated temporary trade events sponsored by Landlord
                  which are conducted in conjunction with the use of permanent
                  showrooms and which include the rental of temporary exhibit
                  space, permanent tenants will be offered a priority selection
                  of such temporary exhibit space based in the initial year upon
                  the dates on which such permanent tenants entered into leases
                  for space within the Building and in subsequent years on a
                  priority basis reasonably determined by Landlord.

         (b)      When temporary exhibit space is used in conjunction with
                  permanent showrooms, non-information processing industry
                  representatives may display wares for specific trade markets.

         (c)      Temporary space areas may be rented independent of permanent
                  showrooms, in which event Landlord will have sole discretion
                  as to exhibition policies.

3.       BUSINESS HOURS. Permanent showrooms bounded by an atrium wholly or
         partially will be open and staffed during all normal business hours of
         the Building. Showrooms must be open during all Landlord sponsored
         trade events with exceptions approved in writing by Landlord. The hours
         of these events will be established by Landlord.

4.       SALES POLICY. Warehousing and on-site delivery to customers is
         prohibited in permanent showrooms and in exhibit space when used in
         conjunction with showrooms. Payment for products or services that are
         of a retail sales nature are prohibited (provided, however, that
         payment or partial payment for orders taken at the Building for future
         delivery to a buyer will be allowed if it is within the applicable
         tenant's normal business practice and is not of a retail sales nature,
         it being the intention hereof to permit payments or partial payments
         intended to bind an order for future delivery without in any way
         qualifying or circumventing the prohibition within the Building against
         retail sales).

                                       37
<PAGE>
 
5.       ACCESS AND ADMISSION OF VISITORS. Landlord reserves the right to
         implement and/or use any or all of the following policies with regard
         to access and admission of visitors into the Building. Tenant will
         receive prior notice of any change in the policy.

         (a)      All entrants to the Building will be registered and issued an
                  identification badge with the exception of visitors with a
                  pre-arranged appointment with a specific tenant.
                  Appointment-only visitors will be issued a badge which
                  requires such visitors to be accompanied by the applicable
                  inviter. Appointment-only visitors will not be required to
                  register on the Building's visitor database.

         (b)      Terms of issuance of badges will be annual with the exception
                  of specific trade events, including without limitation
                  conferences and symposia, in which event badges will be valid
                  only during the scheduled event. Temporary user badges will be
                  issued to non-scheduled daily visitors.

         (c)      A registration fee will be established by Landlord which will
                  defray the cost of registration and better ensure the quality
                  of visitors.

         (d)      Permanent tenants' badges will be issued based on one badge
                  per 500 Usable Square Feet of space, with additional badges
                  available upon payment of a registration fee cost or annual
                  renewal cost.

         (e)      Members of the press and educational institutions will be
                  issued a maximum of five annual complimentary badges with
                  additional badges available upon payment of the standard
                  registration fee.

         (f)      Employees and agents of Landlord will be issued badges at the
                  discretion of Landlord.

         (g)      Visitor registration information deemed appropriate by
                  Landlord will be made available to permanent tenants.
                  Information regarding tenant invitees will be proprietary and
                  not available to tenants. Attendee registration lists of
                  externally sponsored events will be the property of the
                  sponsoring group.

6.       PUBLIC FUNCTION/ON-SITE ACCOMMODATIONS.

         (a)      A visitor information directory system will be provided by
                  Landlord to assist visitors in locating vendors. Tenant and
                  exhibitor listings will be categorized by company name and
                  product offerings. Each tenant is eligible to multiple product
                  listings applicable, up to a maximum of one product category
                  listing per 500 Usable Square Feet of permanent lease space.
                  Additional listings may be issued at a nominal fee subject to
                  product eligibility.

         (b)      Tenant's meeting room use will be coordinated on a reservation
                  basis and all

                                       38
<PAGE>
 
                  tenants will be eligible. Standard fees will be applied and
                  Landlord will control the rental of these areas and the use of
                  the areas will be coordinated by the buyer/tenant services
                  department of Landlord. Reservations for meeting room space
                  within the Building will be on a first-come first-served
                  basis.

7.       MERCHANDISING OF INFOMART. INFOMART reserves the right to list tenants,
         partially or in entirety, of INFOMART in merchandising programs and
         agrees that such use shall not contain editorial references regarding
         specific tenants.

8.       AMENDMENTS TO GENERAL POLICIES AND PROCEDURES. Landlord may amend its
         policies from time to time and such changes shall be binding upon
         Tenant.





                                    INITIALS

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                            -----------            -----------

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<PAGE>
 
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                                       40
<PAGE>
 
                                   EXHIBIT "E"
                                   -----------
            To Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

--------------------------------------------------------------------------------

                                     PARKING
                                     -------

This Exhibit "E" ("Exhibit") describes and specifies Tenant's non-exclusive
right to use non-reserved parking spaces ("Garage Spaces") located inside the
Building's exterior card access parking garage ("Parking Garage"), and reserved
parking spaces ("Lower Level Spaces") on the lower level of the Building's
interior card access parking garage ("Lower Level Garage"). Additionally, spaces
in the surface parking lots associated with the Building and located on the
Property ("Surface Parking") are provided for the non-exclusive and common use
of Landlord, all tenants of the Building, and their respective guests and
invitees. Utilization of the Surface Parking is subject to availability (and
Landlord shall have no obligation to provide available Surface Parking) and to
such rules and regulations as may be promulgated by Landlord from time to time.
Use of the Parking Garage, Lower Level Garage and the Surface Parking is subject
to the terms and conditions set forth below.

1.       DEFINITIONS.    The terms which are defined in the Lease shall have the
         same meaning in this Exhibit.

2.       GRANT AND RENTAL FEE. Provided no event of default has occurred and is
         continuing under the Lease, Tenant shall be permitted non-exclusive use
         of nineteen (19) Garage Spaces in the Parking Garage during the Lease
         Term at such monthly rates and subject to such terms, conditions, and
         regulations as are, from time to time, promulgated by Landlord and
         charged or applicable to patrons of said parking Garage for spaces
         similarly situated within said Parking Garage. The parking rate for
         each of the Garage Spaces as of the date hereof is $45.00. Provided no
         event of default has occurred and is continuing under the Lease, Tenant
         shall be permitted use of six (6) reserved Lower Level Spaces in the
         Lower Level Garage during the Lease Term at such monthly rates and
         subject to such terms, conditions, and regulations as are, from time to
         time, promulgated by Landlord and charged or applicable to patrons of
         said Lower Level Garage. The parking rate for each of the Lower Level
         Spaces as of the date hereof is $60.00 for spaces designated on the
         attached Schedule 1 "Lower Level Garage Pricing Schedule" as "circle"
         parking and $90.00 for spaces designated as "diamond" parking. Tenant
         shall also have the right to use the Surface Parking, free of charge,
         during the Lease Term.

3.       RISK.  All motor vehicles (including all contents thereof) shall be
         parked in the Garage Spaces, Lower Level Spaces or in the Surface
         Parking, as applicable, at the sole risk of Tenant, its employees,
         agents, invitees and licensees, it being expressly agreed and
         understood that Landlord has no duty to insure any of said motor
         vehicles (including the contents thereof), and that Landlord is not
         responsible for the protection and security of such vehicles.  Landlord
         shall have no liability whatsoever for any property damage and/or
         personal injury which might occur as a result of or in connection with
         the parking

                                       41
<PAGE>
 
         of said motor vehicles in any of the Garage Spaces, Lower Level Spaces
         or in the Surface Parking, as applicable, and Tenant hereby agrees to
         indemnify and hold Landlord harmless from and against any and all
         costs, claims, expenses, and/or causes of action which Landlord may
         incur in connection with or arising out of Tenant's use of the Garage
         Spaces, Lower Level Spaces or the Surface Parking pursuant to this
         Agreement.

4.       RULES AND REGULATIONS. In its use of the Garage Spaces, Lower Level
         Spaces and the Surface Parking, Tenant shall follow all of the Rules
         and Regulations of the Building (attached to the Lease as Exhibit "B")
         applicable thereto, as the same may be amended from time to time. Upon
         the occurrence of any breach of such rules or default by Tenant under
         the Lease, Landlord shall be entitled to terminate this Exhibit, in
         which event Tenant's right to utilize the Garage Spaces, Lower Level
         Spaces and/or the Surface Parking shall thereupon automatically cease.

5.       SECURITY. Landlord shall be entitled to utilize whatever access device
         Landlord deems necessary (including but not limited to the issuance of
         parking stickers or access cards), to insure that only tenants
         authorized to use spaces in the Parking Garage and Lower Level Garage
         are using such spaces.  In the event Tenant, its agents or employees
         wrongfully park in any of the Parking Garage's or Lower Level Parking
         Garage's spaces, Landlord shall be entitled and is hereby authorized to
         have any such vehicle towed away, at Tenant's sole risk and expense,
         and Landlord is further authorized to impose upon Tenant a penalty of
         $25.00 for each such occurrence.  Tenant hereby agrees to pay all
         amounts falling due hereunder upon demand therefor, and the failure to
         pay any such amount shall additionally be deemed an event of default
         under the Lease, entitling Landlord to all of its rights and remedies
         thereunder.

6.       ADDITIONAL SPACES. In the event that Tenant expands the Leased
         Premises, Tenant shall be entitled to additional Garage Spaces within
         the Parking Garage based upon a ratio of one (1) additional Space per
         additional 1,000 Usable Square Feet incorporated into the Leased
         Premises and additional Lower Level Spaces within the Lower Level
         Garage based upon a ratio of one reserved parking space per 3,000
         Rentable Square Feet incorporated into the Lease Premises.  Such
         additional Garage Spaces and Lower Level Spaces shall be subject to
         such monthly rates, terms, conditions, and regulations as are, from
         time to time, promulgated by Landlord and charged or applicable to
         patrons of said Parking Garage or Lower Level Garage for spaces
         similarly situated within said Parking Garage or Lower Level Garage.

Schedule 1- Lower Level Garage Price Schedule



                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------

                                       42
<PAGE>
 
                                  SCHEDULE "1"
                                  ------------
      To Exhibit E to Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                        Lower Level Garage Price Schedule
                        (For illustrative purposes only)


                            [Floorplan appears here]


                                       43
<PAGE>
 
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                                       44
<PAGE>
 
                                   EXHIBIT "F"
                                   -----------

            To Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

--------------------------------------------------------------------------------

                      TELECOMMUNICATIONS EQUIPMENT LICENSE
                      ------------------------------------

This Exhibit "F" describes the license to install and operate certain specified
"Telecommunications Equipment" in the Building which is being granted by
Landlord to Tenant upon the following terms and conditions:

1.       DEFINED TERMS. For purposes of this Exhibit, all terms defined in this
         Lease (including other exhibits in this Lease) will be used in this
         Exhibit without further definition. In addition, when delineated with
         initial capital letters, the following terms will have the following
         respective meanings:

         (a)      "Building Grade" shall mean the type, brand and/or quality of
                  materials Landlord designates from time to time to be the
                  minimum quality to be used in the Building or the exclusive
                  type, grade, or quality of material to be used in the
                  Building.

         (b)      "Cable" shall mean only (i) optical fibers encased in an
                  aluminum sleeve, (ii) EMT conduit, (iii) copper cable, or (iv)
                  other materials approved by Landlord. The Cable (or conduit)
                  shall not exceed four inches in diameter.

         (c)      "License Fees" shall mean, collectively, the Service Fee, the
                  Raceway Fee, and the Antenna License Fee as defined in
                  Paragraph 3 of Exhibit "G" attached hereto, and other sums of
                  money becoming due and payable to Landlord hereunder.

         (d)      "License Term" shall mean a term commencing on the
                  Commencement Date and shall expire upon the expiration or
                  earlier termination of the Lease Term, unless sooner
                  terminated pursuant to the provisions of this Exhibit.

         (e)      "Normal Business Hours" for the Building shall mean 8:30 a.m.
                  to 5:00 p.m. Mondays through Fridays, exclusive of normal
                  business holidays.

         (f)      "Raceway" shall mean a vertical and/or horizontal space and
                  pathway within the Project of no more than four inches in
                  diameter (unless a greater size is approved in writing by
                  Landlord) used for routing telecommunications cables and
                  ancillary equipment from Tenant's point of presence in the
                  Building. The precise location of the Raceway applicable to
                  this Telecommunications License will be designated by Landlord
                  and the Telecommunications Equipment (as defined herein) will
                  be installed only as designated by Landlord.

         (g)      "Raceway Fee" shall mean the sum calculated for all installed
                  Cable or conduits from Tenant's point of presence to other
                  locations or customers in the Building at the rates identified
                  in Schedule A per month.

                                       45
<PAGE>
 
         (h)      "Service Fee" shall mean the sum calculated for all installed
                  services from Tenant's point of presence to other locations or
                  customers in the Building at the rates identified in Schedule
                  B.

         (i)      "Telecommunications Equipment" shall mean the Cable, junction
                  boxes, hangers, pull boxes, grounding wiring and related
                  equipment used in the normal course of Tenant's business,
                  which will be installed by Tenant, after approval by Landlord,
                  into the equipment room and Raceway to be used by Tenant,
                  pursuant to the terms of this License.

2.       GRANT OF LICENSE. Subject to and upon the terms set forth in this
         Exhibit, Landlord grants Tenant a license to use the Raceway and to
         install Cable in the Raceway which connects to various tenants of the
         Building on a non-exclusive basis, all as is more particularly
         described in this Exhibit for the purpose of providing access for
         Tenant's Telecommunications Equipment from the Leased Premises to the
         points of entry for the Building ("Telecommunications License"). Tenant
         acknowledges that this Telecommunications License gives Tenant the
         right to provide services to any tenant in the Building listed on
         Schedule C. Landlord specifically reserves the right to contract with
         competitors of Tenant for the same or similar services in the Building
         and acknowledges that it has entered into any such contracts prior to
         the date of this Telecommunications License. Landlord shall have no
         obligation to assist Tenant in marketing its equipment and/or services
         to the Building or to any other property owned by Landlord.

3.       LICENSE TERM. This License shall be in effect during the Lease Term. If
         the Raceway is not available for the commencement of Tenant's
         installation operations by the Commencement Date, due to the omission,
         delay or default of Tenant or of anyone acting under or for Tenant,
         Landlord shall have no liability; and the obligations of Tenant under
         this License (including, without limitation, the obligation to pay
         License Fees shall nonetheless commence as of the Commencement Date.
         Prior to the expiration or earlier termination of this
         Telecommunications License, Tenant shall remove all its
         Telecommunications Equipment that can be removed without causing any
         material damage to the Building and shall surrender and deliver the
         Raceway to Landlord in the same condition in which it existed at the
         Commencement Date, excepting only ordinary wear and tear and damage
         arising from any cause not required to be repaired by Tenant. In the
         event that  Tenant  fails to comply with the terms of this Paragraph 3,
         (i) all such Telecommunications Equipment remaining within the Raceway
         or Building may, at Landlord's option, become the sole property of
         Landlord or (ii) Landlord may, if it so elects, perform any act which
         Tenant is required to perform and/or remove the Telecommunications
         Equipment and other property at Tenant's cost, and Tenant shall pay
         Landlord promptly all costs incurred in removing said property upon
         demand.

4.       USE. The Raceway and mechanical room shall be used solely for the
         installation, operation and maintenance of the Telecommunications
         Equipment and for no other purpose whatsoever. Any use of the Raceway
         for any other purpose or any attempt by Tenant to allow the use or
         occupation of the Raceway by anyone other than Tenant shall,

                                       46
<PAGE>
 
         unless otherwise agreed to by Landlord in writing shall be a default;
         and Landlord shall have the right to immediately terminate this License
         pursuant to the provision of Section 6.1 of the Lease. Tenant shall not
         use or permit the use of the Raceway for any purpose which is illegal,
         dangerous to life, limb or property, or which, in Landlord's reasonable
         opinion, creates a nuisance or which would increase the cost of
         insurance coverage with respect to the Building. In particular, no
         semiconductors or other electronic equipment containing polychlorinated
         biphenyls (PCB's) or other environmentally hazardous materials will
         either be used or stored in or around the Raceway; and no such
         materials will be used in any of the Telecommunications Equipment
         installed by Tenant in the Raceway. Tenant will not permit unauthorized
         persons or persons with insufficient expertise or experience to enter
         any mechanical room in which the Raceway is located to maintain or
         operate its Telecommunications Equipment. Tenant understands that the
         mechanical rooms must be kept locked and secure at all times must not
         be available or open to the public. Landlord may, at Landlord's
         discretion, authorize other licensees and tenants of the Building to
         use portions of the Raceway, or to use portions of other raceways in
         the Building, whether for the installation of telecommunications
         equipment or otherwise, so long as such uses would not require Tenant
         to remove its previously installed Cable from the Raceway. Tenant
         acknowledges that interruptions in utility services are not uncommon in
         facilities such as the Building and that any sensitive electronic
         equipment which may be used in the Raceway should be protected by
         Tenant from utility service interruptions through the use of backup
         power supplies, surge protectors and other appropriate safety systems.

5.       INSTALLATION. The point of presence and network interface will be in
         accordance with the rules and regulations established by (a) Landlord
         and (b) the Public Utility Commission or other governmental authority
         with jurisdiction over such matters in the State of Texas. Each
         horizontal Raceway (including the installation of Cable therein) shall
         be constructed and/or installed at the sole cost and expense of Tenant.
         Tenant shall provide Landlord with a site diagram depicting the
         distribution system to all end users no less frequently than once every
         year and in conjunction with each new service installation. Failure to
         provide such a diagram on a timely basis shall be an event of default.
         Schedule D depicts the Cable  locations in the Raceway and terminal
         points on all floors.  All  specifications, Telecommunications
         Equipment, and Cable will be installed in a good and workmanlike
         manner, and the installation must be approved by Landlord's technical
         representative prior to the commencement of use of the
         Telecommunications Equipment by Tenant. No activities of Tenant
         associated with the construction of the Raceway and the installation of
         the Telecommunications Equipment shall be conducted during Normal
         Business Hours.

6.       LICENSE FEE PAYMENT.

         (a)      The License Fees applicable to Tenant's initial installation
                  of four (4), four inch (4") cables shall be paid to Landlord
                  during the License Term, without any setoff or deduction
                  whatsoever. Except as otherwise provided in this Exhibit, the
                  License Fees for each calendar month or portion thereof during
                  the License Term shall be due and payable in advance on the
                  first day of each month during the

                                       47
<PAGE>
 
                  License Term; and Tenant shall pay such Service Fee and
                  Raceway Fee as provided in Schedule F monthly, in advance, on
                  or before the first day of each calendar month, and without
                  demand. All installments of the License Fees which are not
                  paid when due will bear interest and be subject to a late
                  charge as provided in Section 3.2 of the Lease, and Landlord
                  may exercise the remedies provided in Section 3.3 of the Lease
                  in the event of consecutive late payments of any of the
                  License Fees.

         (b)      Tenant shall keep an accurate set of books and records of all
                  installed service from business conducted in the Building or
                  Raceway, and all supporting records such as, work orders and
                  other records which are necessary to verify and substantiate
                  the amount of Tenant's License Fees at Tenant's business
                  office located in the Premises. All such books and records
                  shall be retained and preserved for at least twenty-four (24)
                  months after the end of the calendar year to which they
                  relate, and shall be subject to inspection and audit by
                  Landlord and its agents at all reasonable times. The
                  acceptance by Landlord of payments of any License Fees shall
                  be without prejudice to Landlord's right to an examination of
                  Tenant's books and records in order to verify the computation
                  of the Raceway Fee and Service Fee provided by Tenant. In the
                  event Landlord is not satisfied with any monthly statement or
                  annual statement submitted by Tenant, Landlord shall have the
                  right to have its auditors make a special audit of all books
                  and records, wherever located, pertaining to sales made in or
                  from the Building or Raceway during the period in question. If
                  such statements are found to be incorrect to an extent of more
                  than two percent (2%) over the figures submitted by Tenant,
                  Tenant shall pay for such audit. Tenant shall promptly pay to
                  Landlord any deficiency or Landlord shall promptly credit to
                  Tenant any overpayment, as the case may be, which is
                  established by such audit.

         (c)      In addition to the License Fees, Tenant shall pay Landlord if,
                  and when due, any sales, use or other taxes or assessments
                  which are assessed or due by reason of this License or
                  Tenant's use of the Building hereunder.

         (d)      Upon each anniversary date of this License, including any
                  renewal term, the License Fees payable by Tenant shall
                  increase as follows and using the following definitions:

                  "Consumer Price Index" - The monthly indexes of the National
                  Consumer Price Index for All Urban Consumers (CPI-U) - All
                  Items, issued by the Bureau of Labor Statistics.

                  "Base Price Index Number" - The Consumer Price Index as of
                  December of the year in which the Commencement Date occurs.

                  "Current Index Number" - The Consumer Price Index as of
                  December of the year in which the calculations are being done.

                                       48
<PAGE>
 
                  If the Current Index Number is greater than the Base Price
                  Index Number, then the "Percentage of Increase" shall be
                  calculated as follows:

                  (Current Index Number - Base Price Index Number) , Base Price
                  Index ' 100 = Percentage of Increase

                  This Percentage of Increase shall be multiplied by the License
                  Fee defined above to obtain the new rate to go into effect on
                  each anniversary of the Commencement Date of this License. The
                  fee increase shall in no case be less than 3.5% annually of
                  the most recent past License Fees amount, as increased. This
                  Percentage of Increase applies to License Fee only and does
                  not apply to any charges in the Base Rent.

7.       CONDITION OF THE RACEWAY. Tenant accepts the Raceway "as is" without
         benefit of any improvements to be constructed or made by Landlord.

8.       MAINTENANCE AND REPAIR BY LANDLORD. Except as otherwise expressly
         provided herein, Landlord shall not be required to make any repairs to
         the Raceway other than repairs to exterior and load-bearing walls of
         the Building, floors of the mechanical rooms (but not to floor
         coverings), and the roof of the Building, which may be required from
         time to time, but only after such required repairs have been requested
         by Tenant in writing. In no event shall Landlord be responsible for the
         maintenance or repair of improvements which are not composed of
         Building Grade materials.

9.       SERVICE AREA ACCESS. Except in the case of an emergency, Tenant shall
         not enter or attempt access to any of the Service Areas (including air,
         electrical, mechanical or telecommunications risers, ducts, closets,
         conduits, duct work, rooms or other horizontal or vertical spaces in
         the Building) without notifying Landlord in writing at least two (2)
         days in advance. In the case of an emergency, Tenant may enter or seek
         access to the Raceway through the Service Areas provided it gives
         Landlord at least two (2) hours prior notice and provided that a
         Building security guard must unlock such Service Area. If Landlord is
         also experiencing an emergency situation in the Building at the same
         time that Tenant has notified Landlord of an emergency, Landlord shall
         have no obligation to first address or respond to Tenant's emergency
         and shall only be obligated to accommodate Tenant's concerns as time
         permits thereafter. Tenant also agrees to furnish Landlord, within two
         (2) business days thereafter, a written report explaining all repairs
         and procedures which were conducted during any such emergency
         operations, in sufficient detail to permit Landlord's engineers to
         evaluate same. Any access to the Service Areas shall require Tenant to
         sign in at the security department console, and Tenant shall permit the
         Landlord's security guard or a representative of Landlord to accompany
         Tenant during any such work within a Service Area, if Landlord so
         desires. No installation, alterations or repairs shall be initiated
         without first delivering to Landlord's engineers plans and
         specifications of the proposed changes,  in substance and form
         acceptable to Landlord.  No oral approval of these plans and
         specifications shall be effective. No electrical grounding shall be
         permitted to other equipment in the mechanical rooms without Landlord's
         specific written approval of the method and location of such grounding.
         No monitoring or inspection of Tenant's work by Landlord's

                                       49
<PAGE>
 
         representatives shall be deemed supervision of Tenant's employees or
         shall be deemed to be a representation or warranty of any particular
         level of telecommunications expertise attained by Landlord's
         representative. Tenant shall monitor and supervise its own employees
         and shall assume responsibility for the expertise and quality of its
         work and shall not rely upon Landlord for same.

10.      NO ACCESS TO OTHER TENANTS' PREMISES. Tenant acknowledges that nothing
         in this License entitles it to enter and connect its Telecommunications
         Equipment to any tenant's premises in the Building without the prior
         written consent of Landlord. Tenant also acknowledges that it has been
         informed that telecommunications connections to individual tenant's
         premises in the Building will normally require removal of ceiling
         panels, at each tenant's expense, with such removal operations only
         being performable by Landlord's agents or employees.

11.      LICENSES AND PERMITS. Prior to commencing any work on the Raceway,
         Tenant shall obtain all necessary licenses, permits and consents and
         provide copies of same to Landlord. Landlord shall have the right to
         monitor all such work, at its own expense.

12.      COSTS. Tenant shall be responsible for any and all cost, damage or
         expense arising from the installation, maintenance, or repair to the
         Raceway or the Telecommunications Equipment, including, without
         limitation, any and all cost, damage or expense to the Building or the
         property of Landlord or other licensees or tenants of the Building
         arising from such installation, maintenance or repair operations.
         Tenant will make any and all repairs necessary in a timely manner. If
         Tenant does not make required repairs to Landlord's satisfaction within
         twenty-four (24) hours of notification from Landlord that said repairs
         are necessary within a tenant's space or within ten (10) days of
         notification from Landlord that said repairs are necessary pursuant to
         Paragraph 13 of this Exhibit, then Landlord will have the right but not
         the obligation to perform any such repairs at Tenant's sole cost and
         expense. Tenant shall on demand pay to Landlord as additional License
         Fees (i) the cost of such work plus fifteen percent (15%) thereof as
         administrative costs; plus (ii) interest thereon at the rate of 12% per
         annum from the date of demand.

13.      MAINTENANCE, REPAIRS, AND ALTERATIONS BY TENANT. Tenant shall not
         commit any waste or allow any waste to be committed within or on any
         portion of the Raceway or in any Service Area and will maintain the
         Raceway in a clean, attractive condition and in good repair. At the
         termination of this License, Tenant agrees to deliver up the Raceway to
         Landlord in as good condition as at the Commencement Date, ordinary
         wear and tear and damage by casualty excepted (unless caused by
         Tenant). Tenant will remove all excess cable, tools, and equipment and
         will keep all areas neat and clean at all times. Landlord shall have
         the right, at its option, at Tenant's own cost and expense, to repair
         or replace any damage done to the Building, or any part thereof, caused
         by Tenant (or by any Affiliate, contractor, agent, or employee of
         Tenant), and Tenant shall pay the reasonable cost thereof to Landlord
         on demand as additional License Fees. Tenant shall not make or allow
         any alterations to the Raceway without the prior written consent of
         Landlord. Tenant shall not place signs on any of the doors or corridors

                                       50
<PAGE>
 
         leading to the Raceway, without first obtaining the prior written
         consent of Landlord in each such instance, which consent may be given
         or arbitrarily withheld on such conditions as Landlord may elect.
         Landlord shall have the right, at its option, at Tenant's own cost and
         expense, to remove any signs placed by Tenant without Landlord's prior
         written consent, and to repair any damage caused by the such signs. Any
         and all alterations to the Raceway shall become the property of
         Landlord upon termination of this License.

14.      USE OF ELECTRICAL SERVICES BY TENANT. All electrical usage associated
         with the Telecommunications Equipment will be governed by the
         provisions of Section 4.3 of the Lease.

15.      LAWS AND REGULATIONS. Tenant shall comply with all Legal Requirements
         and Insurance Requirements and shall, at Tenant's sole cost, take all
         measures necessary to assure that the Telecommunications Equipment
         strictly complies with all Legal Requirements and Insurance
         Requirements. Tenant shall also pay promptly when due all royalties or
         other fees due in connection with the operation of the
         Telecommunications Equipment. In the event compliance with this
         paragraph shall require modifications or alterations of the
         Telecommunications Equipment or the Raceway, no modification or
         alteration shall be made without Landlord's prior written consent,
         which consent may be withheld in Landlord's sole judgment or granted on
         such terms and conditions as Landlord may determine in its sole
         judgment. Tenant shall take all measures necessary to assure that the
         Telecommunications Equipment does not interfere with or disturb the
         operation of any other equipment or business of Landlord or of any
         other licensee, tenant, or occupant of the Building.

16.      SITE TECHNICAL STANDARDS. Tenant will strictly comply with the Site
         Technical Standards (Schedule E) as adopted and altered by Landlord
         from time to time and will cause all of the Tenant Related Parties to
         do so. All changes to such standards will be sent by Landlord to Tenant
         in writing.

17.      ENTRY BY LANDLORD. Tenant shall permit Landlord or its employees,
         agents, contractors, or representatives to have access to any portion
         of the Raceway at all times to inspect the same, to clean or make
         repairs, alterations or additions thereto, and Tenant shall not be
         entitled to any abatement or reduction of License Fees by reason of any
         such entry.

18.      INDEMNIFICATION. In addition to the indemnification obligations of
         Tenant under the Lease, Tenant shall protect, defend, indemnify, and
         hold Landlord harmless from all liability and claims for any injury to
         person or damage to property caused by any act, omission, or neglect of
         Tenant, its agents, servants, employees, or contractors, relative to
         the Telecommunications License, including, without limitation, the
         installation, operation, repair, and maintenance of the
         Telecommunications Equipment.

19.      DAMAGE. Landlord shall not be liable to Tenant for any loss or damage
         to all or any part of the Telecommunications Equipment occasioned by
         theft, fire, act of God, public

                                       51
<PAGE>
 
         enemy, injunction, riot, vandalism, malicious mischief, earthquake,
         flood, strike, insurrection, war, court order, requisition, or order of
         governmental body or authority, or by any other cause whatsoever. Nor
         shall Landlord be liable for any damage or inconvenience which may
         arise through the repair or alteration of any part of the Building or
         through termination of the Telecommunications License. Further,
         Landlord shall not be liable for any damage which may arise through the
         repair or alteration of any part of the Building or through termination
         of this Telecommunications License other than damage caused by the
         gross negligence or willful misconduct of Landlord, its agents or
         employees.

20.      INSURANCE. In addition to the insurance obligations of Tenant under the
         Lease, Tenant shall maintain a policy or policies of fire and extended
         coverage insurance on the Telecommunications Equipment, in such amounts
         as Tenant may deem appropriate; provided, however, that Tenant shall
         never have any claim against Landlord for any loss or damage that may
         occur to the "Antenna Equipment" (as defined in Exhibit "G") which
         could be covered by insurance.

21.      TRANSFERS BY TENANT. Tenant shall not assign, convey, mortgage, pledge,
         hypothecate, encumber, or otherwise transfer the Telecommunications
         License or grant any license, concession, or other right with respect
         to the Telecommunications License without the prior written consent of
         Landlord, which consent may be granted or withheld in Landlord's sole
         discretion. In addition, the Telecommunications License shall
         terminate, in Landlord's sole discretion, upon the assignment of the
         Lease or a subletting of the Premises, unless such transfer is to an
         Affiliate of Tenant in connection with an assignment or sublease to an
         Affiliate as provided in Section 4.7 of the Lease.

22.      DEFAULT BY TENANT. In addition to provisions of Articles 8 and 6 of the
         Lease, Tenant shall be deemed to be in default with respect to the
         Antenna License in the event that (a) Tenant shall fail to pay the
         Telecommunications License Fees when due; or (b) Tenant shall fail to
         maintain the Telecommunications Equipment in good order and repair and
         in a safe condition as provided in this Exhibit; or (c) Tenant shall
         fail to maintain all necessary licenses and permits with respect to the
         operation of the Telecommunications Equipment. Upon a default by Tenant
         with respect to the Telecommunications License, Landlord may, at
         Landlord's sole election, pursue the remedies granted to Landlord for
         default under the Lease or, in the alternative, terminate the
         Telecommunications License granted hereunder without terminating the
         Lease or terminating Tenant's right to possession of the Premises under
         the Lease.

23.      SURVIVAL. Certain provisions of this Exhibit relate to the rights and
         obligations of Landlord and Tenant subsequent to the termination or
         expiration of the Lease Term. Such provisions include, without
         limitation, the restoration obligations of Tenant under Paragraph 13
         hereof and the indemnification obligations of Tenant under Paragraph 19
         hereof. Such provisions shall survive the expiration or other
         termination of the Lease Term and the Telecommunications License
         granted to Tenant hereunder.



                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------

                                       52
<PAGE>
 
                                   SCHEDULE A
                                   ----------

      To Exhibit F to Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

--------------------------------------------------------------------------------

                                   RACEWAY FEE

Monthly charges for the initially installed three (3) for fee and one (1) for
free 4" conduits are included in the Base Rental.

Any additional Raceway Fees for conduits shall mean the total installed conduits
of conductors at the following rates:
<TABLE>
<CAPTION>
         SIZE                               up to .5"  .5 to 1"  1 to 1.5"  1.5 to 2"  2 to 2.5"  2.5 to 4"  Over 4"
         <S>                                <C>        <C>       <C>        <C>        <C>        <C>        <C>
         $/linear foot horizontal           $0.10      $0.20     $0.30      $0.40      $0.75      $1.00      Quote
         $/linear foot horizontal           $0.15      $0.30     $0.45      $0.60      $1.13      $1.50      Quote
                  (out of Building)
         $/linear foot vertical (12'min)    $1.00      $2.00     $3.00      $4.00      $7.50      $10.00     Quote
</TABLE>

per month.

Discount based on Rentable Square Feet:

0-10,000                    0%
10,001-20,000               5%
20,001-30,000              10%
30,001-40,000              15%
40,001-50,000              20%



                                       53
<PAGE>
 
                                   SCHEDULE B
      To Exhibit F to Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                                  SERVICE FEE


The Service Fee shall mean the total installed services at the following rates:

                           a. DS-O = Zero Dollars per month,
                           b. DS-1 = Zero Dollars per month, and
                           c. DS-3 = Zero Dollars per month

                                       54
<PAGE>
 
                                   SCHEDULE C

      To Exhibit F to Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

              List of Tenants to Whom Licensee May Provide Services


                                       55
<PAGE>
 
                                   SCHEDULE D

      To Exhibit F to Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                  Telecommunications Equipment Systems Diagram



    This schematic describes the telecommunications equipment systems to be
       installed for the limited purpose of the license described herein.


                                       56
<PAGE>
 
                                   SCHEDULE E

      To Exhibit F to Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                            Site Technical Standards

                  1. The fiber transmission cables and all copper telephone
                  cables must be Teflon (or of City approved fire retardant
                  material) jacketed type cable, and secured by either stainless
                  steel clamps or approved equal when not run in EMT type
                  conduit. Excess transmission line must be removed.

                  2. Each fiber or copper telephone line or conduit shall be
                  identified with stainless steel tags that identifies the
                  user/Licensee: (1) at the equipment cabinet; (2) at each side
                  of horizontal/vertical penetration (3) as the line traverses
                  the Building at a minimum of 72' intervals, coincident with
                  column lines, and (4) at the termination point(s).

                  3. The location and installation of all equipment and conduit
                  will be designated by the site coordinator. These locations
                  will be shown on the License. Changes must be approved in
                  writing by the site coordinator. Any conduit or cable failing
                  to meet the above standards will be immediately removed from
                  the Building at Licensee's expense. In the event Licensee
                  fails to promptly remove any such conduit or cable, Licensor
                  may do so at Licensee's expense.

                  4. On a 24-hour notice, the Site Equipment will be made
                  available for inspection by the site coordinator to assure
                  compliance with the above standards.

                  5. The following information is essential for site
                  coordination and must be provided. Any and all changes must
                  have prior approval and be reported to the site coordinator.

                           A.       Manufacturer and model number of all end
                                    equipment.

                           B.       Type and length of all cable and lines.

                           C.       The name, address and telephone number of
                                    the person or group directly responsible for
                                    the day-to-day maintenance.

                           D.       The name, address and telephone number of
                                    the person or group directly responsible for
                                    the License Agreement.

                                   SCHEDULE F
     To  Exhibit F to Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                           Calculation of License Fee

                                       57
<PAGE>
 
         The License Fee shall be paid in monthly installments pursuant to
         Paragraph 6 of the License. The License Fee shall be equal to:

                  1.  The Service Fee, plus

                  2.  The Raceway Fee, plus

                  3.  The Satellite/Antenna Fee.

         This License Fee does not include construction costs, installation
         costs, utilities or any other costs associated with occupancy and
         operation in the Building.




                                       58
<PAGE>
 
                                   EXHIBIT "G"

            To Lease Agreement By and Between INFOMART-Dallas L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                            SATELLITE/ANTENNA LICENSE
                            -------------------------

This Exhibit "G" describes the license to install certain specified
satellite/antenna communications equipment upon the roof of the Building which
is being granted by Landlord to Tenant upon the following terms and conditions:

1.       DEFINED TERMS. For purposes of this Exhibit, all terms defined in this
         Lease (including other exhibits in this Lease) will be used in this
         Exhibit without further definition. In addition, other terms which are
         defined in this Exhibit will, when delineated with initial capital
         letters, have the respective meanings specified in this Exhibit.

2.       GRANT OF LICENSE. Subject to the terms and provisions of this Exhibit,
         Landlord grants to Tenant a license ("Antenna License") to maintain a
         satellite antenna dish together with related wiring and equipment
         (collectively, the "Antenna Equipment") on the roof of the Building.
         The term of the Antenna License shall commence on the Commencement Date
         and shall terminate upon the expiration or the earlier termination of
         the Lease Term, unless sooner terminated pursuant to the provisions of
         this Exhibit. The size, location, and manner of installation of the
         Antenna Equipment shall be determined at Landlord's sole discretion,
         which discretion will take into consideration (a) the functional
         requirements of the Antenna Equipment and of any other satellite
         antenna dishes located on the roof of the Building and (b) standards of
         architectural integrity with respect to the Building (and, in that
         regard, the Antenna Equipment shall be located so as not to be visible
         except from above the Building, shall match the Building color, and
         shall have no visible marking or logo). The Antenna License shall
         commence on the Commencement Date and shall expire upon the expiration
         or earlier termination of the Lease Term.

3.       ANTENNA LICENSE FEE. The Antenna License Fee allowed by this Exhibit is
         One Hundred Ten and 59/100ths Dollars ($110.59) . License Fees will not
         go into effect until the later of the Rental Commencement Date or the
         commencement of construction for such antenna. Additional satellite
         dishes shall be at a fee of one hundred fifty (150) percent of the
         established initial Satellite/Antenna Fee. The Antenna License Fee
         includes the connection of the Antenna Equipment to Tenant's
         Telecommunications Equipment in the Leased Premises, and no additional
         fees will be charged for the use of a raceway to make such connection.

4.       INSTALLATION, MAINTENANCE, AND REPAIR. Tenant shall, at Tenant's sole
         cost and expense, provide for the installation of the Antenna Equipment
         and for all service, repairs, and maintenance to the Antenna Equipment.
         With respect to the installation of the Antenna Equipment, the Antenna
         Equipment shall not be affixed to the

                                       59
<PAGE>
 
         roof of the Building by nail, bolt, screw, or other device which
         penetrates the roof; and all wiring penetrations shall be made by
         Landlord's roofing contractor at Tenant's sole cost and expense. The
         Antenna Equipment shall be maintained by Tenant in good order and
         repair and in a safe condition. In the event that Tenant fails to
         maintain the Antenna Equipment and Landlord, in Landlord's sole
         judgment, determines that such failure presents a danger of injury to
         persons or damage to property in or about the Building. Landlord may,
         after prior written notice to Tenant (except in the event of
         emergencies) (but without being under any obligation to do so) arrange
         for such repairs or maintenance to be done and recover the cost
         incurred from Tenant. Any such sums shall be due and payable by Tenant
         on demand. Tenant shall provide Landlord with prior written notice of
         Tenant's need to service, maintain, or repair the Antenna Equipment.
         Landlord shall then provide Tenant with access to the roof of the
         Building and other non-tenant areas of the Building at such times and
         under such conditions as may be reasonably determined by Landlord.
         Landlord may charge Tenant for any of Landlord's equipment and material
         used by Tenant and any time spent by Landlord, Landlord's management
         staff, or Landlord's consultants arising out of Tenant's failure to
         properly maintain or repair the Antenna equipment. In that regard, the
         Antenna Equipment shall be of a type so as to constitute a permanent,
         as opposed to a temporary, installation.

5.       INTERFERENCE. Tenant shall modify the Antenna Equipment or relocate the
         Antenna Equipment to another area approved by Landlord in the event
         that the Antenna Equipment, in Landlord's sole judgment, causes any
         interference with or disturbs the operation of any other antenna
         equipment or business of Landlord or of any other occupants of the
         Building or creates or results in any noise, odor, or nuisance to any
         other occupant of the Building, or areas adjacent to the Building.
         Tenant must immediately shut off the Antenna Equipment upon
         notification of interference and may restart, modify, or relocate the
         Antenna Equipment to test for interference only with Landlord's
         permission.

6.       USE. The Antenna Equipment shall be designed, installed, and operated
         solely for the purpose of passive receptions of transmissions and is
         solely for use in Tenant's internal business. The benefits of the
         Antenna Equipment may not be provided by Tenant to third parties. The
         Antenna Equipment may not be sold or rented by Tenant to third parties,
         nor may Tenant sublet or assign this License. Tenant shall operate the
         Antenna Equipment in compliance with all Legal Requirements and
         Insurance Requirements and shall maintain all necessary licenses and
         permits with respect to its operation of the Antenna Equipment. Any
         electrical usage associated with the Antenna Equipment shall be
         governed by the provisions of Section 4.3 of the Lease.

7.       INTERRUPTION. Landlord may, at Landlord's sole discretion but without
         cost to Landlord, after prior written notice to Tenant (except in the
         event of emergencies), require interruption of service or relocation or
         removal of the Antenna Equipment for repairs, maintenance, or
         modifications to the Building, including, but not limited to, roofing,
         structural, electrical, or mechanical repairs. Landlord shall use
         reasonable efforts not to interfere with Tenant's business.

                                       60
<PAGE>
 
8.       MODIFICATION AND SUBSTITUTION. Tenant may not substitute or modify the
         Antenna Equipment or any part thereof, without the prior written
         approval of Landlord. Any substitution or modification approved by
         Landlord shall automatically become a part of the Antenna Equipment,
         and all terms of this Exhibit shall apply to such substitution and/or
         modification.

9.       RESTORATION. Tenant shall, at Tenant's sole cost and expense, remove
         the Antenna Equipment and restore the Building to its currently
         existing condition upon the termination or expiration of the Lease
         Term, and/or this Antenna License. Such removal and restoration work
         shall be completed by Tenant within thirty (30) days of the date on
         which the condition requiring such removal and restoration work occurs.
         In the event that Tenant fails to complete the removal of the Antenna
         Equipment and the restoration of the Building within such thirty (30)
         day period, Landlord shall have the right (but without any obligation
         to do so) to remove the Antenna Equipment and restore the Building, in
         which event, Tenant shall reimburse Landlord for all costs incurred by
         Landlord in performing such removal and restoration work on demand. In
         addition, any Antenna Equipment remaining at the Building subsequent to
         the expiration of such thirty (30) day period and upon prior written
         notice to Tenant's last known address, shall be deemed to have been
         abandoned by Tenant, so that in no event shall Landlord have any duty
         to preserve or restore the Antenna Equipment on Tenant's behalf. If
         Landlord does choose to store the Antenna Equipment on Tenant's behalf,
         the cost of storage incurred by Landlord shall be reimbursed by Tenant
         on demand. In such event, Landlord shall also have the right to sell
         such Antenna Equipment for salvage value and to apply the proceeds
         derived from such sale to sums owing by Tenant to Landlord under this
         Lease (including this Exhibit). The provisions of this Paragraph 9
         shall survive the expiration of this Antenna License and of the Lease
         Term.

10.      REPRESENTATIONS OF TENANT. As a material inducement to Landlord to
         grant the Antenna License to Tenant, Tenant hereby represents and
         warrants to Landlord that (a) the Antenna Equipment has received
         approval from the Underwriters Laboratory and from the Federal
         Communications Commission; (b) the Antenna Equipment will be installed
         in accordance with the plans and specifications approved by Landlord,
         in Landlord's sole discretion; (c) the Antenna Equipment is designed to
         require minimum maintenance; (d) the Antenna Equipment will not
         interfere with other like antenna equipment located on the rooftop of
         the Building or in the Building; and (e) Tenant will have received all
         necessary licenses and permits from all applicable governmental
         authorities regarding the installation and operation of the Antenna
         Equipment, prior to such installation and operation.

11.      INDEMNIFICATION. In addition to the indemnification obligations of
         Tenant under the Lease, Tenant shall protect, defend, indemnify, and
         hold Landlord harmless from all liability and claims for any injury to
         person or damage to property caused by any act, omission, or neglect of
         Tenant, its agents, servants, employees, or contractors, relative to,
         or arising in connection with, the Antenna License, including, without
         limitation, the installation, operation, repair, and maintenance of the
         Antenna Equipment.

                                       61
<PAGE>
 
12.      DAMAGE. Landlord shall not be liable to Tenant for any loss or damage
         to all or any part of the Antenna Equipment occasioned by theft, fire,
         act of God, public enemy, injunction, riot, vandalism, malicious
         mischief, earthquake, flood, strike, insurrection, war, court order,
         requisition, or order of governmental body or authority or by any other
         cause whatsoever. Further, Landlord shall not be liable for any damage
         or inconvenience which may arise as a result of the repair or
         alteration of any part of the Building or through termination of the
         Antenna License, other than damage caused by the gross negligence or
         willful misconduct of Landlord, its agents or employees.

13.      INSURANCE. In addition to the insurance obligations of Tenant under the
         Lease, Tenant shall maintain a policy or policies of fire and extended
         coverage insurance on the Antenna Equipment, in such amounts as Tenant
         may deem appropriate; provided, however, that Tenant shall never have
         any claim against Landlord for any loss or damage that may occur to the
         Antenna Equipment which could be covered by insurance.

14.      TRANSFERS BY TENANT. Tenant shall not assign, convey, mortgage, pledge,
         hypothecate, encumber, or otherwise transfer the Antenna License or
         grant any license, concession, or other right with respect to the
         Antenna License without the prior written consent of Landlord, which
         consent may be granted or withheld in Landlord's sole discretion,
         unless such transfer or assignment is to an Affiliate in connection
         with Tenant's transfer of the Lease to an Affiliate pursuant to Section
         4.7 of the Lease. In addition, the Antenna License shall terminate, in
         Landlord's sole discretion, upon the assignment of the Lease or a
         subletting of the Premises, except as provided in Section 4.7.

15.      DEFAULT BY TENANT. In addition to provisions of Article 8 of the Lease,
         Tenant shall be deemed to be in default with respect to the Antenna
         License in the event that (a) Tenant shall fail to maintain the Antenna
         Equipment in good order and repair and in a safe condition as provided
         in this Exhibit; or (b) Tenant shall fail to maintain all necessary
         licenses and permits with respect to the operation of the Antenna
         Equipment. Upon a default by Tenant with respect to the Antenna
         License, Landlord may, at Landlord's sole election, pursue the remedies
         granted to Landlord for default under the Lease or, in the alternative,
         terminate the Antenna License granted hereunder without  terminating
         the Lease or terminating Tenant's right to possession of the Premises
         under the Lease.

16.      SURVIVAL. Certain provisions of this Exhibit relate to the rights and
         obligations of Landlord and Tenant subsequent to the termination or
         expiration of the Lease Term. Such provisions include, without
         limitation, the restoration obligations of Tenant under Paragraph 9
         hereof and the indemnification obligations of Tenant under Paragraph 11
         hereof. Such provisions shall survive the expiration or other
         termination of the Lease Term and the Antenna License granted to Tenant
         hereunder.


                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------

                                       62
<PAGE>
 
                                   EXHIBIT "H"
                                   -----------

            To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                                    GENERATOR
                                    ---------

This Exhibit "H" describes and specifies Tenant's use of additional areas in the
Building and on the Property for purposes of the installation and operation of
Tenant's Generator, upon the following terms and conditions:

1.       DEFINED TERMS. For purposes of this Exhibit "H", all terms defined in
         the Lease (including other Exhibits to the Lease) will be utilized
         herein without further definition. In addition, when delineated with
         initial capital letters, the following terms shall have the following
         respective definitions and meanings:

         a.       "Cable" shall mean only optical fibers, copper wires (also
                  known as "copper pairs"), coaxial cable, and other similar
                  transmission wiring customarily used in the industry, unless
                  other materials are expressly approved in writing by Landlord.

         b.       "Conduit" shall mean a plastic or metal sleeve, no more than
                  4" in aggregate diameter, unless a larger size is expressly
                  approved by Landlord in writing, in which Cable is encased
                  and/or through which Cable passes.

         c.       "Generator" shall mean the 750KW generator with automatic
                  transfer switch and load bank equipment to be installed by
                  Tenant on the 8 foot by 16 foot pad at the location designated
                  on Schedule "1" attached to this Exhibit.

         d.       "Tank" shall mean the 2,000 gallon fossil fuel tank and
                  associated transfer pumps to be installed by Tenant at the
                  location designated on Schedule "1" attached to this Exhibit.
                  The term "Tank" shall also be deemed to include the fuel oil
                  pipes to be installed by Tenant from the Tank to the
                  Generator. The Tank must have self-contained spill and leak
                  control features, it must be installed in a secure and safe
                  location above-ground, and it must conform to all legal
                  requirements concerning tank tightness, spill control and
                  monitoring features required by state and federal laws.

         e.       "Equipment" shall mean the Cable, Conduit, junction boxes,
                  hangers, pull boxes, grounding wiring and related equipment
                  used in the normal course of Tenant's business, which will be
                  installed by Tenant into the Leased Premises and into the
                  Raceway to be utilized by Tenant, pursuant to the terms
                  hereof.

                                       63
<PAGE>
 
2.       GRANT OF LICENSE. Landlord grants Tenant a license, subject to the
         terms and conditions herein, to (i) use such locations on the Property
         as are approved in writing by Landlord in order for Tenant to install
         Tenant's generator cabling to, and core drilling of, the Building core
         structural wall (it being acknowledged that Landlord has made no
         representation that Tenant will be able to utilize any Southwestern
         Bell or other utility easements in this regard) and use such Raceways
         as are approved in writing by Landlord in order for Tenant to install
         its Generator cabling from the Generator to the points of entry at the
         Building core structural wall to the Leased Premises, (ii) use such
         Raceways as are approved in writing by Landlord for the purpose of
         Tenant's installing, maintaining, repairing, replacing and operating
         the Equipment, (ii) use the designated location of the Generator for
         the purpose of installing, maintaining, repairing and operating the
         Generator, and (iii) use the designated location of the Tank for the
         purpose of installing, maintaining, repairing and operating the Tank.

3.       LICENSE TERM. The term of this license shall be coterminous with the
         Lease Term and shall continue in force during a period beginning on the
         Commencement Date and continuing until the expiration or earlier
         termination of the Lease Term.

4.       USE. Any Raceway to be utilized by Tenant pursuant to the terms hereof
         shall be used for the installation, maintenance, repair, replacement
         and operation of the Equipment as provided herein and for no other
         purpose. Tenant agrees not to use or permit the use of any such Raceway
         for any purpose which is illegal, dangerous to life, limb or property
         or which, in Landlord's reasonable opinion, creates a nuisance or which
         would increase the cost of insurance coverage with respect to the
         Building. In particular, no semiconductors or other electronic
         equipment containing polychlorinated biphenyls (PCB's) or other
         environmentally hazardous materials will either be used or stored in or
         around the Leased Premises or any such Raceway and no such materials
         will be used in any of the Equipment installed by Tenant in the Leased
         Premises or any such Raceway. Tenant will not permit any unauthorized
         person or persons with insufficient expertise or experience to enter
         the Raceways or maintain or operate the Equipment. Nothing herein
         should be construed as permitting Tenant to use any Raceway for a
         purpose not specifically described herein, or to install new or
         substitute Cable or Conduit into a Raceway which consists of materials
         other than those defined in Paragraph 1 hereof. It is also acknowledged
         that Landlord may, at Landlord's discretion, authorize other tenants
         and licensees of the Building to use portions of any Raceways or risers
         in the Building, whether for the installation of telecommunications
         equipment or otherwise, so long as such uses would not require Tenant
         to remove its Cable or Conduit, which has already been installed by
         Tenant, from such Raceway. Tenant acknowledges that interruptions in
         utility services are not uncommon in facilities such as the Building
         and Tenant acknowledges that any sensitive electronic equipment which
         may be used in the Leased Premises or a Raceway will be protected by
         Tenant from utility service interruptions through the use of the
         Generator and other backup power supplies, surge protectors and other
         safety systems. Tenant acknowledges that it has taken all precautionary
         steps it deems necessary to protect such equipment in the Leased
         Premises and in any Raceway, including the acquisition of insurance if
         applicable. Tenant agrees to release Landlord from any damages or
         losses (including attorney's fees and expenses) sustained to any of

                                       64
<PAGE>
 
         Tenant's Equipment as a result of utility service interruptions, unless
         solely caused by the gross negligence or willful misconduct of
         Landlord, its employees and agents, and agrees to indemnify, defend and
         hold Landlord harmless from any damages or losses (including attorneys'
         fees and expenses) caused by Tenant's Equipment as a result of utility
         service interruptions, unless such interruptions arise primarily by
         reason of the negligence or willful misconduct of Landlord, its agents
         or employees. This release and indemnity is in addition to and not in
         substitution of any other release and indemnity in the Lease.

5.       ADDITIONAL TERMS OF LICENSE. The license granted herein to Tenant shall
         additionally be subject to and expressly conditioned upon the
         following:

         a.       Construction of Raceways. Tenant shall install its generator
                  cabling and the other Equipment, including the Conduit and all
                  necessary Raceways, to the extent not already in place, in the
                  Building at Tenant's sole cost and expense and shall provide
                  Landlord with "as-built" drawings and specifications of same.

         b.       HVAC and Mechanical Room Access. Except in the case of an
                  emergency, Tenant shall not enter or attempt access to any of
                  the Building's air, electrical, mechanical or
                  telecommunications risers, ducts, closets, Conduits, duct
                  work, rooms, Raceways, or other horizontal or vertical spaces
                  in the Building, including the Service Areas, without
                  notifying Landlord in writing at least two (2) days in
                  advance. In the case of an emergency, Tenant may enter or seek
                  access to a Raceway utilized by Tenant pursuant to the terms
                  hereof through mechanical rooms or Service Areas provided
                  Tenant gives Landlord at least two (2) hours prior written
                  notice and provided further that a Building security guard or
                  engineer must unlock and accompany Tenant's employees into
                  such Service Areas and mechanical rooms; if Landlord is also
                  experiencing an emergency situation in the Building at the
                  same time that Tenant has notified Landlord of an emergency,
                  Landlord shall have no obligation to first address or respond
                  to Tenant's emergency and shall only be obligated to
                  accommodate Tenant's concerns as time permits thereafter.
                  Tenant also agrees to furnish Landlord, within two (2)
                  business days thereafter, a written report explaining all
                  repairs and procedures which were conducted  during any such
                  emergency operations, in sufficient detail to permit
                  Landlord's engineers to evaluate the same. Any access to the
                  Building mechanical rooms, whether during an emergency or
                  otherwise, shall require Tenant to sign in at the Building
                  manager's office or the security desk, and Tenant shall permit
                  the Building manager's security guard or representative of
                  Landlord of the Building manager to accompany Tenant during
                  any such mechanical room, if Landlord so desires. No
                  installation, alterations or repairs shall be initiated
                  without first delivering to Landlord's engineers, plans and
                  specifications of the proposed changes, in substance and form
                  acceptable to Landlord. No oral approval of these plans and
                  specifications shall be deemed effective; only evidence of
                  written approval which has been received by Landlord's
                  Building manager will be binding against Landlord. No
                  electrical grounding shall be permitted to other equipment in
                  the mechanical rooms without Landlord's specific written
                  approval of the method and locations of such grounding.

                                       63
<PAGE>
 
         c.       Insurance; Mechanic's Liens. No construction, alteration or
                  removal operations shall be initiated by Tenant hereunder
                  unless Tenant has first obtained workers compensation and
                  builders risk insurance in limits acceptable to Landlord. Any
                  construction or mechanic's liens filed or claimed against the
                  Leased Premises, any Raceway or the Building as a result of
                  Tenant's operations shall be immediately paid, released or
                  bonded over in a manner acceptable to Landlord.

         d.       Hours of Operations. No activities of Tenant associated with
                  the construction and the installation of the Equipment shall
                  be conducted during the normal business hours of the Building.

         e.       Licenses and Permits. Prior to commencing any work on and/or
                  installations hereunder in the Leased Premises, at the
                  locations of the Generator and Tank, or of any Raceway, Tenant
                  shall obtain all necessary licenses, permits and consents and
                  provide copies of the same to Landlord. Landlord shall have
                  the right to supervise all such work, at its own expense.

         f.       Generator License Fee. The license fee for the one (1)
                  Generator allowed by this Exhibit is included in the Base
                  Rental. Additional pads may be added with the approval of
                  Landlord (the size and location of which will be at Landlord's
                  sole option) and the Generator License Fee will be at the then
                  current Landlord's charge for each such additional pad.

         g.       Costs. Licensee shall be responsible for any and all costs,
                  damages or expenses arising from the installation,
                  maintenance, or repair to any Raceway utilized by Tenant
                  pursuant to the terms hereof, the Leased Premises or the
                  Equipment, the Generator or the Tank and any and all costs,
                  damages or expenses to the Building or the property of
                  Landlord or other licensees or tenants of the Building arising
                  from such installation, maintenance or repair operations.
                  Tenant agrees to indemnify and hold Landlord harmless from and
                  against any loss, cost, damage or expense arising out of or in
                  connection with Tenant's installation, maintenance, repair or
                  operation of the Equipment, the Generator or the Tank, unless
                  such loss, cost, damage or expense arises primarily from the
                  negligence or willful misconduct of Landlord, its agents or
                  employees. This indemnity is in addition to and not in
                  substitution of any other indemnity in the Lease. Tenant shall
                  also pay all ad valorem taxes attributable to the Equipment,
                  the Generator and the Tank.

6.       CARE OF GENERATOR, TANK AND RACEWAYS BY TENANT. Tenant agrees not to
         commit any waste or allow any waste to be committed on the areas where
         the Generator and the Tank are located or in any of the mechanical or
         equipment rooms in which any Raceway utilized by Tenant pursuant to the
         terms hereof is located. At the termination of this license, Tenant
         agrees to deliver the areas on which the Generator and Tank are located
         and each Raceway to Landlord in as good condition as at the date of
         this Lease, subject to ordinary wear and tear and damages, fire, and
         other casualties caused by any third parties not affiliated with
         Tenant, its agents, employees or contractors.

                                       66
<PAGE>
 
7.       REPAIRS AND ALTERATIONS BY TENANT. Landlord shall have the right, at
         its option, and at Tenant's cost and expense, to repair or replace any
         damage done to the Building or the Property, or any part hereof, caused
         by Tenant or Tenant's agents, employees, invitees, or visitors, and
         Tenant shall pay the reasonable cost thereof to Landlord on demand as
         additional rent. Tenant agrees with Landlord not to make or allow to be
         made any alterations to any Raceway without the prior written consent
         of Landlord, other than routine Cable installation operations in a
         Raceway utilized by Tenant pursuant to the terms hereof, but even
         routine Cable installations shall not be permitted without prior
         written notice of at least two (2) days to Landlord. Any and all
         alterations to any Raceway shall become the property of Landlord upon
         termination of this license, except for the Equipment installed by
         Tenant. All of the Equipment, the Generator and the Tank (but not the
         Conduit) shall be removed by Tenant from the Leased Premises, the
         Building and the Property immediately upon termination of the license
         in compliance with the provisions of Section 4.4 of the Lease. Tenant
         shall be responsible for repairing any damage to the Leased Premises,
         any Raceway, the Building and/or the Property resulting from the
         removal of any Equipment, the Generator, the Tank or other personal
         property, and Tenant shall be responsible for restoring the Leased
         Premises, any Raceway, the Building and/or the Property, as applicable,
         to good condition. If Tenant fails to remove the Equipment, the
         Generator, the Tank or any other property required to be removed
         herein, Landlord may, if Landlord so elects, remove the Equipment, the
         Generator, the Tank and any other property at Tenant's cost and Tenant
         shall pay Landlord promptly upon demand all costs in removing said
         property.

8.       LAWS AND REGULATIONS. Tenant agrees to comply with all applicable laws,
         ordinances, rules and regulations of any governmental entity or agency
         having jurisdiction with respect to the Equipment, the Generator and
         the Tank, specifically including, without limitation, all applicable
         environmental laws. In particular, prior to installation of the
         Equipment, Tenant shall provide Landlord written evidence, satisfactory
         to Landlord in its sole discretion, of the representations and
         warranties set forth in this Paragraph 8. The parties  acknowledge that
         the Equipment should emit no amounts of radiation whatsoever. Tenant
         shall, at Tenant's sole cost, take all measures necessary to insure
         that no radiation is emitted from the Equipment and that the Equipment,
         the Generator and the Tank strictly complies with all laws, rules,
         regulations, ordinances and codes, whether now or hereafter existing,
         of all federal, state and local governmental authorities and that the
         Equipment strictly complies with all contractual obligations to which
         Tenant is bound in connection with such Equipment, specifically
         including, without limitation, regulations of the Federal
         Communications Commission, the Environmental Protection Agency, and the
         Occupational Safety and Health Administration, applicable to the
         emission of radiation from active transmission equipment or similar
         facilities. Tenant shall also pay promptly when due all royalties or
         other fees due in connection with the operation of the Equipment. In
         the event compliance with this paragraph shall require modifications or
         alteration of the Equipment, any Raceway utilized by Tenant pursuant to
         the terms hereof, the Leased Premises, the Generator or the Tank, no
         modification or alteration thereof shall be made without Landlord's
         prior written consent, which consent may be withheld in Landlord's

                                       67
<PAGE>
 
         sole discretion or granted on such terms and conditions as Landlord may
         determine in its sole discretion. Tenant shall take all measures
         necessary to insure that the Equipment, the Generator and the Tank do
         not interfere with or disturb the operation of any other equipment or
         business of Landlord or of any other tenant or occupant of the
         Building.

9.       RELEASE OF LIABILITY AND INDEMNITY. Landlord shall not be liable to
         Tenant, or to Tenant's agents, servants, employees, customers or
         invitees for any injury to person or damage to property caused by any
         act, omission, or neglect of Tenant, its agents, servants, employees,
         invitees or any other person entering the Building under the invitation
         of Tenant or arising out of the use of the Raceways, the Generator or
         the Tank by Tenant and the conduct of its business or out of a default
         by Tenant in the performance of its obligations hereunder, other than
         injury or damage which is solely attributable to Landlord's gross
         negligence or willful misconduct. Tenant hereby indemnifies and holds
         Landlord harmless from all liability and claims for any such damage or
         injury. In addition, Tenant shall protect, defend and indemnify
         Landlord, and its mortgagees and Building manager, and each of their
         respective directors, officers, joint venturers, employees and agents,
         and hold them harmless from and against all liability and claims for
         any injury to person or damage to property or business caused by any
         act, omission, or neglect of Tenant, its agents, servants, employees,
         or contractors, or any condition or circumstance relative to the
         license granted hereunder, specifically including, without limitation,
         (i) the installation, maintenance, operation, repair, modification or
         removal of the Equipment, the Generator or the Tank, or (ii) any tort,
         including, without limitation, slander and tortious interference with
         business arising out of the use and operation of the Equipment, or, as
         a result of the content of the transmissions and receptions sent or
         received over or through the Equipment, unless such liability and
         claims arise primarily by reason of the negligence or willful
         misconduct of Landlord, its agents or employees. Landlord and its
         mortgagees, the Building manager and each of their respective
         directors, officers, joint venturers, employees and agents shall also
         not be liable for any consequential or special damages arising out of
         any such injury to person or damage to property or business. This
         release and indemnity is in addition to and not in substitution of any
         other release and indemnity in the Lease.

10.      TRANSFERS BY TENANT. Tenant shall not transfer, convey, mortgage,
         pledge, hypothecate, or encumber Tenant's license interest hereunder or
         grant any license, concession or any other right to use a portion of
         the Raceways without the prior written consent of Landlord, which may
         be granted or withheld in Landlord's sole discretion. Notwithstanding
         the foregoing, Tenant may assign the license granted hereby in
         connection with any assignment of Tenant's entire interest in the Lease
         or a subletting of the entire Leased Premises which is permitted under
         Section 4.7 of the Lease without the necessity of obtaining any further
         approval from Landlord.

11.      COMMENCEMENT OF OPERATIONS. The commencement of operations in the
         Leased Premises by Tenant shall constitute the acknowledgment and
         agreement of Tenant that Tenant is fully familiar with the physical
         condition of the Building (including the mechanical rooms and Raceway
         space of the Building), that Tenant has accepted the same in good order
         and condition, and that the Raceway space complies in all respects

                                       68
<PAGE>
 
         with the requirements hereof and is suitable for the purposes for which
         the same is hereby licensed. In that regard, Landlord hereby disclaims,
         and Tenant hereby waives, any warranty of suitability with respect to
         the Raceway space, and any warranty of fitness for a particular
         purpose.




                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------

                                       69
<PAGE>
 
                                  SCHEDULE "1"
                                  ------------

    To Exhibit "H" to Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                              LOCATION OF GENERATOR
                              ---------------------
                        (For illustrative purposes only)

                            [Floorplan appears here]




                                       70
<PAGE>
 
                                   EXHIBIT "I"
                                   -----------

            To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                                 RENEWAL OPTION
                                 --------------


1.       DEFINED TERMS. For purposes of this Exhibit "I", all terms defined in
         the Lease (including other exhibits to the Lease) will be utilized
         herein without further definition. In addition, when delineated with
         initial capital letters, the following term shall have the following
         definition and meaning:

         (a) "Renewal Date" shall mean the first day next following the
             expiration date of the Lease Term.


2.       GRANT OF OPTION. Tenant shall have the following option ("Option") to
         renew this Lease:

         Tenant may, by notifying Landlord of its election in writing at least
         six (6) full calendar months prior to the end of the Lease Term, renew
         this Lease for an additional lease term (the "Second Lease Term")
         beginning on the first (1st) Renewal Date and continuing for five (5)
         years thereafter. Such renewal shall be on all of the terms and
         conditions of this Lease which are not inconsistent herewith.

         The Base Rental payable beginning on the first (1st) Renewal Date and
         continuing thereafter shall be at then current market rate.

         If Tenant exercises its option to renew the Lease for a Second Lease
         Term, Tenant may, by notifying Landlord of its election in writing at
         least six (6) full calendar months prior to the end of the Second Lease
         Term, renew this Lease for an additional lease term (the "Third Lease
         Term") beginning on the second (2nd) Renewal Date and continuing for
         five (5) years thereafter. Such renewal shall be on all of the terms
         and conditions of this Lease which are not inconsistent herewith,
         except that no renewal option shall exist during the Third Lease Term.

         The Base Rental payable beginning on the second (2nd) Renewal Date and
         continuing thereafter shall be at then current market rate.


         Failure by Tenant to notify Landlord of Tenant's election to exercise
         the renewal option herein granted within the time limits set forth for
         such exercise shall constitute a waiver of such Option. Notwithstanding
         the foregoing, the Option shall not be applicable at any time when
         there is a default under the Lease which has not been cured after
         applicable

                                       71
<PAGE>
 
         notice and cure periods. In addition, the Option shall automatically
         terminate upon the termination of the Lease Term, whether by Landlord
         upon the occurrence of an event of default or otherwise or, at the
         option of Landlord, in its sole discretion, upon the assignment,
         subletting, or other transfer by Tenant, whether or not with the
         approval of Landlord to any person or entity other than an Affiliate.




                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------


                                       72
<PAGE>
 
                                   EXHIBIT "J"
                                   -----------
            To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS, as Tenant

-------------------------------------------------------------------------------

                             WORK LETTER (ALLOWANCE)
                             -----------------------


This Work Letter ("Work Letter") describes and specifies the right and
obligations of Landlord and Tenant with respect to certain allowances granted to
Tenant hereunder and rights and responsibilities of Landlord and Tenant with
respect to the design, construction and payment for the completion of Tenant's
initial leasehold improvements ("Initial Improvements") within the Leased
Premises.

1.       DEFINITIONS. Terms which are defined in the Lease shall have the same
         meaning in this Work Letter. Additionally, as used in this Work Letter,
         the following terms (when delineated with initial capital letters)
         shall have the respective meaning indicated for each as follows:

         (a)      "Allowance" shall mean an amount not to exceed Two Hundred
                  Seventy-two Thousand Nine Hundred Forty and 00/100ths Dollars
                  ($272,940.00)

         (b)      "Basic Construction of the Building" shall mean the structure
                  of the Building as built on the date of this Work Letter.

         (c)      "Landlord's Architect" shall mean the architect designated by
                  Landlord as its architect, from time to time, to perform the
                  functions of Landlord's Architect hereunder.

         (d)      "Plans and Specifications" shall mean collectively, the plans,
                  specifications and other information prepared or to be
                  prepared by Tenant's Architect and, where necessary, by
                  Landlord's electrical, mechanical and structural engineers,
                  all at Tenant's expense, which shall detail the Work required
                  by Tenant in the Premises and which shall be approved in
                  writing by both Tenant and Landlord prior to the commencement
                  of such Work.

         (e)      "Tenant's Architect" shall mean EPB Design Center LTD, who is
                  an architect licensed to practice in the State of Texas.

         (f)      "Work" shall mean all materials and labor to be added to the
                  Basic Construction of the Building in order to complete the
                  installation of the Initial Improvements within the Leased
                  Premises for Tenant in accordance with the Plans and
                  Specifications, including, without limitation any
                  modifications to the Building,

                                       73
<PAGE>
 
                  any electrical or plumbing work required to meet Tenant's
                  electrical and plumbing requirements, and any special air
                  conditioning work required to be performed in the Leased
                  Premises.

         (g)      "Cost of the Work" shall mean the cost of all materials and
                  labor to be added to the Basic Construction of the Building in
                  order to complete the installation of the Initial Improvements
                  within the Leased Premises in accordance with the Plans and
                  Specifications.

         (h)      "Tenant's Costs" shall mean that portion of the Cost of the
                  Work in excess of Allowance.

         (i)      "Change Costs" shall mean all costs or expenses attributable
                  to any change in the Plans and Specifications which, when
                  added to other costs and expenses incurred in completing the
                  Work, exceed Allowance, including, without limitation, (i) any
                  cost caused by direction of Tenant to omit any item of Work
                  contained in the Plans and Specifications, (ii) any additional
                  architectural or engineering services, (iii) any changes to
                  materials in the process of fabrication, (iv) the cancellation
                  or modification of supply or fabricating contracts, (v) the
                  removal or alteration of any Work or any plans completed or in
                  process, or (vi) delays affecting the schedule of the Work.

         (j)      "Working Days" shall mean all days of the week other than
                  Saturday, Sunday, and legal holidays.

         (k)      "Contractor" shall mean the contractor or contractors engaged
                  by Tenant to perform the Work in accordance with the
                  provisions of Section 4.2(b) of the Lease.

2.       PROCEDURE AND SCHEDULES FOR THE COMPLETION OF PLANS AND SPECIFICATIONS.
         The Plans and Specifications shall be completed in accordance with the
         following procedure and time schedules:

         (a)      Design Drawings. Within sixty (60) Working Days from execution
                  of the Lease, Tenant shall submit to Landlord four (4) sets of
                  prints of design drawings, specifying the intended design,
                  character and finishing of the Initial Improvements within the
                  Leased Premises. Such package shall include separate drawings
                  for signs in accordance with Landlord's sign criteria. The
                  design drawings shall set forth the requirements of Tenant
                  with respect to the installation of the Initial Improvements
                  within the Leased Premises, and such drawings shall include,
                  without limiting their scope, a Tenant approved space plan,
                  architectural design of the space, including office front,
                  plans, elevations, sections, and renderings indicating
                  materials, color selections and finishes.

                  (i)      After receipt of design drawings, Landlord shall
                           return to Tenant one set of Prints of design drawings
                           with Landlord's suggested modifications

                                       74
<PAGE>
 
                           and/or approval within ten (10) Working Days of
                           receipt thereof from Tenant. If, upon receipt of
                           approved design drawings bearing Landlord's comments,
                           Tenant wishes to take exception thereto, Tenant may
                           do so in writing, by certified mail addressed to
                           Landlord, within five (5) Working Days from the date
                           of receipt of Landlord's comments on the design
                           drawings. Unless such action is taken, Tenant will be
                           deemed to have accepted and approved all of
                           Landlord's comments on the design drawings.

                  (ii)     If design drawings are returned to Tenant with
                           comments, but not bearing approval of Landlord, the
                           design drawings shall be immediately revised by
                           Tenant and resubmitted to Landlord for approval
                           within ten (10) Working Days of their receipt by
                           Tenant.

         (b)      Completion of Plans and Specifications. All Plans and
                  Specifications shall be prepared in strict compliance with
                  applicable Building standards and requirements, this Work
                  Letter and otherwise, and shall also adhere to the design
                  drawings approved by Landlord. In order to assure the
                  compatibility of Tenant's electrical and mechanical systems
                  and the compatibility of Tenant's structural requirements with
                  the existing Building and in order to expedite the preparation
                  of Tenant's electrical, mechanical and structural drawings,
                  Tenant or Tenant's Architect shall deliver to Landlord's
                  Architect, not later than thirty (30) Working Days from the
                  date of Landlord's approval of design drawings, a detailed
                  plan setting forth any and all electrical, mechanical and
                  structural requirements, and Landlord's Architect shall
                  retain, at Tenant's expense, Landlord's electrical, mechanical
                  and structural engineers to prepare all necessary electrical,
                  mechanical and structural construction drawings which shall be
                  included as a part of the Plans and Specifications. All
                  construction documents and calculations prepared by Tenant's
                  Architect shall be submitted by Tenant, in the form of four
                  (4) sets of blueline prints, to Landlord for approval within
                  ten (10) Working Days after the date of receipt by Tenant of
                  Landlord's approval of design drawings, electrical, mechanical
                  and structural drawings. If the Plans and Specifications are
                  returned to Tenant with comments, but not bearing approval of
                  Landlord, the Plans and Specifications shall be immediately
                  revised by Tenant and resubmitted to Landlord for approval
                  within fifteen (15) Working Days of their receipt by Tenant.

                  (i)      The fees for Tenant's Architect and any consultants
                           or engineers retained by or on behalf of Tenant or
                           Tenant's Architect (including, but not limited to,
                           the electrical, mechanical and structural engineers
                           required to be retained under this paragraph) shall
                           be paid by Tenant. Tenant shall also pay for any
                           preliminary drawings by Landlord's Architect for
                           review of the design drawings, the Plans and
                           Specifications, and any revisions to such documents,
                           and the fees and expenses of Landlord's Architect for
                           inspection of the Work, as required by Landlord.
                           Tenant may use funds from the Allowance to make such
                           payments.

                                       75
<PAGE>
 
                  (ii)     Tenant shall have the sole  responsibility  for
                           compliance  of the Plans and Specifications with all
                           applicable statutes, codes, ordinances and other
                           regulations, and the approval of the Plans and
                           Specifications or calculations included therein by
                           Landlord shall not constitute an indication,
                           representation or certification by Landlord that such
                           Plans and Specifications or calculations are in
                           compliance with said statutes, codes, ordinances and
                           other regulations. In instances where several sets of
                           requirements must be met, the requirements of
                           Landlord's insurance underwriter or the strictest
                           applicable  requirements shall apply where not
                           prohibited by applicable codes.

3.       TERMINATION RIGHT. If for any reason Landlord and Tenant have not
         agreed in writing upon final Plans and Specifications on or before the
         date which is ninety (90) days from the date of submission to Landlord,
         then Landlord or Tenant shall have the right to terminate the Lease by
         providing the other party with written notice of the electing parties'
         decision to terminate this Lease within thirty (30) days from the
         expiration of such ninety (90) day period. The failure of either party
         to exercise such termination right in the manner and within the time
         period specified above shall be deemed to be an irrevocable waiver of
         such right.

4.       PAYMENT. In the event Landlord acts as the general contractor for the
         Initial Improvements in the Leased Premises, the Allowance will be
         applied to offset the amounts due Landlord as reflected in the monthly
         invoices therefor submitted by Landlord to Tenant. In the event
         Landlord does not act as the general contractor for the Initial
         Improvements in the Leased Premises, Landlord shall pay the Allowance
         to Tenant within thirty (30) days of Landlord's receipt of invoices
         submitted by Tenant to Landlord.

5.       PERFORMANCE OF WORK AND DELAYS. Tenant shall cause the Contractor to
         perform the Work in strict accordance with the Plans and
         Specifications. If a delay shall occur in the completion of the Work by
         Tenant as the probable result of (i) any failure to furnish when due
         Tenant's design drawings, Tenant's electrical, mechanical and/or
         structural requirements, Tenant's Plans and Specifications or any
         revision to any such documents, (ii) any change by Tenant in any of the
         Plans and Specifications, (iii) any state of facts which gives rise to
         a change referred to in the definition of Change Costs or any changes
         resulting in a Change Cost, (iv) any other act or omission of Tenant,
         its agents or employees, including any violation of the provisions of
         the Lease or any delay in giving authorizations or approvals pursuant
         to this Work Letter, or (v) any other cause except (a) as specified in
         Section 8.1 of the Lease or (b) arising from a default by Landlord,
         then any such delay shall not justify any extension of the Commencement
         Date of the Lease.

6.       CHANGE ORDERS. All changes and modifications in the Work from that
         contemplated in the Plans and Specifications, whether or not such
         change or modification gives rise to a Change Cost, must be evidenced
         by a written Change Order executed by both Landlord and Tenant. In that
         regard, Tenant shall submit to Landlord such

                                       76
<PAGE>
 
         information as Landlord shall require with respect to any Change Order
         requested by Tenant. After receipt of requested Change Order, together
         with such information as Landlord shall require with respect thereto,
         Landlord shall return to Tenant either the executed Change Order, which
         will evidence Landlord's approval thereof, or the Plans and
         Specifications with respect thereto with Landlord's suggested
         modification.

7.       WHOLE AGREEMENT; NO ORAL MODIFICATION. This Work Letter embodies all
         representations, warranties and agreements of Landlord and Tenant with
         respect to the matter described herein, and this Work Letter may not be
         altered or modified except by an agreement in writing signed by the
         parties.

8.       PARAGRAPH HEADINGS. The paragraph headings contained in this Work
         Letter are for convenient reference only and shall not in any way
         affect the meaning or interpretation of such paragraphs.

9.       NOTICES. All notices required or contemplated hereunder shall be given
         to the parties in the manner specified for giving notices under the
         Lease.

10.      BINDING EFFECT. This Work Letter shall be construed under the laws of
         the State of Texas and shall be binding upon and shall inure to the
         benefit of the parties hereto and their respective permitted successors
         and assigns.

11.      CONFLICT. In the event of conflict between this Work Letter and any
         other exhibits or addenda to this Lease, this Work Letter shall
         prevail.







                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------


                                       77
<PAGE>
 
                       THIS PAGE INTENTIONALLY LEFT BLANK


                                       78
<PAGE>
 
                                   EXHIBIT "K"
                                   -----------
            To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS as Tenant

-------------------------------------------------------------------------------

                           ROOFTOP COOLING PROVISIONS
                           --------------------------

This Exhibit "K" describes and specifies the right to install dry cooling
equipment upon the roof of the Building hereby granted by Landlord to Tenant,
which is being granted upon the following terms and conditions:

1.        DEFINED TERMS. Terms defined in the Lease and delineated herein by
          initial capital letters shall have the same meaning ascribed thereto
          in the Lease, except to the extent that the meaning of such term is
          specifically modified by the provisions hereof. In addition, other
          terms not defined in the Lease but defined herein will, when
          delineated with initial capital letters, have the meanings ascribed
          thereto in this Exhibit. Terms and phrases which are not delineated by
          initial capital letters shall have the meanings commonly ascribed
          thereto.

2.        GRANT OF LICENSE. Subject to the terms and provisions of this Exhibit,
          Landlord hereby grants to Tenant the right and license to install,
          operate, maintain, repair and replace a maximum of [NUMBER (##)] dry
          cooling units (the dimension of each of which shall not exceed a
          maximum surface area of 1,500 square feet), together with related
          wiring, piping, vents and equipment, including [# OF PIPES] [#INCHES]"
          pipes to and from the Leased Premises and such equipment
          (collectively, the "Cooling Equipment") on the roof of the Building.
          The size, location, and manner of installation of the Cooling
          Equipment shall be approved by Landlord as part of the approval of the
          Plans and Specifications pursuant to Exhibit "J" hereto,  which
          approval shall take into  consideration the functional requirements of
          the Cooling Equipment and of any other equipment located on the roof
          of the Building and shall be subject to standards of structural and
          architectural integrity with respect to the Building (and, in that
          regard, the Cooling Equipment shall be located so as not to be visible
          except from above the Building, shall complement the Building color,
          and shall have no visible marking or logo). It is understood and
          acknowledged that the area to be covered by the Cooling Equipment,
          including necessary walkways and required airspace, shall not exceed
          [###] square feet. The license granted to Tenant shall commence on the
          Commencement Date and shall expire upon the expiration or earlier
          termination of the Lease Term.

3.        LICENSE FEE. Four Thousand Two Hundred eighteen and 75/100th Dollars
          ($4,218.75) per month, commencing on the Rental Commencement Date.
          Additional dry cooling units may be added with the approval of
          Landlord (the size and location of which will be at Landlord's
          reasonable option) and the License Fee for each such additional
          cooling equipment will be at the then current Landlord charge for such
          additional cooling equipment.

                                       79
<PAGE>
 
4.        INSTALLATION, MAINTENANCE, AND REPAIR. Tenant shall, at Tenant's sole
          cost and expense, provide for the installation of the Cooling
          Equipment and for all service, repairs, and maintenance to the Cooling
          Equipment. With respect to the installation of the Cooling Equipment,
          the Cooling Equipment shall not be affixed to the roof of the Building
          by nail, bolt, screw, or other device which penetrates the roof
          (except as approved in writing by Landlord), and all wiring
          penetrations , venting penetrations and piping penetrations shall be
          made by Landlord's roofing contractor at Tenant's sole cost and
          expense, which shall be limited to the actual and reasonable cost
          thereof. The Cooling Equipment shall be maintained by Tenant in good
          order and repair and in a safe condition. In the event that Tenant
          fails to so maintain the Cooling Equipment and Landlord, in Landlord's
          sole discretion, deems such failure to present a danger of injury to
          persons or damage to property in or about the Building, Landlord may
          affect such repairs or maintenance and recover the cost thereof from
          Tenant, which sum shall be due and payable within ten (10) days after
          Landlord's written notice to Tenant with respect thereto. Tenant shall
          provide Landlord with prior written notice of Tenant's need to effect
          service, maintenance, or repairs hereunder and shall thereupon have
          access to the roof of the Building and other non-tenant areas of the
          Building at such times and under such conditions as may be prescribed
          by Landlord. Landlord may charge Tenant for any equipment and material
          of Landlord used by Tenant with respect thereto, at Landlord's
          standard rates. The Cooling Equipment shall be a Trade Fixture as
          defined in Section 1.18 of the Lease.

5.       INTERFERENCE. Landlord assumes no responsibility for any interference
         in the use or installation of the Cooling Equipment. Tenant shall, at
         Tenant's option, either modify the Cooling Equipment or relocate the
         Cooling Equipment to another area approved by Landlord in the event
         that the Cooling Equipment, in Landlord's reasonable judgment causes
         any interference with or disturbs the operation of any other equipment
         or business of Landlord or any other occupant of the Building or
         creates or results in any noise, odor, or nuisance to any other
         occupant of the Building, or the areas adjacent thereto. However, if
         the need to modify or relocate the Cooling Equipment is caused by the
         exercise of any right granted by Landlord to other occupants of the
         Building after the Commencement Date or changes to any other equipment
         of Landlord of any other occupant of the Building after the
         Commencement Date, then Landlord shall bear the cost of modifying or
         relocating the Cooling Equipment. In the event that Landlord determines
         that the Cooling Equipment causes any interference with or disturbs the
         operation of any other Cooling Equipment or business of Landlord or any
         other occupant of the Building, and Tenant fails to modify or is unable
         to modify the Cooling Equipment so as to eliminate such interference,
         Landlord shall, prior to making a decision that the Cooling Equipment
         needs to be shut off, take into account the nature of the interference
         and the reasonable business needs of both Tenant and the other occupant
         of the Building (including Landlord) being affected and shall consult
         with Tenant regarding this matter. Tenant must shut off the Cooling
         Equipment within a reasonable period of time after notification of the
         interference or disturbance from Landlord, taking into account the
         nature of the interference and Tenant's reasonable business needs; and
         Tenant may restore, modify, or relocate the Cooling Equipment to test
         for interference only with

                                       80
<PAGE>
 
         Landlord's permissions, which shall not be unreasonably withheld,
         delayed or conditioned.

6.        USE. The Cooling Equipment is solely for use for Tenant's internal
          business and the benefits of the Cooling Equipment may not be provided
          by Tenant to third parties except Tenant's Customers. The Cooling
          Equipment may not be sold or rented by Tenant to third parties, nor
          may Tenant sublet or assign the license granted to Tenant hereunder,
          except in connection with any assignment or sublease which is
          permitted under Section 4.7 of the Lease. Tenant shall operate the
          Cooling Equipment in compliance with all applicable laws, regulations,
          and rules of the governmental authorities having jurisdiction thereof
          and shall maintain all necessary licenses and permits with respect
          thereto. Any electrical usage associated with the Cooling Equipment
          shall be governed by the provisions of Section 2.5 of the Lease.

7.        INTERRUPTION. Landlord may, at Landlord's sole discretion but without
          cost to Landlord, upon such advance notice to Tenant as is reasonable
          under the circumstances, taking into account the nature of the
          repairs, maintenance or modification needing to be made by Landlord
          and (after consultation with Tenant) Tenant's reasonable business
          needs, require interruption of service or relocation or removal of the
          Cooling Equipment for repairs, maintenance, or modification of the
          Building, including, but not limited to, roofing, structural,
          electrical, or mechanical repairs.

8.        MODIFICATION AND SUBSTITUTION. Tenant may not substitute or modify the
          Cooling Equipment or any part thereof, without the prior written
          approval of Landlord, which consent shall not be unreasonably withheld
          or delayed. It is understood and agreed that it shall not be
          unreasonable if Landlord disapproves any such substitution or
          modification because of a material increase in the size or area
          covered by the Cooling Equipment. Any substitution or modification
          approved by Landlord shall automatically become a part of the Cooling
          Equipment, and all terms of this Exhibit shall apply to such
          substitution and/or modification.

9.        RESTORATION. Tenant shall, at Tenant's sole cost and expense, remove
          the Cooling Equipment and repair any damage to the Building caused by
          such removal if the Lease Term, and the license herein granted,
          terminates,  whether by expiration of the Lease Term or otherwise.
          Such removal and restoration work shall be completed by Tenant within
          sixty (60) days of the date on which the condition requiring such
          removal and restoration work occurs. In the event that Tenant shall
          fail to complete the removal of the Cooling Equipment and the
          restoration of the Building within such sixty (60) day period,
          Landlord shall have the right to effect such removal and restoration,
          but shall not be obligated to do so, in which event, Tenant shall
          reimburse Landlord for all costs incurred by Landlord in performing
          such removal and restoration work. In addition, any Cooling Equipment
          remaining at the Building subsequent to the expiration of such sixty
          (60) day period and upon prior written notice to Tenant shall be
          deemed to have been abandoned by Tenant, so that in no event shall
          Landlord have any duty to preserve or restore the Cooling Equipment on
          Tenant's behalf. If Landlord does choose to store the Cooling
          Equipment on Tenant's behalf, the cost of storage incurred by Landlord
          shall be

                                       81
<PAGE>
 
          reimbursed by Tenant on demand. In such event, Landlord shall also
          have the right to sell such Cooling Equipment for salvage value and to
          apply the proceeds derived from such sale to sums owing by Tenant to
          Landlord hereunder. The provisions of this Paragraph 9 shall survive
          the expiration of the license granted hereunder and of the lease Term.

10.       REPRESENTATIONS OF TENANT. As a material inducement to Landlord to
          enter into this Exhibit and to grant the license to Tenant described
          in Paragraph 2 hereof, Tenant hereby represents and warrants to
          Landlord that (a) the Cooling Equipment has received all necessary
          approvals; (b) the Cooling Equipment will be installed in accordance
          with the plans and specifications therefore as approved by Landlord in
          accordance with the provisions of Exhibit "J"; (c) the Cooling
          Equipment is designed to require minimum maintenance; (d) the Cooling
          Equipment will not emit any fluids or gases which constitute hazardous
          materials or substances under applicable environmental laws or
          regulations, or which could otherwise cause damage to persons or
          property; (e) the Cooling Equipment will not interfere with other
          equipment presently located on the rooftop of the Building or in the
          Building; and (f) Tenant will have received all necessary licenses and
          permits from all applicable governmental  authorities regarding the
          installation and operation of the Cooling Equipment, prior to such
          installation and operation, and (g) the Cooling Equipment will
          strictly comply with all laws, rules, ordinances and codes whether now
          or existing of all Federal, State, and all local government
          authorities.

11.       INDEMNIFICATION. In addition to the indemnification and obligations of
          Tenant under the Lease, Tenant shall protect, defend, indemnify, and
          hold Landlord harmless from (i) all liability and claims for any
          injury to person or damage to property caused by any act, omission, or
          neglect of Tenant, its agents, servants, employees, or contractors,
          relative to the license, including, without limitation, the
          installation, operation, repair, and maintenance of the Cooling
          Equipment, unless such liability and claims arise primarily by reason
          of the negligence or willful misconduct of Landlord, its agents,
          contractors or employees, and (ii) all losses, damages or liabilities
          that may be suffered or incurred by Landlord in connection with
          Landlord's inability to complete any necessary repairs, maintenance,
          or modification of the Building, including, but not limited to,
          roofing, structural, electrical or mechanical repairs, as a result of
          Tenant's failure to allow interruption of service or relocation or
          removal of the Cooling Equipment pursuant to Section 7 of this
          Exhibit.

12.       CASUALTY DAMAGE. Landlord shall not be liable to Tenant for any loss
          or damage to all or any part of the Cooling Equipment occasioned by
          theft, fire, act of God, public enemy, injunction, riot, vandalism,
          malicious mischief, earthquake, flood, strike, insurrection, war,
          court order, requisition, or order of governmental body or authority
          or by any other cause whatsoever, other than that solely caused by the
          gross negligence or willful misconduct of Landlord, its agents,
          contractors or employees. Nor shall Landlord be liable for any damage
          or inconvenience which may arise through the repair or alteration of
          any part of the Building or through termination of the license granted
          pursuant to this Exhibit "L" following a default under Section 15
          hereof, other than

                                       82
<PAGE>
 
                                   EXHIBIT "L"
                                   -----------

            To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS as Tenant

-------------------------------------------------------------------------------

             SUBORDINATION, NON-DISTURBANCE AND ATTORNMENT AGREEMENT
             -------------------------------------------------------


NOTICE: THIS SUBORDINATION, NON-DISTURBANCE AND ATTORNMENT AGREEMENT RESULTS IN
YOUR LEASEHOLD ESTATE IN THE PROPERTY BECOMING SUBJECT TO AND OF LOWER PRIORITY
THAN THE LIEN OF SOME OTHER OR LATER SECURITY INSTRUMENT.

THIS AGREEMENT is entered into by and among Tenant, Landlord, and Beneficiary
and effects the Property described in Schedule "A" attached hereto. The terms
"Tenant", "Landlord", "Beneficiary", "Premises", "Lease", "Property", "Loan",
"Note", and "Mortgage" are defined in the schedule of Definitions attached
hereto as Schedule "B". This Agreement is entered into with reference to the
following facts:

(a)      Landlord and Tenant have entered into the Lease covering the Premises
         in the Property.

(b)      Beneficiary has agreed to make the Loan to Landlord to be evidenced by
         the Note, which Note is to be secured by the Mortgage covering the
         Property, provided that the Lease is subordinate to the lien of the
         Mortgage.

(c)      For the purposes of completing the Loan, the parties hereto desire
         expressly to acknowledge the subordination of the Lease to the lien of
         the Mortgage, it being a condition precedent to Beneficiary's
         obligation to consummate the Loan that the lien of the Mortgage be
         unconditionally and at all times prior and superior to the leasehold
         interests and estates created by the Lease.

(d)      Tenant has requested that Beneficiary agree not to disturb Tenant's
         possessory rights in the Premises in the event Beneficiary should
         foreclose the Mortgage; provided that Tenant is not then in default
         under the Lease and provided further that Tenant attorns to Beneficiary
         or the purchaser at any foreclosure or trustee's sale of the Property.

NOW, THEREFORE, in consideration of the mutual covenants contained herein and of
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties agree as follows:

1.       SUBORDINATION. Notwithstanding anything to the contrary set forth in
         the Lease, Tenant hereby acknowledges that the Lease and the leasehold
         estate created thereby and all of Tenant's rights thereunder shall be
         and shall at all times remain subject, subordinate and inferior to the
         Mortgage and the lien thereof, and all rights of Beneficiary thereunder
         and to any and all renewals, modifications, consolidations,
         replacements and extensions thereof.

                                       83
<PAGE>
 
2. ACKNOWLEDGEMENT AND AGREEMENT BY TENANT. Tenant acknowledges that:

                  (a) Beneficiary would not make the Loan without this
                  Agreement;

                  (b) It acknowledges the Mortgage and the agreements evidencing
                  and securing the Loan; and

                  (c) Beneficiary, in making any disbursements to Landlord, is
                  under no obligation or duty to oversee or direct the
                  application of the proceeds of such disbursements, and such
                  proceeds may be used by Landlord for purposes other than
                  improvement of the Property.

                  (d) From and after the date hereof, in the event of any act or
                  omission by Landlord which would give Tenant the right, either
                  immediately or after the lapse of time, to terminate the Lease
                  or to claim a partial or total eviction, Tenant will not
                  exercise any such right:

                           (i) until it has given written notice of such act or
                           omission to Beneficiary, which notice shall be given
                           not later than the number of days required by the
                           Lease for such notice to the Landlord; and

                           (ii) until the same period of time as is given to
                           Landlord under the Lease to cure such act or omission
                           shall have elapsed following such giving of notice to
                           Beneficiary and following the time when Beneficiary
                           shall have become entitled under the Mortgage to
                           remedy the same.

                  (e) It has notice that the Lease and the rent and all other
                  sums due thereunder have been assigned or are to be assigned
                  to Beneficiary as security for the Loan secured by the
                  Mortgage. In the event that Beneficiary notifies Tenant of a
                  default under the Mortgage and demands that Tenant pay its
                  rent and all other sums due under the Lease to Beneficiary,
                  Tenant shall honor such demand and pay its rent and all other
                  sums due under the Lease directly to Beneficiary or as
                  otherwise required pursuant to such notice.

                  (f) It shall send a copy of any notice or statement under the
                  Lease to Beneficiary at the same time such notice or statement
                  is sent to Landlord.

                  (g) It has no right or option of any nature whatsoever,
                  whether pursuant to the Lease or otherwise, to purchase the
                  Premises or the Property, or any portion thereof or any
                  interest therein, and to the extent that Tenant has had, or
                  hereafter acquires, any such right or option, the same is
                  hereby acknowledged to be subject and subordinate to the
                  Mortgage and is hereby waived and released as against
                  Beneficiary.

                                       84
<PAGE>
 
                  (h) This Agreement satisfies any condition or requirement in
                  the Lease relating to the granting of a non-disturbance
                  agreement.

3.       FORECLOSURE AND SALE. In the event of foreclosure of the Mortgage, or
         upon a sale of the Property pursuant to the trustee's power of sale
         contained therein, or upon a transfer of the Property by conveyance in
         lieu of foreclosure, then:

                  (a) Non-Disturbance. So long as Tenant complies with this
                  Agreement and is not in default under any of the terms,
                  covenants, or conditions of the Lease after applicable notice
                  and cure periods (if any), the Lease shall continue in full
                  force and effect as a direct lease between the succeeding
                  owner of the Property and Tenant, upon and subject to all of
                  the terms, covenants and conditions of the Lease, for the
                  balance of the term of the Lease including renewal terms.
                  Tenant hereby agrees to adhere to and accept any such
                  successor owner as landlord under the Lease, and to be bound
                  by and perform all of the obligations imposed by the Lease and
                  Beneficiary, or any such successor owner of the Property, will
                  not disturb the possession of Tenant, and will be bound by all
                  of the obligations imposed on the Landlord by the Lease,
                  provided, however, that Beneficiary, or any purchaser at a
                  trustee's or sheriff's sale or any successor owner of the
                  Property shall not be:

                           (i) liable for any act or omission of a prior
                           landlord (including Landlord); or

                           (ii) subject to any offsets or defenses which Tenant
                           might have against any prior landlord (including
                           Landlord); or

                           (iii) bound by any rent or additional rent which
                           Tenant might have paid in advance to any prior
                           landlord (including Landlord) for a period in excess
                           of one (1) month, except for scheduled payments of
                           additional rent, or by any security deposit, cleaning
                           deposit, or other prepaid charge which Tenant might
                           have paid in advance to any prior landlord (including
                           Landlord); or

                           (iv) bound by any agreement or modification of the
                           Lease made without the written consent of
                           Beneficiary.

                  (b) New Lease. Upon the written request of either Beneficiary
                  or Tenant to the other given at the time of any foreclosure,
                  trustee's sale or conveyance in lieu thereof, the parties
                  agree to execute a lease of the Premises upon the same terms
                  and conditions as the Lease between Landlord and Tenant, which
                  lease shall cover any unexpired term of the Lease existing
                  prior to such foreclosure, trustee's sale or conveyance in
                  lieu of foreclosure.

                  (c) Beneficiary shall have no responsibility to provide (or
                  liability for not providing) any additional space for which
                  Tenant has any option or right under

                                       85
<PAGE>
 
                  the Lease if, as a result of any lease or leases entered into
                  with other tenants prior to Beneficiary acquiring title to the
                  Property, a conflict exists between such other lease or leases
                  and the right or option of Tenant with respect to additional
                  space, unless Beneficiary at its option elects to provide the
                  same and Tenant hereby releases Beneficiary from any
                  obligation it may otherwise have to provide the same, and
                  agrees that Tenant shall have no right to cancel the Lease,
                  abate rent or assert any claim against Beneficiary as a result
                  of the failure to provide any option space.

                  (d) Beneficiary shall have no liability to Tenant or any other
                  party for any conflict between the provisions of the Lease and
                  the provisions of any other lease affecting the Property,
                  including, but not limited to, any provisions relating to
                  renewal options and options to expand, and in the event of
                  such a conflict, Tenant shall have no right to cancel the
                  Lease or take any other remedial action against Beneficiary or
                  action against any other party for which Beneficiary would be
                  liable.

4.       ACKNOWLEDGEMENT AND AGREEMENT BY LANDLORD. Landlord, as landlord under
         the Lease and mortgagor or trustor under the Mortgage, acknowledges and
         agrees for itself and its heirs, successors and assigns, that:

                  (a) This Agreement does not:

                           (i) constitute a waiver by Beneficiary of any of its
                           rights under the Mortgage; and/or

                           (ii) in any way release Landlord from its obligations
                           to comply with the terms, provisions, conditions,
                           covenants, agreements and clauses of the Mortgage;

                  (b) The provisions of the Mortgage remain in full force and
                  effect and must be complied with by Landlord; and

                  (c) In the event of a default under the Mortgage, Tenant may
                  pay all rent and all other sums due under the Lease to
                  Beneficiary as provided in this Agreement.

5.       NO OBLIGATION OF BENEFICIARY. Beneficiary shall have no obligation or
         incur any liability with respect to the erection or completion of the
         improvements in which the Premises are located or for completion of the
         Premises or any improvements for Tenant's use and occupancy, either at
         the commencement of the term of the Lease or upon any renewal or
         extension thereof or upon the addition of additional space, pursuant to
         any expansion rights contained in the Lease.

6.       NOTICE. All notices, consents, waivers or other communications which
         this Lease requires or permits any party to give to another shall be in
         writing and shall be given only by registered, certified or "Express"
         mail, or by Federal Express or other similar courier

                                       86
<PAGE>
 
         service, return receipt requested, postage prepaid, to the recipient
         party at the addresses set forth in Schedule "B" to this Agreement. Any
         party may change its notice address at any time by giving written
         notice of such change to the other party in the manner provided herein
         at least ten (10) days prior to the date such change is effected. All
         notices under this Lease shall be deemed given, received, made or
         communicated on the delivery date or attempted delivery date shown on
         the return receipt or similar document utilized by the courier service
         for the purpose of indicating delivery.

7.       MISCELLANEOUS.

         (a)      This Agreement supersedes any inconsistent provision of the
                  Lease.

         (b)      Nothing contained in this Agreement shall be construed to
                  derogate from or in any way impair or affect the lien and
                  charge or provisions of the Mortgage.

         (c)      Beneficiary shall have no obligations nor incur any liability
                  with respect to any warranties of any nature whatsoever,
                  whether pursuant to the Lease or otherwise, including, without
                  limitation, any warranties respecting use, compliance with
                  zoning, Landlord's title, Landlord's authority, habitability,
                  fitness for purpose or possession.

         (d)      In the event that Beneficiary shall acquire title to the
                  Premises or the Property, Beneficiary shall have no
                  obligation, nor incur liability, beyond Beneficiary's then
                  equity interest, if any, in the Premises, and Tenant shall
                  look exclusively to such equity interest of Beneficiary, if
                  any, in the Premises for the payment and discharge of any
                  obligations imposed upon Beneficiary hereunder or under the
                  Lease, and Beneficiary is hereby released and relieved of any
                  other obligations hereunder and under the Lease.

         (e)      This Agreement shall inure to the benefit of the parties
                  hereto, their respective successors and permitted assigns;
                  provided, however, that in the event of the assignment or
                  transfer of the interest of Beneficiary, all obligations and
                  liabilities of Beneficiary under this Agreement shall
                  terminate, and thereupon all such obligations and liabilities
                  shall be the responsibility of the party to whom Beneficiary's
                  interest is assigned or transferred; and provided further that
                  the interest of Tenant under this Agreement may not be
                  assigned or transferred without the prior written consent of
                  Beneficiary.

         (f)      This Agreement shall be governed by and construed in
                  accordance with the laws of the state in which the Property is
                  located.

                                       87
<PAGE>
 
IN WITNESS WHEREOF, the parties have executed this Subordination,
Non-Disturbance, and Attornment Agreement as of the ________ day of
____________________, 19____.

IT IS RECOMMENDED THAT, PRIOR TO THE EXECUTION OF THIS SUBORDINATION,
NON-DISTURBANCE AND ATTORNMENT AGREEMENT, THE PARTIES CONSULT WITH THEIR
ATTORNEYS WITH RESPECT THERETO.

                             B E N E F I C A R Y
                             -------------------

                             ____________________, a _______________________

                             By:
                                -----------------------------------
                             Name:
                                  ---------------------------------
                             Title:
                                   --------------------------------


                             L A N D L O R D
                             ---------------

                             INFOMART-Dallas, L.P., a Texas limited partnership

                             By: INFOMART USA, L.P., a Texas limited partnership

                                   By: /s/ ????????????????????
                                      -----------------------------------
                                   Name:
                                        ---------------------------------
                                   Title: Authorized Agent
                                         --------------------------------

                             T E N A N T
                             -----------

                             FOCAL COMMUNICATIONS CORPORATION OF
                             TEXAS, a Delaware corporation

                             By: /s/ Brian F. Addy
                                -----------------------------------
                             Name:   Brian F. Addy
                                  ---------------------------------
                             Title:  Executive Vice President
                                   --------------------------------

Schedule "A" - Property Description
Schedule "B" - Schedule of Definitions

                                    INITIALS

                    Landlord                 Tenant
                            -----------            -----------

                                       83
<PAGE>
 
                                  SCHEDULE "A"
                                  ------------
    To Exhibit "L" To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS as Tenant

-------------------------------------------------------------------------------

                                LEGAL DESCRIPTION
                                -----------------

BEING a 25.454 acre tract of land situated in the City of Dallas, Dallas County,
Texas and out of the James A. Sylvester Survey, Abstract No. 1383 and being a
part of City of Dallas Block No. 6053, also being the same tract of land
conveyed to Dallas Market Center Company by a Special Warranty Deed recorded in
Volume 82113, Page 3240 of the Deed Records of Dallas County, Texas, said 25.454
acre tract of land being more particularly described as follows:

BEGINNING at a 1/2 inch iron rod found for the point of intersection of the
southwesterly right-of-way line of the Chicago Rock Island and Pacific Railroad
with the northwesterly right-of-way line of Oak Lawn Avenue;

THENCE with the northwesterly right-of-way line of Oak Lawn Avenue the
following:

         South 3131'40" West a distance of 366.74 feet to an "X" chiseled in
         concrete found for corner in a curve to the right, the radius point of
         said curve bearing North 5008'58" West a distance of 241.00 feet from
         said "X";

         Southwesterly with said curve to the right through a central angle of
         0309'20" an arc distance of 13.27 feet to an "X" chiseled in concrete
         set for the point of reverse curvature of a curve to the left having a
         radius of 259.00 feet;

         Southwesterly with said curve to the left through a central angle of
         1128'43" an arc distance of 51.89 feet to a 1/2 inch iron rod found for
         the point of reverse curvature of a curve to the right having radius of
         129.00 feet;

         Southwesterly with said curve to the right through a central angle of
         2406'22" an arc distance of 138.22 feet to a 1/2 inch iron rod set for
         the point of compound curvature of a curve to the right having a radius
         of 50.00 feet;

         Northwesterly with said curve to the right through a central angle of
         2406'22" an arc distance of 21.04 feet to a 1/2 inch iron rod found in
         the northeasterly right-of-way line of Stemmons Freeway for the point
         of compound curvature of a curve to the right having a radius of
         1130.92 feet;

THENCE with the northeasterly right-of-way line of Stemmons Freeway the
following:

         Northwesterly with said curve to the right through a central angle of
         0724'40" an arc distance of 146.28 feet to a 1/2 inch iron rod found
         for the point of tangency of said curve;

                                       89
<PAGE>
 
         North 5533'45" West a distance of 816.18 feet to a 1/2 inch iron rod
         found for point of curvature of a curve to the left having a radius of
         3289.04 feet;

         Northwesterly with said curve to the left through a central angle of
         0123'21" an arc distance of 79.74 feet to a bolt in concrete found for
         the most southerly corner of a tract of land leased to Southwestern
         Furniture Mart Co. from Industrial Properties Corporation as recorded
         in Volume 67076, Page 0690 of the Deed Records of Dallas County, Texas;

THENCE departing the northerly right-of-way line of Stemmons Freeway with the
easterly line of the Southwestern Furniture Mart Company tract, North 0921'30"
East a distance of 1064.46 feet to a 1/2 inch iron rod found for corner in the
curving southwesterly right-of-way line of the Chicago, Rock Island and Pacific
Railroad, the radius point of said curve being situated South 3311'48" West a
distance of 1599.88 feet;

THENCE with the southerly right-of-way lien of the Chicago, Rock Island and
Pacific Railroad the following:

         Southeasterly with said curve to the right through a central angle of
         0241'48" an arc distance of 75.30 feet to a 1/2 inch iron rod found for
         corner;

         North 5207'00" East a distance of 30.11 feet to a 1/2 inch iron rod
         found for corner in a curve to the right, the radius point of said
         curve being situated South 3219'18" West a distance of 1553.95 feet;

         Northwesterly with said curve to the right through a central angle of
         2126'39" an arc distance of 581.59 feet to a 1/2 inch iron rod set for
         corner;

         North 4516'10" East a distance of 53.07 feet to 1/2 inch iron rod set
         for corner; South 3148'40" East a distance of 976.20 feet to the POINT
         OF BEGINNING;

CONTAINING an area of 25.454 acres of land.


                                       90
<PAGE>
 
                                  SCHEDULE "B"
                                  ------------

    To Exhibit "L" To Lease Agreement By and Between INFOMART-Dallas, L.P.,
      as Landlord and FOCAL COMMUNICATIONS CORPORATION OF TEXAS as Tenant

--------------------------------------------------------------------------------

                             SCHEDULE OF DEFINITIONS
                             -----------------------


Beneficiary shall mean ___________________________________, a
___________________________________ . All notices to Beneficiary shall be mailed
to:

         ----------------------------------------

         ----------------------------------------

         ----------------------------------------

         ----------------------------------------

with copy to:

         ----------------------------------------

         ----------------------------------------

         ----------------------------------------

         ----------------------------------------

and a copy to:

         ----------------------------------------

         ----------------------------------------

         ----------------------------------------

         ----------------------------------------

Mortgage shall mean a first lien Mortgage or Deed of Trust and Security
Agreement with Assignment of Rents dated as , encumbering the Property, executed
by Landlord, as Mortgagor or Trustor, to , as Trustee, in favor of Beneficiary,
securing repayment of the Loan evidenced by the Note, to be recorded in the
records of the county in which the Property is located.

91
<PAGE>
 
Landlord shall mean _______________________,  a _____________, having an office
at ___________________.

Lease shall mean a certain lease entered into by and among Landlord and Tenant
dated as of ________________, 19__, covering the premises.

Loan shall mean a first mortgage loan in an amount up to _____________________
from Beneficiary to Landlord.

Note shall mean that certain ____________________ Note executed by Landlord in
favor of _______________, a _________________, dated as of _______________,
19__, in the amount of _____________________.

Premises shall mean certain space in the Improvements located in and upon the
Property.

Property shall mean the real property described in Schedule "A" attached hereto
together with the improvements thereon.

Tenant shall mean ___________________, a ________________ corporation, having an
office located at ________________.

                                       92
<PAGE>
 
         [LETTERHEAD OF DATA CENTER DESIGN and DEVELOPMENT CORPORATION]




February 5, 1999

Chris Rehor
FOCAL COMMUNICATIONS CORPORATION
200 N. La Salle Street
Chicago, IL  60601

                                                        VIA: U.S.P.S. PRIORITY

                        Re:  1950 Stemmons Freeway, Dallas, TX
                             FOCAL-DALLAS   DCDDC#98-149

                   Subject:  Change Order #1


Dear Mr. Rehor:

Enclosed are three (3) original copies of Change Order #1 dated February 5, 1999
for credits due to Focal Communications Corporation because of the deletion of
one (1) UPS and related equipment markup, one (1) fire-rated glass window and
related installation, and DCDDC's overhead & profit. Total value of the 
aforementioned is: $(126,722.30) CREDIT.

These original documents have been singed and dated by the Architect and Data
Center Design and Development Corporation. If acceptable, please execute and
date all three copies, and return the fully executed Architect and Contractor
copies to my attention for distribution.

Please contact me immediately if you have any questions. Thank you for your
assistance.

Sincerely,
Data Center Design and Development Corporation

/s/ Louis D. Blatnick

Louis D. Blatnick, CSI
Manager, Project and Business Administration



LDB/ldb

C:  Administrative
     Jeff Wells, VP - Focal

98-149 TRANS-CHRIS REHOR-REV'D CO#1
<PAGE>
 
                           STANDARD FORM OF AGREEMENT
                       BETWEEN OWNER AND DESIGNER-BUILDER
                      FOR DESIGN AND CONSTRUCTION SERVICES

THIS AGREEMENT made as of 12 January 1999 by and between Focal Communications
(thereinafter called "Owner"), and Data Center Design and Development
Corporation, 630 North Court, Palatine, Illinois 60067 (hereinafter called
"Designer-Builder").

WITNESSETH:

That in consideration of the mutual covenants and agreements herein contained,
the parties hereto do hereby covenant and agree as follows:

ARTICLE 1:  DEFINITIONS

1.1      OWNER: "Owner" shall mean the above-named organization designated as
         "Owner", its successors and assigns.

1.2      OWNER'S REPRESENTATIVE: "Owner's Representative" shall mean such person
         designated by the Owner on written notice to Designer-Builder, to act
         as its representative.

1.3      DESIGNER-BUILDER: "Designer-Builder" shall mean the above-named
         organization designated as "Designer-Builder", its successors and
         assigns.

1.4      DESIGNER-BUILDER'S REPRESENTATIVE: "Designer-Builder's Representative"
         shall mean or such other person as may be designated by
         Designer-Builder on written notice to Owner to act as its
         representative.

1.5      WORK: "Work" shall cover all labor, material and services necessary to
         produce the design services and the construction required by the
         contract documents.

1.6      DATE OF SUBSTANTIAL COMPLETION: "Date of "Substantial Completion" of
         the work or a designated portion thereof is the date when: (i) the
         construction is sufficiently completed in accordance with the contract
         documents so that Owner may occupy the work or designated portion
         thereof for the use for which it is intended, or (ii) Owner obtains a
         temporary certificate or a permanent certificate of occupancy for the
         work, and Designer-Builder has obtained a waiver of all liens,
         mechanics and otherwise and has presented same to Owner.

1.7      CONSTRUCTION CHANGE DIRECTIVE: A "Construction Change Directive" is a
         written order to the Designer- Builder, prepared by the
         Designer-Builder signed by the Owner's Representative, issued after the
         execution of this contract, authorizing the Designer-Builder to make
         additions, deletions or modifications in the work to be performed by
         Designer-Builder.

1.8      CHANGE ORDER: A "Change Order" is a written order to the
         Designer-Builder, prepared by Designer-Builder, signed by the Owner's
         Representative, countersigned by Designer-Builder's Representative,
         issued after the execution of the contract, authorizing an adjustment
         (if any) in the contract sum to be paid to Designer-Builder or an
         adjustment in the time for the completion of the work.

ARTICLE 2:  CONTRACT DOCUMENTS

2.1      The contract documents shall consist of the following:

2.1.1    This contract, together with Appendices attached hereto, and any
         amendments or modifications thereto, including any signed work order or
         signed exchange order.

2.1.2    All other documents described in Appendices attached hereto and made
         part hereof.
<PAGE>
 
2.2      In the event of conflict between this contract and the provisions of
         the other contract documents, this contract shall control.  In the
         event of any conflict between the provisions of the contract documents,
         other than this contract, the latest approved document shall control.

2.3      All contract documents hereinafter submitted by Designer-Builder for
         the approval of Owner in connection with the performance of the work
         shall be deemed approved unless Owner makes written objection thereto
         within five (5) working days after receipt thereof.

2.4      The drawings, specifications and other documents furnished by
         Designer-Builder are instruments of service and are the property of
         Designer-Builder whether or not the work for which they are made is
         commenced. Drawing, specifications and other documents furnished by
         Designer-Builder shall not be used by Owner or other prospects, for
         completion of the work by others, except by written agreement relating
         to use, liability and compensation.

2.5      Submission or distribution of documents to meet official regulatory
         requirements or for other purposes in connection with the work is not
         to be construed as publication in derogation of the Designer-Builder's
         or its architect's common law copyrights or other reserved rights.
         Owner shall own neither the documents nor the copyrights.

ARTICLE 3:  SCOPE OF THE WORK

3.1      The Designer-Builder shall provide all necessary design services and
         furnish all necessary supplies, materials and equipment and all
         necessary labor and service required for the design and construction of
         the project to completion to Owner's satisfaction in accordance with
         the contract documents.

3.2      The design services to be provided by Designer-Builder under this
         contract shall cover all of the following to the extent required for
         the performance of the work; as defined in the "Concept Design
         Program".

3.2.1    Preparation of working drawings and specifications setting forth in
         detail the requirements for the execution and completion of the work.

3.2.2    Review of all shop drawings, samples and other submissions for
         conformance with the design concept of the work and for compliance with
         the drawings and specifications.

3.3      The Designer-Builder shall not be liable to Owner or otherwise
         responsible for damage or injury to the work or for additional costs or
         delays in the performance of the work arising out of or caused by
         concealed, latent or unforeseen surface or sub-surface conditions:  (i)
         differing from those indicated in the contract documents, or (ii) not
         specifically disclosed to Designer-Builder in writing prior to the
         commencement of the work.

3.4      The Designer-Builder shall not be liable to Owner or otherwise
         responsible for damage or injury to the work or for additional costs or
         delays in the performance of the work arising out of or caused by
         reason of the Designer-Builder relying upon or acting upon incorrect or
         inaccurate written information furnished the Designer-Builder by Owner
         or its agents and representatives.

3.5      See Appendix G

ARTICLE 4:  COMMENCEMENT AND COMPLETION

4.1      The work to be performed under this contract shall be commenced
         following the execution of this contract and shall be substantially
         completed at a date to be jointly agreed upon. Those areas as
         designated in the Milestone Schedule and Appendix F, Significant
         Milestones shall be completed on those dates, and in the manner
         described.

ARTICLE 5:  CONTRACT SUM

5.1      The owner shall pay the Designer-Builder for the performance of the
         work, subject to additions and deductions by change order as provided
         in Article 12, in current funds, the contract sum of Two Million Seven
         Hundred Sixty One Thousand Three Hundred Thirty Eight and No/100
         Dollars ($2,761,338.00).  The said contract sum shall cover the entire
         charge for design and construction services as set forth in the
         Appendices.  The said
<PAGE>
 
         contract sum is a GMP (Guaranteed Maximum Price) with the
         Designer-Builders Fees Fixed and a Cost of Savings Split of 50-50
         between the Owner and the Designer-Builders on the balance of the
         contract.

5.2      The contract sum is comprised of the following:

         A.  Phase 1 Construction
         B.  Critical Support Equipment

5.3      All additions and/or deductions not covered by change orders will be
         paid from the cost of savings on a line item basis.

5.4      Per agreement the Owner shall pay the Designer-Builder an amount equal
         to Twenty percent (20%) ($552,268.00) of the contract sum upon
         execution of the contract.

ARTICLE 6:  PROGRESS PAYMENTS

6.1      On or before the 15th day of the first full month following the
         commencement of the work and continuing thereafter on or before the
         15th day of each succeeding month, Designer-Builder shall submit to
         Owner an invoice for payment and made a part hereof, covering all work
         performed during the preceding month, including all work performed
         pursuant to change orders.

6.2      The invoice for payment shall show:

6.2.1    Charge for all design and construction work performed during the
         preceding month, including the value of construction material and
         equipment, the payment of which Designer-Builder is liable for, whether
         or not such construction material is delivered to the project site.

6.3      Within twenty (20) days after submission of the invoice for payment,
         Owner shall make a progress payment to Designer-Builder in an amount
         equal to the aggregate sum of the following:

6.3.1    100% of the aggregate amount of the charge for design, construction and
         equipment as shown on the invoice for payment.

6.3.2    Interest at the rate of one and one-half (1-1/2%) percent per month
         shall be paid on the unpaid balance of any invoice for payment, not
         paid by the Owner within twenty (20) days from the date of receipt
         thereof. Such interest shall be in addition to the contract sum to be
         paid Designer-Builder pursuant to this agreement.

6.4      Each invoice for payment shall contain Designer-Builder's
         certifications that the work described in the invoice has been
         completed in accordance with the contract documents, that all due items
         are paid for, for which previous certificates were issued and payments
         received, and that the amount of the payment shown on the invoice now
         due Designer-Builder.

ARTICLE 7:  PAYMENT TO DESIGNER-BUILDER FOLLOWING SUBSTANTIAL COMPLETION

7.1      When the Designer-Builder determines that the work or a designated
         portion thereof is substantially complete, the Designer-Builder shall
         prepare and submit to Owner a certificate of substantial completion,
         which shall fix the date of substantial completion.  The
         Designer-Builder shall attach to the certificate a list of items to be
         completed or corrected by Designer-Builder before final payment.  The
         Owner shall have seven (7) days after receipt of the certificate of
         substantial completion to make written objection thereto.  Any
         objection on the part of the Owner shall be in such specific detail as
         to adequately apprise Designer-Builder of the basis for the objection.
         In the event Owner does not make objection within the prescribed time,
         the work shall be deemed to be substantially complete as of the date
         specified in the certificate of substantial completion.

ARTICLE 8:  FINAL PAYMENT
<PAGE>
 
8.1      The Owner shall make final payment within thirty (30) days after
         completion of the work, provided the contract be then fully performed.

8.2      The making of final payment shall constitute a waiver of all claims by
         Owner except those arising from: (1) unsettled claims; (2) faulty or
         defective work appearing after substantial completion; and (3) failure
         of the work to comply with requirements of the contract documents.

8.3      Acceptance of final payment shall constitute a waiver of all claims by
         Designer-Builder except those previously made in writing and identified
         by Designer-Builder as unsettled at the time of final payment.

ARTICLE 9:  DESIGNER-BUILDER'S GUARANTY

9.1      Designer-Builder, subject to the provisions of 9.2 hereof, guarantees
         that the work shall be performed in a skillful and workmanlike manner,
         free from defects in material and workmanship and in conformance with
         the contract documents.  This guaranty is expressly in lieu of all
         other guaranties and warranties, express or implied, including any
         warranties of merchantability and fitness.  Designer-Builder's
         liability for its guaranty hereunder shall be limited to remedying, any
         defect in material and workmanship approximately resulting from the
         failure of Designer-Builder to perform the work in a skillful and
         workmanlike manner, provided however: (i) that such defect shall
         manifest itself on or before the expiration of the first year next
         following the date of substantial completion, and (ii) that Owner shall
         give notice in writing to Designer-Builder of such defect on or before
         the expiration of the first year next following the date of substantial
         completion.  In lieu of remedying such defect in the work,
         Designer-Builder, with the concurrence of Owner, may pay Owner the cost
         of repair thereof.

9.2      Designer-Builder does not guarantee or warrant, either expressly or
         impliedly, the materials in or workmanship of supplies, materials,
         equipment or machinery manufactured by third parties and furnished and
         installed by Designer-Builder in the performance of the work, but
         Designer-Builder shall endeavor to obtain from all vendors and
         suppliers and assign to Owner the customary warranties and guaranties
         of such vendors and suppliers with respect thereto and Designer-Builder
         shall, at the sole cost and expense of Owner, render reasonable
         assistance to Owner when requested in order to enable Owner to enforce
         such warranties and guaranties by the third party manufacturers or
         suppliers.

ARTICLE 10:  RESPONSIBILITIES OF OWNER

The Owner, at its sole cost and expense, shall:

10.1     Cooperate with the Designer-Builder in all respects.

10.2     Designate in writing a person to act as Owner's Representative with
         respect to the work to be performed under this agreement and such
         person shall have complete authority to transmit instructions to the
         Designer-Builder through Designer-Builder's Representative, receive
         information and interpret and define Owner's policies and decisions
         with respect to all aspects of the work covered by this agreement.

10.3     Provide full information with respect to the scope of the work to be
         performed by Designer-Builder.

10.4     Give prompt written notice to Designer-Builder whenever the Owner
         observes or otherwise becomes aware of any defect in the work covered
         by this agreement.

10.5     Guarantee access to and make all provisions for the Designer-Builder to
         enter upon public and private lands as required for the
         Designer-Builder to perform its work under this agreement.

10.6     Make available a convenient and adequate source of water and
         electricity and furnish telephone service and all other utilities
         required for the performance of the work.

10.7     Furnish all property and topographic surveys describing the physical
         characteristics and legal limits of the job site and necessary utility
         locations at the job site required for the performance of the work.

10.8     Furnish necessary documentation engineering structural suitability of
         existing floor slabs required for the performance of the work.

10.9     Provide at the job site for the benefit and use of Designer-Builder in
         the performance of the work, sufficient
<PAGE>
 
         office space, parking space and storage space for materials and
         construction equipment.

10.10    Make application to all municipal jurisdictional agencies and obtain
         all governmental approvals, permits, license and variances required for
         the performance of the work.

10.11    Not cause any other construction to be undertaken in the immediate area
         of the job site where the work is to be performed by Designer-Builder
         under this agreement without the consent of Designer-Builder.

10.12    Owner to provide tax exempt status certification to Designer-Builder.

ARTICLE 11:  RESPONSIBILITIES OF DESIGNER-BUILDER

The Designer-Builder shall:

11.1     Cause the work to be performed in accordance with the contract
         documents, using its best skill and attention.

11.2     Provide and pay for all labor, materials, equipment, tools,
         construction equipment and machinery, transportation and other
         facilities and services necessary for the execution and completion of
         the work.

11.3     At all times during the performance of the work, keep the premises free
         from accumulation of waste materials or rubbish caused by its
         operations.  At the completion of the work, Designer-Builder shall
         remove all its waste materials and rubbish from and about the job site
         as well as its tools, construction equipment, machinery and surplus
         materials and shall leave the job site "room clean" or its equivalent.

11.4     Assist Owner in making application to all municipal jurisdictional
         agencies for the purpose of obtaining all governmental approvals,
         permits, licenses and variances required for the performance of the
         work, provided nevertheless, that Owner shall be responsible for the
         actual obtainment of all such approvals, permits, licenses and
         variances.

11.5     Obtain and furnish to the Owner all waivers of lien, including but not
         limited to, waiver of mechanics liens.

ARTICLE 12:  CHANGES IN THE WORK

12.1     The Owner, without invalidating the contract, may order changes in the
         work consisting of additions, deletions or modifications and the
         contract sum and the time for the completion of the work shall be
         adjusted accordingly. All such changes in the work shall be authorized
         by a construction change directive and thereafter confirmed by a change
         order in the manner hereinafter provided for.

12.2     In the event that any building landlord of the owner or any
         governmental official orders changes in the scope of work, the contract
         sum and the time for the completion of the work shall be adjusted
         accordingly. All such changes in the work shall be authorized by a
         construction change directive and thereafter conformed by a change
         order in the manner hereinafter provided for.

12.3     The Designer-Builder shall not be required to proceed with any
         additions, deletions or modifications to the work unless pursuant to a
         construction change directive. Should the work to be performed by
         Designer-Builder under any construction change directive result in an
         increase in the cost of the work, the Owner shall be obligated to pay
         Designer-Builder the total cost of the additional work.

12.4     The cost or net credit to the Owner resulting from a change in the work
         as above provided shall be confirmed by a change order.  Thereupon, the
         time for the completion of the work shall be adjusted accordingly. The
         contract sum shall be adjusted upwards or downwards by the amount of
         any additional cost to Owner or credited to Owner resulting from any
         change in the work caused by change order. The refusal by Owner to
         execute a change order shall not relieve Owner of its obligation to pay
         Designer-Builder the full cost of any additional work performed by
         Designer-Builder pursuant to a construction change directive.
<PAGE>
 
ARTICLE 13:  INDEMNIFICATION BY DESIGNER-BUILDER

13.1     Designer-Builder, within the limited of its insurance coverage as
         provided in Article 16 hereof, shall indemnify and save Owner harmless
         from and against any loss, liability or damages (including reasonable
         attorney fees, court and/or arbitration expenses) which may be
         sustained by Owner by reason of sickness, disease, bodily injury or
         death to persons or damages to tangible property occurring during the
         performance of the work and resulting from the negligence of
         Designer-Builder, its subcontractors, agents or employees, including,
         but not limited to, damages caused directly or indirectly by any
         substances, conditions, elements, material or any combination of the
         foregoing either intentionally or unintentionally emitted or released
         or caused to be emitted or released by Designer-Builder, its
         subcontractors, agents or employees from or at work site.

13.2     The Designer-Builder shall indemnify and save harmless Owner from all
         costs, fees, damages and expenses arising out of or resulting from any
         charge or encumbrance in the nature of a laborer's, mechanic's or
         material man's lien asserted by a party or parties other than Designer-
         Builder in connection with the performance of the work.

ARTICLE 14:  INDEMNIFICATION BY OWNER

14.1     Owner shall indemnify and save Designer-Builder harmless from and
         against any loss, liability or damages (including reasonable attorney
         fees, court and/or arbitration expenses) which may be sustained
         by Designer- Builder by reason of sickness, disease, bodily injury or
         death to persons or damages to tangible property occurring during the
         performance of the work and resulting from the negligence of Owner, its
         subcontractors, agents or employees.

ARTICLE 15:  PATENT INDEMNIFICATION

15.1     The Owner shall not cause Designer-Builder to use any design, process
         or equipment which would infringe upon any patent right held by any
         third party.

15.2     Owner shall indemnify and save Designer-Builder harmless from and
         against all liability, damages and expenses arising out of any suit or
         action brought against Designer-Builder for use of any design, process
         or equipment required by the Owner, based upon a claim that any design,
         process or equipment incorporated in the work infringes upon any
         invention, design, process or device which is the subject of a patent.

ARTICLE 16:  DESIGNER-BUILDER INSURANCE

16.1     The Designer-Builder shall purchase and maintain all of the insurance
         described in 16.1.1 through 16.1.3 hereof, for not less than the limits
         of liability therein specified so as to protect Designer-Builder from
         claims under workmen's compensation acts and other employee benefits
         acts, from claims for damage because of bodily injury, including death,
         and from claims for damage to property which may arise out of or result
         from the Designer-Builder's operations under this contract, whether
         such operations be by Designer-Builder or by any subcontractor or
         anyone directly or indirectly employed by any of them.

16.1.1   Workmen's compensation, including occupational disease in accordance
         with the statutory requirements set forth by the state which the work
         is to be performed and employer's liability insurance covering all
         Designer-Builder's employees engaged in the performance of this
         contract in the minimum sum of $500,000.00.

16.1.2   Comprehensive general liability insurance, including Designer-Builder's
         protective liability contractual liability insurance covering death or
         bodily injury with minimum limits of $250,000.00 per person and
         $1,000,000.00 for any one accident and property damage coverage with
         minimum limits of $1,000,000.00 in the aggregate. The contractual
         liability insurance shall insure the hold harmless and indemnification
         agreement contained in 13.1 of this contract.

16.1.3   Comprehensive automobile liability insurance covering Designer-Builder
         for claims arising from owned, hired and non-owned vehicles covering
         death or bodily injury with minimum limits of $1,000,000.00 for any one
         accident and property damage coverage with minimum limits of
         $1,000,000.00 per accident.

16.2     Certificates of insurance covering all of the insurance required to be
         maintained by Designer-Builder shall be filed with the Owner.
<PAGE>
 
ARTICLE 17:  OWNER'S INSURANCE

17.1     Owner, at its own expense, shall obtain and maintain until completion
         of the work, comprehensive general liability insurance and property
         insurance which shall insure against the perils of fire, flooding,
         extended coverage, theft, vandalism and malicious mischief, containing
         a limit of not less than the full amount of the contract sum.  This
         insurance shall include the interest of the Owner, Designer-Builder and
         subcontractors. The loss, if any, under such insurance is to be made
         adjustable with and payable to Owner, Designer-Builder and
         subcontractors, as their interest may appear.  The policy for such
         insurance shall be issued by an insurance carrier authorized to do
         business in the state in which the work is to be performed and shall
         not be canceled or substantially modified without at least thirty (30)
         days advance written notice to the Designer-Builder.

17.2     Certificates of insurance, naming Designer-Builder as an additional
         insured under the above described policies, shall be filed with
         Designer-Builder.

ARTICLE 18:  TERMINATION OF THE AGREEMENT BY OWNER

18.1     The Owner, may, upon ten (10) days written notice and upon the
         occurrence of any event of default by Designer-Builder as provided in
         18.2, terminate this agreement. Upon receipt of such notice of
         termination, Designer-Builder shall also take steps necessary for its
         orderly and safe disengagement from and protection of the work.

18.2     The Designer-Builder shall be in default under this agreement upon the
         occurrence of any of the following mentioned events:

18.2.1   Insolvency or bankruptcy of Designer-Builder or the making of any
         assignment for the benefit of creditors, or the filing of any petition
         for bankruptcy or reorganization by Designer-Builder.

18.2.2   The willful breach of any material provisions of this agreement by
         Designer-Builder and the failure by Designer-Builder to take prompt
         action to remedy same with ten (10) days of the receipt of written
         notice from Owner.

18.3     In the event of Designer-Builder's default, Owner shall have the right
         to terminate the employment of Designer-Builder to take possession of
         the work and of all materials, tools and appliances thereon with the
         right to their continued use, without obligation, until such time as
         the work is completed, and to finish the work by any reasonable method
         Owner may deem expedient.

18.4     Should Owner terminate the contract, Owner shall pay Designer-Builder a
         sum equal to the value of the total work completed to date of
         termination, including:  (i) all design services, and (ii) all
         construction work.

ARTICLE 19:  TERMINATION OF THE AGREEMENT BY DESIGNER-BUILDER

19.1     The Designer-Builder may, upon the occurrence of any event of default
         by Owner as provided in 19.3 and upon ten (10) days written notice to
         Owner, terminate this agreement without prejudice to any legal remedies
         or rights it may possess, and recover from Owner payment for the entire
         unpaid balance of the aggregate sum due Designer-Builder for the
         performance of the work completed as of the date of termination,
         including the cost of all supplies, materials, equipment and design
         services, the cost of which Designer-Builder shall be legally obligated
         to satisfy.

19.2     In addition to such sums as due Designer-Builder under the provisions
         of Article 5, Owner shall also pay Designer-Builder:

19.2.1   All charges for actual costs incurred by Designer-Builder to date of
         termination, including all costs and expenses incurred for the orderly
         and safe disengagement from and protection of the work.

19.2.2   All cancellation charges which Designer-Builder shall become obligated
         to pay any subcontractor or material man of Designer-Builder.

19.2.3   The amount of such damages as Designer-Builder shall become obligated
         to pay any subcontractor or material man of Designer-Builder.
<PAGE>
 
19.3     The Owner shall be in default of this agreement upon the occurrence of
         any of the following mentioned events:

19.3.1   Insolvency or bankruptcy of Owner or the making of any assignment for
         the benefit of creditors or the filing of any petition for bankruptcy
         or reorganization by Owner.

19.3.2   The willful breach of any provision of this agreement by Owner and the
         failure by Owner to remedy same within ten (10) days of the receipt of
         written notice thereof from Designer-Builder.

19.3.3   The willful failure without cause to make payment on any application
         for payment within thirty (30) days from submission thereof.  Should
         Owner in good faith dispute a portion of any application for payment,
         it shall still be required to make timely payment of the undisputed
         portion of such application for payment and its failure to do so shall
         constitute an act of default.

19.4     In lieu of terminating the contract as above provided, upon the
         happening of any of the events above mentioned, Designer-Builder may
         stop the performance of the work or the progress of the work until such
         time as the default is remedied without prejudice to any other legal
         remedy or right it may possess.  The election by Designer-Builder to
         stop the work as herein provided shall not be deemed a waiver of
         Designer-Builder's right to terminate the agreement.

19.5     All monies due Designer-Builder upon termination of this agreement
         shall bear interest at the rate of 1-1/2 percent per month, calculated
         from the date of termination and all such interest shall belong
         exclusively to Designer-Builder.

ARTICLE 20:  DISPUTES

20.1     All claims, disputes or other matters in controversy arising out of or
         relating to this agreement shall be determined in the manner provided
         for in Appendix E attached hereto and made a part hereof.

ARTICLE 21:  DELAYS

21.1     Designer-Builder shall not be responsible for any delay in the
         performance or progress of the work or liable for any costs or damages
         sustained by Owner resulting from such delay caused by any act or
         neglect of the Owner or Owner's Representative, or by any employee of
         Owner, or by any third party acting as the agent, servant or employee
         of the Owner, or by changes ordered in the work, or by labor disputes,
         including but not limited to strikes, slowdowns, job actions, picketing
         and secondary boycotts, or by fire or other casualty loss, or by
         unusual delay in transportation, or by acts of God, or as a result of
         compliance with any order or request of any federal, state or municipal
         governmental authority or any person purporting to act therefore, or by
         acts of declared or undeclared war, or by public disorder, riot or
         civil commotion, or by any other cause beyond the control and without
         the fault or negligence of Designer-Builder. In the event of any such
         delay, Designer-Builder shall proceed with due diligence to alleviate
         such delay and continue the performance of all obligations hereunder.
         The time during which Designer-Builder is delayed in the performance of
         the work, as herein provided, shall be added to the time for completion
         of the work as set forth in Article 4. All additional costs or damages
         resulting from any delay in the performance or progress of the work
         caused by any act or neglect of the Owner, its agents or
         representatives, shall be borne entirely by the Owner.

ARTICLE 22:  MISCELLANEOUS PROVISIONS

22.1     Any notices required or permitted under this contract shall be deemed
         to be fully given if sent by telecopy, messenger service or certified
         mail.

         If to the Owner, addressed:    If to the Designer-Builder addressed:

         Brain Addy                     Raymond Noesges
         Focal Communications           Data Center Design and Development
         200 North LaSalle Street       630 North Court
         Chicago, Illinois 60601        Palatine, Illinois 60067
<PAGE>
 
22.2     It is agreed between the parties hereto that the interpretation and
         enforcement of this agreement shall be in conformity with the laws of
         the State of Illinois.

22.3     In the event one or more, but not all of the provisions of this
         agreement are declared unlawful and/or unenforceable by a court of
         competent jurisdiction, such determination shall not affect the
         legality or enforce ability of the remainder of the terms hereof.

22.4     This agreement may be executed in several counterparts, each of which
         so executed shall be deemed to be an original, and such counterparts
         shall, together, constitute and be one and the same agreement.

22.5     This agreement shall inure to the benefit of and be binding upon the
         parties hereto and their successors and assigns.

22.6     The Owner and Designer-Builder, by and through their respective
         signatories to this agreement, each represent to the other that they
         are authorized to enter into this agreement.

ARTICLE 23:  OFFER OF EMPLOYMENT

23.1     Owner and Designer-Builder mutually agree that each will not offer
         employment to, nor accept for employment, the employees of the other
         party for a period of six (6) months after final payment and after the
         completion of such other assignments as may follow arising out of this
         contract.

IN WITNESS WHEREOF, the parties hereto have caused these present to be signed by
their proper corporate officers and have affixed their corporate seals hereto
the day and year first above written.

ATTEST:

Focal Communications
200 North LaSalle Street
Chicago, Illinois 60601


                            By Brian F. Addy             1/29/99
                              -----------------------  -----------
                              Duly Authorized Officer      Date

Data Center Design and
Development Corporation
630 North Court
Palatine, Illinois 60067


                            By Raymond Noesges           1/29/99
                              -----------------------  -----------
                              Duly Authorized Officer      Date
<PAGE>
 
                                   Appendix A

                              FOCAL COMMUNICATIONS
                                    Info Mart
                                   Dallas, Tx.
                                Project No:98-149

General Conditions                                                     131,886
         Includes; field supervision, project management, accounting
         & clerical support, project clean up, dumpsters, insurance,
         permit allowance, site materials and services.

Demolition                                                              14,240
         Includes; selective demolition of walls, doors, hardware,
         acoustical ceiling, glazing, mechanical and electrical.

Premium Time (Allowance)                                                40,000

Cutting and Patching (Allowance)                                         5,000

Site Work                                                                7,708
         Includes; generator pad, excavation, fill and landscape
         restoration.

Structural Steel                                                        50,200
         Includes; installation structural steel condenser rack on
         roof, structural steel battery support in power room,
         rigging and crane time.

Millwork                                                                14,300
         Includes; break room cabinets and counters, copy room
         cabinets and counters.

Door, Frames and Hardware                                               33,880
         Includes all hollow metal and wood doors, frames, hardware,
         Herculite double entry door and rated conference room
         window 4'0"X 6'0".

Access Floor                                                           274,637
         Includes; 2-tiered access floor in Co-Lo area, single
         tiered access floor in switch/transport area.  Both areas
         complete with ramps, stairs, railings, plenum barriers,
         perforated air flow panels and accessories.

GWB                                                                     51,644
         Includes; all new walls, all in fill of existing openings
         as required, extension of rated wall assemblies in various
         areas, all office and demising partitions.

Acoustical Ceilings                                                     47,471
         Includes; clean room acoustical grid ceiling system in
         Co-Lo And Switch/Transport, and 2X2 lay in acoustical
         ceiling system in all finished support area including break
         room and corridor.
<PAGE>
 
Floor Finishes                                                          26,064
         Includes; 18"X18" carpet tile in general office, private
         office, conference and copy rooms.  VCT at landings,
         corridors and break room.  Vinyl wall base as indicated in
         all finished areas.  Painting of exposed concrete floor
         slabs.

Wall Finishes                                                           14,756
         Includes; interior painting of walls, doors and frames.
         Wall coverings as indicated.

Appliances                                                                 800
         Includes; microwave and under counter refrigerator.

Window Treatment (allowance)                                             8,430
         Includes; mini-blinds or coverings as indicated on room
         finish schedule.

Corner Guards                                                            2,400
         Includes; installation of protective guards on all exposed
         corners in the finished areas.

Signage (Allowance)                                                      3,000
         Includes; door, electrical and mechanical equipment signage
         within the finished areas.  Interior/Exterior LOGO or
         directional signage is not included.

HVAC-Mechanical                                                        206,850
         Includes; installation of five (5) 20 ton process air
         conditioners with roof top condensing units, one (1) 15
         ton process air conditioner with roof top condensing unit
         sheet metal duct work for process air conditioners. Roof
         portals for refrigerant lines and future refrigerant lines
         to finished equipment rooms. There are no provisions for
         additional refrigerant lines to unfinished areas.

Fire     Protection-Detection                                          257,230
         Includes; new water line for pre-action and modifications
         to existing wet pipe sprinkler systems. Values and
         manifolds for pre-action systems. Pre-action piping systems
         and zones as indicated. Wet pipe system in common area and
         office areas. Smoke/heat detection with addressable panel
         with tie in to building fire alarm panel.

Plumbing                                                                13,600
         Includes; New/reworked waste and water lines. Condensate
         and humidification lines for processed air conditioners.
         Break room waste and water, fixtures and lines.

Security-Card Access-Monitoring (allowance)                             18,143
         Includes; furnish and install back boxes and conduit;
         installation only of cable, devices and systems furnished
         by other. All terminations are by others.

Voice and Data Cabling (allowance)                                      14,200
         Includes; installation of cabling furnished and terminated
         by others.

<PAGE>
 
Electrical                                                             507,657
         Includes; installation of all electrical equipment,
         grounding, TVSS, service feeder from building 4000A
         vertical bus duct, feeder from new generator system, under
         floor spot leak detection, EPO system, step down
         transformers, all lighting and receptacles, panel boards for
         distribution, all AC power distribution, feeders,
         breakers. Installation of UPS and generator systems. Four
         (4) 4" and Two (2) 2" conduits for fiber and GPS. There is
         no DC conduit, cabling, racking or installation included.
<PAGE>
 
                                   Appendix B



                                 RECAP PROPOSAL
                              FOCAL COMMUNICATIONS
                                   Dallas, Tx
                                January 11, 1999

<TABLE>
<CAPTION>

DESCRIPTION
<S>                                                                                 <C>
GENERAL CONDITIONS                                                                  $131,886.00
DEMOLITION                                                                            14,240.00
PREMIUM TIME (ALLOWANCE)                                                              40,000.00
CUTTING AND PATCHING (ALLOWANCE)                                                       5,000.00
SITE WORK                                                                              7,708.00
STRUCTURAL STEEL                                                                      50,200.00
MILLWORK                                                                              14,300.00
DOORS, FRAMES AND HARDWARE                                                            33,880.00
ACCESS FLOOR                                                                         274,637.00
GWB                                                                                   51,644.00
ACOUSTICAL CEILINGS                                                                   47,471.00
FLOOR FINISHES                                                                        26,064.00
WALL FINISHES                                                                         14,756.00
APPLIANCES                                                                               800.00
WINDOW TREATMENT (ALLOWANCE)                                                           8,430.00
CORNER GUARDS                                                                          2,400.00
SIGNAGE (ALLOWANCE)                                                                    3,000.00
HVAC/MECHANICAL                                                                      206,850.00
FIRE PROTECTION-DETECTION                                                            257,230.00
PLUMBING                                                                              13,600.00
SECURITY-CARD ACCESS-MONITORING (ALLOWANCE)                                           18,143.00
VOICE AND DATA CABLING (ALLOWANCE)                                                    14,200.00
ELECTRICAL                                                                           507,657.00
CRITICAL SUPPORT EQUIPMENT                                                           474,202.00
                                                                                  -------------
                                    SUB-TOTAL                                     $2,218,298.00
                                    CONTINGENCY                                      221,830.00
                                    FEE (OVERHEAD AND PROFIT) 6%                     133,098.00
                                    DESIGN FEE 8%                                    188,112.00
                                                                                ---------------
                                    TOTAL                                         $2,761,338.00
</TABLE>
<PAGE>
 
                                   Appendix C

                         GENERAL CONDITIONS COST SUMMARY
                              FOCAL COMMUNICATIONS
                                   Dallas, Tx.
                                January 11, 1999
<TABLE>
<CAPTION>
DESCRIPTION                              QUANTITY   UNIT      UNIT PRICE        COST
----------------------------------------------------------------------------------------
<S>                                         <C>      <C>      <C>             <C>
Superintendent                              60       MD          515.00       30,900.00

Second Shift Supervisor                     12       MD          592.00        7,104.00

Project Coordinator                         45       MH           58.00        2,610.00

Project Manager                             23       MD          634.00       14,582.00

Clean up                                    42       MD          290.00       12,180.00

Dumpsters                                    5       EA.         450.00        2,250.00

Vendor Dumpsters                             3       EA.         450.00        1,350.00

Second Shift Clean up                       10       MD          290.00        2,900.00

Vendor Clean up                             10       MD          335.00        3,350.00

Lay out                                      8       MD          340.00        2,720.00

Misc. materials & supplies                   1       LS        2,500.00        2,500.00

Temporary Protection                         1       LS        1,200.00        1,200.00

Job Site Telephone                           1       LS        1,500.00        1,500.00

Trucking & Deliveries                        1       LS        1,140.00        1,140.00

Final Clean up (Above/below floor)                   LS        2,500.00        2,500.00

Permit (Allowance)                                   LS       20,000.00       20,000.00

Travel                                      15       ea          500.00        7,500.00

Auto                                        12       ea          300.00        3,600.00

Per Diem                                    60       ea          200.00       12,000.00
----------------------------------------------------------------------------------------

                                                        TOTAL               $131,886.00
</TABLE>
LEGEND
MD = Man Days
MH = Man Hours
LS = Lump Sum
<PAGE>
 
                                   Appendix D

                              FOCAL COMMUNICATIONS
                                   DALLAS, TX.
                           CRITICAL SUPPORT EQUIPMENT
                               PROJECT NO: 98-149



PROCESS ENVIRONMENTAL CONTROL SYSTEMS                  $ 143,844.00

         - One (1) UH199AUAAM
         - Two (2) UH245AUAAM
         - Three(3) FH245AUAAM
         - Six (6) condensing units
         - Level Three processors
         - Floor stands
         - Spot leak detection
         - Freight, Tax and Start-up

UNINTERRUPTABLE POWER SUPPLY (ups)                       105,941.00

         - One (1) 225k VA (180kW) 600T UPS Module
         - Two (2) battery cabinets with C&D sealed
           batteries rated for 15 minutes at 180kW load
         - One Maintenance Bypass (4 breaker) in
           matching cabinet
         - Freight, Tax and Start-up

EMERGENCY POWER SYSTEM                                   196,719.00

         - One (1) 750 kW Model 3412 diesel engine
           driven package generator
         - One 1200A Automatic Transfer Switch with by pass isolation
           2700 gallon (under belly) fuel tank
         - Sound attenuating housing
         - Freight, Tax and Start-up

                                       Sub-total      $  446,504.00
                                       DCDDC mark-up      27,698.00
                                                      -------------
                                       Total          $  474,202.00
<PAGE>
 
                                   APPENDIX E


RESOLUTION OF DISPUTES BY ARBITRATION

1.       All claims, disputes or other matters in controversy arising out of or
         relating to this agreement shall be determined by three (3)
         arbitrators, one appointed by the Owner, one appointed by the
         Designer-Builder and the third by the arbitrators so appointed.

2.       Notice of demand for arbitration by either party to this agreement
         shall be served in writing upon the other party to this agreement and
         shall be made within sixty (60) days after the dispute has arisen.

3.       Within thirty (30) days after either party shall have given notice of
         demand for arbitration, Owner and Designer-Builder shall each appoint
         his respective arbitrator and give actual notice thereof to the other
         party. If either party shall fail to appoint an arbitrator within the
         aforesaid initial thirty (30)-day period and give actual notice thereof
         to the other party, then said defaulting party shall be deemed to have
         irrevocably waived its right to appoint an arbitrator and the
         arbitrator appointed by the party not in default shall, within thirty
         (30) days next following the expiration of the initial thirty (30) day
         period, appoint the second arbitrator and such appointment shall be
         binding upon the party in default. Within twenty (20) days after the
         appointment if the two (2) arbitrators cannot select a third arbitrator
         within the prescribed twenty (20)-day period, then either party may
         petition the American Arbitration Association to appoint the third
         arbitrator, subject to his acceptance of the procedure for arbitration
         as herein set forth. Neither party shall have the right to appoint an
         arbitrator unless the person to be appointed shall agree to serve as an
         arbitrator subject to the terms and conditions of this article.

4.       The arbitrators, upon their appointment, and after having been duly
         sworn to do their duties with impartiality and fidelity shall proceed
         with all reasonable dispatch to hear and determine the matter in
         dispute and they shall conduct all arbitration proceedings in the state
         in which the work is to be performed. In the event the taking of
         testimony in the arbitration hearing shall not be concluded within
         sixty (60) days after the third arbitrator shall have been selected,
         the arbitrators shall then expedite the hearing of the matter in
         dispute and take testimony on a continuous weekly basis for at least
         thirty-two (32) hours a week until such time as the taking of all
         testimony is completed and both sides have rested.

5.       The owner and Designer-Builder shall each pay the cost and fees of the
         arbitrator they select or who is selected on their behalf and one-half
         of the costs and fees of the third arbitrator.

6.       Whenever possible and to the extent not inconsistent with the
         provisions of this article, the arbitration proceedings shall be
         conducted in accordance with the Construction Industry Arbitration
         Rules promulgated by the American Arbitration Association. However, the
         failure to comply strictly with such Rules shall not be deemed a breach
         of this agreement.

7.       Any decision or award rendered by at least a majority of the
         arbitrators shall be final and judgment may be entered upon it in
         accordance with the laws of the State of Illinois.

8.       It shall be a condition precedent to the institution of any legal
         proceedings by either party against the other that a decision or award
         shall first have been rendered by the arbitrators.
<PAGE>
 
                                   Appendix F

                               MILESTONE SCHEDULE
                                 PROJECT 98-149




Tuesday 5 January 99       Apply for building/demo permit           DCDDC

Tuesday 5 January 99       Submit drawings for Info Mart review     DCDDC

Tuesday 5 January 99       Receive Permits                          DCDDC

Wed/Thurs 6-7 Jan 99       Interview Sub Contractors                DCDDC

Friday 8 January 99        Order Critical Equipment                 DCDDC

Tuesday 19 January 99      Info Mart comments due                   Info Mart

Thursday 21 January 99     Bids Due from Sub Contractors            All

Monday 1 February 99       Begin construction/demo                  Project Team

Monday 1 February 99       Submit Drawings for City review          DCDDC

Monday 1 March 99          UPS/Process air conditioners delivered   Vendors

Monday 15 March 99         Complete Switch Room general             Project Team
                           construction, ready for Nortel switch
                           installation

Monday 12 April 99         Provide power for Nortel equipment       Project Team

Monday 12 April 99         Provide process cooling to switch room   Project Team

Monday 3 May 99            Generator delivered                      Vendor

Monday 3 May 99            Co-Lo ready to accept cabinets           Project team

Monday 7 June 99           Office area ready for occupancy          Project team

Monday 14 June 99          Construction substantially complete      Project team

Monday 21 June 99          Project closeout                            DCDDC/
                                                                    project team
<PAGE>
 
                                   Appendix G


                          ARTICLE 3: SCOPE OF THE WORK


3.5      The Designer-Builder shall be responsible for obtaining all necessary
         documentation required by the Owner evidencing lien free completion of
         the work. Failure to secure such documentation will result in
         non-payment of the contract sum due to the Designer-Builder.