Document:

<PAGE>

                                                                    Exhibit 10.1
                                                                    ------------

July 27, 2000

Mr. Steven B. Kaufman
116 E. Emerson Road
Lexington, MA  02420

Dear Steve:

In addition to your offer letter Avici will offer the following terms:

If you are terminated without cause, or have a significant reduction in
responsibility, position, or compensation, you will receive a) accelerated
vesting of 75,000 shares of the option granted (Grant 3) of Avici Common Stock
at the exercise price of $31 per share and b) twelve-month continuation of base
salary and benefits to be paid out in accordance with Avici's payroll cycle.

In the event of a change of control of Avici, you will receive accelerated
vesting of 50% of all remaining unvested options.  Further, if due to a Change
of Control, you are terminated without cause or have a significant reduction in
responsibility, position, or compensation, all remaining unvested options would
become fully vested.

If you should die or become permanently physically disabled to an extent where
you are unable to perform you duties, all granted options will vested upon such
an event.

Steve, once again we look forward to joining Avici.

Sincerely,

/s/ Surya Panditi

Surya Panditi
President and CEO
Avici Systems, Inc.<PAGE>

                                                                    Exhibit 10.2

                                      LEASE
                                       BY
                 YVON CORMIER, TRUSTEE OF Y-CEE INVESTMENT TRUST
                                       TO
                               AVICI SYSTEMS INC.

                                TABLE OF CONTENTS

                                                                         Page
                                                                         ----

  1.  Identifications...................................................  3
  2.  Lease; the Premises...............................................  3
  3.  Tenant Fit-Up; Landlord's Work; Verification of Gross Floor Area..  5
  4.  Term..............................................................  8
  5.  Use of the Premises; Licenses and Permits.........................  9
  6.  Basic Rent; Additional Rent.......................................  9
  7.  Real Estate Taxes................................................. 16
  8.  Insurance; Waivers of Subrogation................................. 17
  9.  Utilities......................................................... 19
 10.  Repairs........................................................... 19
 11.  Compliance with Laws and Regulations.............................. 22
 12.  Alterations by Tenant............................................. 23
 13.  Landlord's Access................................................. 24
 14.  Indemnities....................................................... 25
 15.  Casualty Damage................................................... 26
 16.  Condemnation...................................................... 29
<PAGE>

 17.  Landlord's Covenant of Quiet Enjoyment; Title..................... 31
 18.  Tenant's Obligation to Quit....................................... 31
 19.  Transfers of Tenant's Interest.................................... 32
 20.  Transfers of Landlord's Interest.................................. 34
 21.  Mortgagees' Rights................................................ 35
 22.  Tenant's Default; Landlord's Remedies............................. 37
 23.  Remedies Cumulative; Waivers...................................... 40
 24.  Notices........................................................... 41
 25.  Recording......................................................... 42
 26.  Estoppel Certificates............................................. 42
 27.  Hazardous Materials and Compliance with Environmental Laws........ 43
 28.  Other Utilities................................................... 45
 29.  Option to Extend Term............................................. 45
 30.  Security Deposit.................................................. 47
 31.  Brokerage......................................................... 48
 32.  Additional Provisions............................................. 48
 33.  Bind and Inure; Limited Liability of Landlord..................... 49
 34.  Authority......................................................... 50
 35.  Captions.......................................................... 50
 36.  Integration....................................................... 50
 37.  Severability; Choice of Law, and Forum............................ 50

                                    EXHIBITS
                                    --------

Exhibit A - Plan for Tenant's Fit-Up

                                      -2-
<PAGE>

                                     LEASE
                                     -----
1.   Identifications:
     ---------------

     This LEASE made as of the 2nd Day of August, 2000, by and between the
following parties:

                                  THE PARTIES:
                                  -----------

     (a)  The name and address of the Landlord is:

          Yvon Cormier, Trustee of Y-CEE INVESTMENT TRUST, u/d/t dated January
          12, 1979 and recorded with the Middlesex County (Northern District)
          Registry at Book 2348, Page 327, as Amended, 59 Chandler Circle,
          Andover, MA 01810

     (b)  The name and address of the Tenant is:

          Avici Systems, Inc.,
          6 Billerica Office Park
          North Billerica, MA 01862

2.   Lease; the Premises:
     -------------------

     In consideration of the Basic Rent, Additional Rent, and other payments and
covenants of the Tenant hereinafter set forth, and upon the following terms and
conditions, it being the intent of the parties hereto that this instrument be
deemed a "triple net lease" except as hereinafter provided, the Landlord hereby
leases to the Tenant and the Tenant hereby leases from the Landlord certain
premises comprising 81,086 square feet, being the total rentable area in the
Building ("the Premises"), known and otherwise numbered Building 2 of Billerica
Business Center ("Building"), on a parcel of land (the "Property") in Billerica
Business Center, so called in the Town of Billerica, Middlesex County,
Massachusetts, together with the right to use other improvements constructed on
the Property, including the exclusive right to use 320 parking areas adjacent to
the Building.

                                      -3-
<PAGE>

     The Premises are leased together with the right, in common with the
Landlord and all others claiming under the Landlord or otherwise from time to
time lawfully entitled thereto, to use Billerica Park Drive (together with any
additional and substitute ways now or hereafter laid out in Billerica Park for
common use, the "Common Drives") for all purposes for which public ways may now
or hereafter be used in the Towns of Billerica and Tewksbury. The Landlord
reserves the rights to relocate the Common Drives and to install, maintain,
repair, replace and grant easements for utility lines, pipes and conduits in the
Common Drives and across the Property for the benefit of other properties, all
without any measurable interference with or, any interruption of the use and
enjoyment of access to the Premises by the Tenant as contemplated hereby subject
to the same conditions as Landlord's right to relocate common drives and place
and grant easements for utility lines. Landlord further reserves the right at
any time to submit all or a portion of the Common Drives to a declaration of
common easements for the benefit of the Property and the other properties in
Billerica Park serviced by the Common Drives, in which event the Tenant shall,
if (i) all such other properties in Billerica Park are afforded substantially
the same rights and benefits in the common Drives, (ii) the expenses of
maintenance, landscaping, plowing and sanding are fairly and reasonably
allocated among the Property and such other properties and (iii) such
declaration shall not adversely affect the rights of Tenant hereunder
(specifically including the Tenant's rights to use all parking areas adjacent to
the Building) be bound by the same and shall pay as Additional Rent hereunder
its pro-rata (as defined hereafter) share of such expenses as are from time to
time allocable to the Property under such declaration, provided those charges
are typical of those assessed to similar properties

                                      -4-
<PAGE>

at Billerica Park. The Tenant's Pro-Rata Share of expenses as delineated
aforesaid shall be based on a fraction the numerator of which shall be the total
amount of square feet leased to the Tenant at the Building and the denominator
of which shall be the aggregate total of rentable square feet of all buildings
in Billerica Business Center.

3.   Tenant Fit-Up; Landlord's Work; Verification of Gross Floor Area:
     ----------------------------------------------------------------

     Upon execution of this Lease, the Tenant shall be allowed access to the
Premises for purposes of undertaking and completing a fit-up of the interior
portion of the Premises in order to accommodate its proposed business operations
according to that plan marked Exhibit A.

     Prior to the Tenant being allowed access to the Building to begin work on
the fit-up, however, it shall be required to submit to the Landlord for approval
all plans and/or specifications related to the contemplated work, it being
agreed, however, that the Landlord will not unreasonably withhold, condition, or
delay such approval.  If Landlord shall fail to approve the plans and/or
specifications within five (5) business days from receipt of the same from the
Tenant, it shall be conclusively presumed that the Landlord has approved the
plans as presented.  If the Landlord shall disapprove the plans and
specifications, he shall set forth in writing the basis for the same.  In
connection with the approval of such plans and specifications, the parties agree
to cooperate with one another and to work together in good faith so that plans
and specifications submitted by the Tenant in a timely manner are approved
and/or accepted by the Landlord within fifteen (15) days from execution of this
instrument.

     All such plans and specifications shall be drawn in accordance with good
engineering practices.  In addition thereto, the Tenant shall assure that all
work done at the Building is completed in a good and workmanlike manner.  Any
contractor(s) hired by the Tenant to perform the fit-up shall use materials in
connection with the fit-up that are comparable in quality to those already
existing materials used in the construction of the Building.

                                      -5-
<PAGE>

     The Tenant shall bear all costs of preparing all plans, drawings and
specifications and, in addition thereto, the Tenant shall be solely exclusively
liable to all contractors hired to perform work for the fit-up, including,
without limitation, all costs of labor and materials.  Barring an instrument
signed by the Landlord, the Landlord shall have no liability whatsoever for
costs, expenses and/or fees due contractors hired by the Tenant.  In connection
therewith, should the Tenant breach this obligation and should the Landlord be
joined in any action brought by any third party against the Tenant asserting
damages for goods sole and delivered and/or for services rendered (including but
not limited to any action to perfect and/or to record a mechanic's or
materialman's lien against the Premises), then, in such event, (i) the Tenant
shall have thirty (30) days from commencement of such litigation to cause the
same to be Dismissed or to cause such lien to be removed by the posting of a
bond or such other means as the Tenant shall, in its sole discretion, implement,
failing which, the same shall be deemed an event of Default, thereby entitling
the Landlord, without further notice other than that required by law, to avail
itself on the remedies described in (S)22 hereafter (ii) the Tenant shall
indemnify and shall otherwise hold the Landlord harmless from and against any
and all such claims, including, but not limited to, reimbursement to the
Landlord for all reasonable legal fees/costs it might incur in connection with
defense of any such action.

     During the period of the fit-up, the Tenant shall pay all utilities for the
Premises based on readings from meters, submeters or check meters.  In
connection therewith, the Tenant acknowledges that failure to complete the fit-
up shall not delay the Commencement Date of the Lease as defined in (S)4 and the
accompanying responsibility of the Tenant to begin payment of both Basic and
Additional Rent effective on the Commencement Date unless such delay is caused
by Landlord, including, without limitation, Landlord's failure to timely approve
Tenant's plans.

                                      -6-
<PAGE>

     Furthermore, and in conjunction with the Tenant's fit-up, it shall be the
sole and exclusive responsibility of the Tenant to obtain any permits necessary
for it to begin work as may be required by the Town of Billerica and, likewise,
it shall be the sole and exclusive responsibility of the Tenant to assure that
all final inspections which may be required by the Municipality take place in a
timely fashion so as not to delay issuance of any Occupancy Permit which might
be required.

     In addition thereto, it shall be the sole and exclusive responsibility of
the Tenant to assure, effective with its exception of this Lease, that all
insurance required by (S)8 has been obtained and is full force and effect.

     In conjunction with the fit-up, and prior to any contractor commencing work
at the Building, the Tenant shall deliver, in form suitable to the Landlord, if
requested, evidence that each of such contractor commencing work at the
Building, the Tenant shall deliver, in form suitable to the Landlord, if
requested, evidence that each of such contractors has in full force and effect
Workmen's Compensation for the benefit of its employees.  In any all event,
however, the Landlord shall bear no liability for any person injured while on
the Premises during the process of the fit-up except if such injury or damage is
caused by the negligence or other misconduct of the Landlord, its agents,
servants, employees or contractors.

     The sole work of the Landlord shall be the following:  Prior to the
Commencement Date, the Landlord shall,

                                      -7-
<PAGE>

     (a) at its sole expense, crack seal, seal coat and stripe the parking lot
and replant and refurbish the landscaping and entry plaza (according to a
mutually acceptable plan);

     (b) at its sole expense, take any measures it deems necessary to assure
that the Building rooftop HVAC equipment and roof are in good working condition.

     Notwithstanding anything to the contrary contained in (S)(b), the Tenant,
from and after the Commencement Date, shall be solely and exclusively liable for
normal maintenance and repair of the HVAC system, including, but not limited to,
obtaining and paying for a service contract, a copy of which shall be delivered
to the Landlord.

4.   Term:
     ----

     The term of this Lease (the "Term"), shall commence on the earlier of the
date the Tenant shall receive a Certificate of Occupancy from the Town of
Billerica of four (4) months from the date the Landlord shall specify in a
written notice to the Tenant specifying the date on which the Tenant shall have
full access to the Building to begin the fit-up under (S)3 and shall end seven
(7) years from the date the lease term commences ("Commencement Date") as
provided for herein unless sooner terminated per the terms and provisions of
this Lease or unless extended, as provided for in (S)29 hereafter.

     Notwithstanding anything to the contrary contained in this Article, if the
Landlord shall fail to provide full access to the Building for the Tenant to
begin its fit-up under Article 3 by September 1, 2000 (the same to be confirmed
in a written notice from the Landlord to the Tenant specifying therein a date
certain of September 1 or earlier), the Tenant shall have the right to notify
the Landlord in writing of its intent to terminate its obligations under the
Lease, in which event, upon receipt of such notice from the Tenant, this Lease
shall be deemed null and void five (5) days from receipt of the Tenant's notice
and all obligations of the parties hereto shall terminate without recourse by
one against the other.

                                      -8-
<PAGE>

5.   Use of the Premises; Licenses and Permits:
     -----------------------------------------

     The Tenant shall use the Premises only for offices, general engineering,
research and development, light manufacturing, warehousing, (collectively
"Permitted Uses"), all to the extent now and hereafter from time to time
permitted under applicable laws, by-laws, ordinances, codes, rules, regulations,
orders and other lawful requirements of governmental bodies having jurisdiction
related thereto.  The Tenant, its subtenants, licensees, invitees and any other
users of the Premises shall apply in their own names for and obtain at their own
expense any and all licenses, permits and other approvals which may be required
from such governmental bodies solely in connection with operation of the
Tenant's business at the Premises during the Term.

6.   Basic Rent; Additional Rent:
     ---------------------------

     Effective with the Commencement Date, and for the first five (5) years of
the term of this Lease as defined by (S)4 above (except as provided for in the
fourth paragraph of this section), the Tenant shall pay to the Landlord as Basic
Rent, the sum of Nine Hundred Seventy Three Thousand Thirty Two and 00/100
($973,032.00) Dollars per year in monthly installments of Eighty One Thousand
Eighty Six and 00/100 ($81,086.00) Dollars, the same calculated at the rate of
Twelve and 00/100 ($12.00) Dollars per square foot for 81,086 rentable square
feet of area at the Building.

     For years six (6) and seven (7) of the term of this Lease as defined by
(S)4 above (except as provided for in the fourth paragraph of this section), the
Tenant shall pay to the Landlord as Basic Rent, the sum of One Million Fifty
Fourth Thousand One Hundred Eighteen and 00/100 ($1,054,118.00) Dollars per year
in monthly installments of Eighty Seven Thousand Eight Hundred Forty Three and
17/100 ($87,843.17) Dollars, the same calculated at the rate of Thirteen and
00/100 ($13.00) Dollars per square foot for 81,086 rentable square feet of area
at the Building.

                                      -9-
<PAGE>

     As used herein, the term "Year" or "Years" shall mean each twelve (12)
month period beginning on the first day of the calendar month immediately
following the month in which the Commencement Date occurs, if the Commencement
Date is not the first day of the month, and each twelve (12) month period
thereafter, beginning on the anniversary of the first day of the calendar month
immediately following the month in which the Commencement Date occurs, provided,
however, the first "Year" shall include the number of days from the Commencement
date through the last day of the calendar month in which the Commencement Date
occurs.

     As more specifically set forth hereafter, each yearly amount of Basic Rent
is to be Triple Net. Basic Rent shall be payable without counterclaim,
deduction, defense setoff or abatement, except as otherwise expressly provided
herein, in advance on the first day of each month in equal installments (except
in the case of a partial month at the beginning of the Term, in which event the
Tenant shall pay the appropriate pro rata proportion of such installment) to the
Landlord at the address set forth above or to such other party or such other
address as the Landlord may thereafter from time to time specify by notice to
the Tenant. If the name or address of the party entitled to receive Basic Rent
and Additional Rent shall be changed, the Tenant may, until ten (10) days after
receipt of such notice or such change, continue to pay Basic Rent and Additional
Rent to the party and the address to which, and in the manner in which, the
preceding installment of Basic Rent was paid.

     To the end this Lease is Triple Net, Basic Rent shall be received by the
Landlord net of all costs and expenses related to the Premises other than as
expressly set forth herein. The Tenant

                                      -10-
<PAGE>

agrees to pay as "Additional Rent" for that area leased to it and exclusively
dedicated to its use, (i) all real estate taxes issued from the Town of
Billerica (ii) all insurance premiums of the Landlord and (iii) a charge of $.75
per square foot of space in the Premises, such rate to be fixed for the Term of
the Lease provided the original Landlord named herein remains the Landlord,
which totals Sixty Thousand Eight Hundred Fourteen and 50/100 ($60,814.50)
Dollars per year, payable in monthly installments of Five Thousand Sixty Seven
and 88/100 ($5,067.88) Dollars, to cover Tenant's Pro-Rata Share of the costs
(except those of a capital nature) of routine repairs and maintenance,
landscaping, costs of snow plowing and liability insurance premiums for the
parking areas and other exterior common areas of Billerica Business Center
("Operating Costs"). The Additional Rent shall constitute Tenant's full and
complete liability for all real estate taxes, Landlord's insurance and Operating
Costs in any way associated with the Premises, Building and Property other than
Tenant's Pro-Rata Share of water and sewer charges" described in (S)9 below. The
Landlord shall, in each case, at the time of demand for payment, provide the
Tenant with evidence of the payment for any such charges, costs, expenses and
obligations. In connection with the Tenant's obligation to pay Additional Rent,
the Landlord shall provide an annual statement to the Tenant prepared in
accordance with generally accepted accounting principles and certified by an
officer of the Landlord no later than ninety days after the end of each year.

     Any adjustment(s) to Additional Rent shall be based on the annual statement
subject to the Tenant's audit rights as described in the Lease. Notwithstanding
anything in this Lease to the contrary, the Tenant shall have no liability for
the Landlord's mortgage debt service or other financing costs, any brokerage
expenses of the Landlord or any repairs which are the obligation of the Landlord
under Paragraph 10 or any capital improvements or assessments prior to the
Commencement Date of the Lease or within the last year of the Term.

                                      -11-
<PAGE>

     Tenant shall be liable for its share of the cost of repaving and/or
restriping of the parking lot for the Building and repaving and/or restriping of
the roadways at Billerica Business Center. The Tenant's share of the aforesaid
capital improvements or assessments shall be based on a fraction, computed as
follows: for repaving and/or restriping of the parking lot adjacent to the
Building, the Tenant's share shall be 100% based on the fact it shall be the
sole tenant at the Building. For repaving and/or restriping of roadways at
Billerica Business Center, the Tenant's share shall be a fraction the numerator
of which shall be the total amount of rentable square feet leased to the Tenant
in the Building and the denominator shall be the aggregate total of rentable
square feet in all buildings at Billerica Business Center. The assessment for
capital improvements described aforesaid shall be amortized over the economic
useful life of said improvements and shall be paid in equal monthly installments
in the same manner as provided for in the preceding clause of this section. In
the event that the Landlord submits the Common drives to a declaration of common
easements as provided for in (S)2, Tenant's obligations under said declaration
shall supersede it obligations under this paragraph to pay its share of
repairing or restriping of roadways.

     Specifically excluded from Additional Rent are:

     1.  the cost of correcting defects (other than defects in Tenant's work),
except conditions (not occasioned by construction defects) resulting from
ordinary wear and tear shall not be deemed defects for this purpose;

     2.  salaries of officers and executives of the Landlord not connected with
the Operation of the Property;

     3.  the initial cost of tools and equipment used in the Operation of the
Property;

     4.  depreciation;

                                      -12-
<PAGE>

     5.  expenses relating to tenants' alterations, including, without
limitation, the construction of initial leasehold improvement for tenant spaces;

     6.  interest, fees or principal on indebtedness and any ground lease
rents;

     7.  expenses for which the Landlord, by the terms of this Lease or any
other lease, makes a separate charge;

     8.  real estate taxes, other than as provided for by the Lease;

     9.  the cost of any electric current furnished to Tenant and other Building
tenants, exclusive of the lobby which is to be a common area and outside
lighting for the parking lot;

     10. the cost of any services or systems for that portion of the Building
occupied by the Landlord or affiliates of the Landlord (exclusive of space
occupied by the Landlord or affiliates of the Landlord in connection with the
Operation of the Building) and which are not provided generally to other tenants
in the Building;

     11. leasing fees or commissions, advertising and promotional expenses, and
legal fees incurred in connection with leasing space in the Building;

     12. costs incurred in performing work or furnishing services for any tenant
(including Tenant), whether at such tenant's or Landlord's expense, to the
extent that such work or service is in excess of any work or service that
Landlord is obligated to furnish to Tenant at Landlord's expense (e.g., if
Landlord agrees to provide extra cleaning to another tenant, the cost thereof
would be excluded since Landlord is not obligated to furnish extra cleaning to
Tenant);

     13. the cost of repairs or replacements incurred by reason of fire or other
casualty or condemnation;

     14. association fees and dues;

     15. expenses for painting and redecorating for other tenants;

     16. legal expenses in enforcing the terms of the Lease for the Tenant and
any other tenant;

                                      -13-
<PAGE>

     17. penalties and interest for late payment of real estate taxes, water and
sewer charges, or any other obligation of Landlord;

     18. water rents and sewer charges to any tenant on a direct meter basis
which are paid by the tenant;

     19. the cost of any item or service to the extent to which Landlord is
entitled to be reimbursed or compensated by insurance, any tenant, or any third
party, or would have been entitled to reimbursement or compensation if Landlord
maintained the insurance which it is required to maintain hereunder;

     20. expenses due to Landlord's default under any lease or other agreement
relating to the Building or the Premises; and

     21. all other items that, under generally accepted accounting principles,
are properly classified as capital expenditures, except as expressly otherwise
provided in this Lease.

     The Tenant shall pay to the Landlord upon demand as Additional Rent
interest on any payments of Basic Rent or Additional Rent not paid within 10
(ten) days after the same are due at a rate (herein called the "Lease Interest
Rate") equal to five (5%) percent per annum computed from the date such payment
was due until such payment is made, each of such payment(s) of interest being
payable as Additional Rent with the payment of Basic Rent next coming due.
Notwithstanding the foregoing, if no such demand is made in writing delivered in
hand to the Tenant within the period of thirty (30) days following the date any
payment of Basic Rent or Additional Rent was due, the Landlord shall be deemed
to have waived the right to be paid any interest on such payment in respect of
periods prior to such demand.

                                      -14-
<PAGE>

     Rent for any partial month shall be equitably pro-rated. Rent shall be paid
by check mailed by the first business day of each month to the Landlord at the
above address or any other address provided to Tenant by Landlord in accordance
with this Lease.

     The Tenant shall have ninety (90) days from receipt of any year end
statement described above to dispute the inclusion or amount of any item or
items in any such statements provided to it. In such event, the Tenant shall pay
the Operating Expense Payment and the Tax Payment excluding the items or amounts
in dispute. If the parties are unable to settle the dispute between themselves
within sixty (60) days of notice by the Tenant to the Landlord, such dispute
shall be settled by a firm of real estate audit professionals mutually
acceptable to both parties ("Audit Professionals"). If the Landlord and Tenant
cannot agree on the Audit Professionals within thirty (30) days, then each party
shall, within fifteen (15) days thereafter, select one independent firm of
auditors and the two then selected shall choose a third independent Audit
Professional. The third firm thereupon shall settle the dispute and both the
Landlord and Tenant shall be bound its decision. If the Landlord fails to select
its Audit Professional within the 15 day period as aforesaid, then to the extent
the Tenant has previously paid the Landlord for items in Dispute, the Tenant
shall have the right to deduct such amount from any installments of basic or
additional rent or other charges due or becoming due under the Lease ("Set Off
Rights"). If the Tenant fails to select its auditor within such 15 day period,
it shall be conclusively deemed that the Tenant has waived its right to audit
and/or to contest the validity of applicable Year End Statements.

     Audit Professionals shall be entitled to review all records relating to the
disputed items. All costs for the AUDIT Professionals shall be borne by the
party against whom a decision is rendered, subject to the following: if more
than one item is disputed, the expenses shall be apportioned equitably according
to the number of items decided against each party taking into consideration the
amount(s) involved.

                                      -15-
<PAGE>

     If the Audit Professionals determine that the Tenant has made an
underpayment, Tenant shall reimburse Landlord for the amount of the same within
thirty (30) days following such determination. If the Audit Professionals
determine that the Tenant has made an overpayment, the Landlord shall reimburse
the Tenant for the same likewise within thirty (30) days of such determination.

     Should the Landlord fail to pay the overpayment as described aforesaid, the
Tenant shall have the remedy of the Set Off Rights. If the Tenant fails to pay
the Landlord the underpayment, the same shall be deemed a default of non-payment
under the Lease.

7.   Real Estate Taxes:
     -----------------

     See (S)6. In addition: Based on the fact the Tenant is occupying One
Hundred (100%) percent of the Building, Tenant shall pay to Landlord, as
Additional Rent of all taxes levied or assessed by, or becoming payable to any
municipality or any governmental authority having jurisdiction, for or in
respect of the Building, for each tax period wholly included in the Term, all
such payments to be made to Landlord not less than ten (10) days prior to the
last date on which the same may be paid without interest or penalty; provided
that, for any fraction of a tax period included in the Term at the beginning or
end thereof, Tenant shall pay to Landlord, within thirty (30) days after receipt
of invoice therefor, all of the taxes so levied or assessed or becoming payable
which is allocable to such included period.

     Provided Landlord receives Tenant's Pro-Rata Share of such taxes, Landlord
shall pay or cause to be paid, before the same became delinquent, all taxes
imposed with respect to the Property. If Landlord shall receive any tax refund
or rebate with respect to any tax year wholly or partially included within the
Term, then out of any balance remaining thereof, after deducting Landlord's
expenses incurred in obtaining such refund or rebate, Landlord shall pay to
Tenant an amount equal to Tenant's Pro-Rata Share of such balance (or of such
fraction thereof applicable to a tax year partially included within the Term).

                                      -16-
<PAGE>

     Nothing contained in this Lease shall, however, require Tenant to pay any
franchise, corporate, estate, inheritance, succession, capital levy or transfer
tax of Landlord, or any income, profits or revenue tax or charge upon the rent
payable by Tenant under this Lease.

8.  Insurance; Waivers of Subrogation. The Landlord shall carry appropriate
    ---------------------------------
general liability insurance as well as property and casualty insurance on the
Premises, Building, Property and Improvements with the same limits of coverage
as set forth for the Tenant herein. The Landlord shall further obtain insurance
for the Building and Improvements, with full replacement cost as determined by
its Insurer. Tenant shall, at its own cost and expense, obtain prior to the
commencement of the Term and throughout the Term shall maintain, with companies
qualified to do business in Massachusetts and reasonably acceptable to the
Landlord and to any Mortgagee(s), for the benefit as named insureds of the
Landlord, the Tenant and any Mortgagees as their respective interests may
appear, the following:

     1.  comprehensive general liability and property insurance (with
contractual liability rider) against claims for bodily injury, death or property
damage occurring to, upon or about the Premises in limits of $2,000,000.00 per
occurrence for bodily injury or death and property damage or such higher limits
as may result from the operation of clause (iv) below, the risk of loss to all
Tenant's contents of, and personal property and trade fixtures located in, the
Premises being upon the Tenant, and the Landlord having no liability with
respect thereto;

                                      -17-
<PAGE>

     2.  so called "property" or "casualty" insurance, with losses payable to
the Landlord or any first Mortgagee, against loss or damage to the presently
existing (prior to Tenant's occupation) interior Improvements and to any other
buildings, structures and improvements from time to time constituting a part of
the Premises, such as may result from fire and such other casualties as are
normally covered by "extended coverage" and "difference in conditions"
endorsements, such casualty or property insurance to be in an amount equal to
the replacement cost of the Improvements and any other buildings, structures or
improvements from time to time constituting part of the Premises and to include
lost rentals coverage for a period of at least one year. Replacement cost shall
be redetermined as frequently as the Landlord or any Mortgagee may from time to
time reasonably request; and

     3.  at the written request of the Landlord, such other insurance coverages
and such additional coverage amounts as any Mortgagee may reasonably require the
Landlord to obtain provided such coverage is customary and usual with respect to
commercial buildings similar to the subject property.

     At the commencement of the Term and thereafter not less than ten (10) days
prior to the expiration dates of the policies theretofore furnished originals or
duplicates or certificates (contractually binding upon the insurer and not
merely informational) thereof issued by the insurers, shall be delivered to the
Landlord. All such policies and certificates shall provide that they are non-
cancellable without at least thirty (30) days' prior written notice to the
Landlord and any Mortgagees. Tenant shall have the right to substitute insurance
policies of equal coverage.  The Landlord and the Tenant each hereby release the
other from any liability for any loss or damage to the Premises or other
property and for injury to or death of persons occurring on or about the
Premises or in any manner growing out of or connected with the Tenant's use and

                                      -18-
<PAGE>

occupation of the Premises or the condition thereof, whether or not caused by
the negligence or other fault of the Landlord, the Tenant or other respective
agents, employees, subtenants, licensees, invitee or assignees; provided,
however, that this release (i) shall apply notwithstanding the indemnities set
forth in Paragraph 14, but only to the extent that such loss or damage to the
Premises or other property or injury to or death of persons is covered by
insurance which protects the Landlord or the Tenant or both of them as the case
may be; (ii) shall not be construed to impose any other or greater liability
upon either the Landlord or the Tenant than would have existed in the absence
hereof; and (iii) shall be in effect only to the extent and so long as the
applicable insurance policies provide that this release shall not affect the
right of the insureds to recover under such policies, which clauses shall be
obtained by the parties hereto whenever available.

9.   Utilities:
     ---------

     The Tenant expressly acknowledges that its intended use of the Premises is
for commercial purposes and the Landlord shall have no duty to furnish to the
Tenant with utility services of any kind.  Notwithstanding the same, the
Landlord represents that the Premises is served by water, sewer, gas (if
applicable), telephone [subject to telephone company], electric and HVAC.
Separate gas (if applicable), electrical and water meters shall be provided for
the Tenant and the Tenant in turn agrees to be billed directly by the applicable
utility company and the town furnishing water/sewer services.

10.  Repairs:
     -------

     The Landlord shall, subject to the provisions of (S)6 aforesaid and the
third paragraph of this section, and during the term of the Lease, maintain and
make all necessary repairs and replacements of a capital nature to the roof,
foundations, beams, girders, and other structural elements of the Building,
including parking lots and walkways, all other common areas and facilities and
exterior walls of the Premises only (exclusive of signs which may be installed
by the Tenant). However, the Landlord shall have no obligation to repair any
damage resulting directly from negligent acts of the Tenant, its agents,
employees, contractors, and invitees unless covered by Landlord's insurance
policy.

                                      -19-
<PAGE>

     The Landlord shall be responsible for ordinary repairs to the roof so that
the roof is maintained in the same condition as exists on the Commencement Date
of the Lease. However, should the Tenant install any equipment on the roof which
penetrates the membrane, as a consequence of which, damage to the roof is
caused, the Tenant shall be liable to repair any such damage caused by
installation of its equipment.

     In connection herewith, the Landlord represents (i) The Property shall be
maintained consistent with generally accepted practices associated with a first
class research and development building, (ii) it shall provide snow plowing for
the parking areas, (iii) it shall remove snow and ice from walkways and (iv) it
shall provide landscaping services so that the Property is maintained in a neat
and clean condition.

     From and after the commencement of and during the Term, the Tenant shall,
at its own cost and expense: make all other non-structural repairs, interior and
exterior, necessary to keep the Premises, including all electrical, mechanical,
heating, ventilating and air conditioning (including any roof top unit),
plumbing (other than in common areas) and other building systems [excluding
sewer and gas systems, common building systems and all capital repairs and
replacements] serving the Premises, in as good condition, order and repair as
the same are at the commencement of the Term or thereafter may be put,
reasonable wear and use, damage by fire or other casualty caused by the Landlord
and repairs which are expressly the obligation of the Landlord hereunder only
excepted (it being understood, however, that the foregoing exception for
reasonable wear and use shall not relieve the Tenant from the obligation to keep
the Premises in good order, repair and condition).

                                      -20-
<PAGE>

     Landlord shall be responsible for all replacements of a capital nature to
the existing base building air conditioning and other existing base building
systems. Notwithstanding the same, the Tenant shall be liable to pay the
Landlord the cost of such replacements according to the following: The Tenant
shall be assessed for the capital improvements described aforesaid based on an
amortization calculated at the useful life of that improvement. Thereupon, the
Tenant shall pay to the Landlord, as Additional Rent under (S)6, each month
during the remaining Term of the Lease a portion of said cost, based upon said
amortization period. As an example, if a replacement of a capital nature to the
air conditioning or other building systems is made at the conclusion of the
third (3rd) Year of the Term as defined in (S)4, the Tenant would be required to
reimburse the Landlord with interest at the rate of Ten (10%) percent, the cost
incurred for the replacement of the applicable system, calculated at the useful
life of the improvement, for each month during the remainder of the Term or
Option 1 and Option 2, if applicable under (S)29 hereafter. In no event,
however, shall the Tenant be assessed a reimbursement to the Landlord for the
costs of the replacements for the air conditioning and other building systems
beyond the Term of the Lease or applicable Option period.

     Notwithstanding anything to the contrary as contained in this Paragraph,
the Landlord shall be solely and exclusively liable for all repairs and
replacements which are capital in nature, subject to reimbursement by the tenant
as provided for above.

     If the Landlord shall fail to cure any default by the Landlord in its
obligations under this (S)10, within thirty (30) days after notice from the
Tenant to the Landlord of such default (or, in

                                      -21-
<PAGE>

the event of imminent danger of injury to persons or damage to property,
immediately after telephone notice of such default) or to commence such cure and
diligently prosecute the same to completion, in the absence of any Terminable
Default on the part of the Tenant hereunder, the Tenant may, at its option, cure
for the Landlord's account such default as was specified in such notice, in
which event the Tenant may offset against the Basic Rent next accruing under
this Lease, cumulatively until exhausted, the reasonable costs and expenses it
can demonstrate were incurred in good faith in the cure of such default plus
interest thereon at the Lease Interest Rate, but in no event shall any such
offset reduce the amount of Basic Rent payable in any month below the amount
from time to time necessary for the Landlord to make payment of principal,
interest and other expenses payable under any first mortgage perfected against
the Premises, including any real estate taxes or other expenses which are or
could become a lien upon the Premises, except to the extent such taxes and other
expenses are actually paid by the Tenant.

11.  Compliance with Laws and Regulations:
     ------------------------------------

     The Tenant agrees that its obligations to make payment of the Basic Rent,
Additional Rent and all other charges on its part to be paid, and to perform all
of the covenants and agreements on its part to be performed during the Term
hereunder shall not, except as set forth in Paragraph 16 in the event of
condemnation by public authority, be affected by any present or future law, by-
law, ordinance, code, rule, regulation, order or other lawful requirement
regulating or affecting the use which may be made of the Premises.

     During the Term, the Tenant and Landlord shall comply, each at its own cost
and expense, with: all applicable laws, by-laws, ordinances, codes, rules,
regulations, orders, and other lawful requirements of the governmental bodies
having jurisdiction (other than such as are the obligation of the Landlord
hereunder), whether or not foreseeable, and whether or not they

                                      -22-
<PAGE>

involve any changes in governmental policy, which are applicable to the
Premises, the fixtures and equipment therein and thereon, or the Tenant's
particular use thereof; the orders, rules and regulations of the New England
Fire Insurance Rating Association or any other body hereafter constituted
exercising similar functions, which may be applicable to the Premises, the
fixtures and equipment therein or thereon or the use thereof; and the
requirements of all policies of public liability, fire and all other types of
insurance at any time in force with respect to the Premises and the fixtures and
equipment therein and thereon. Notwithstanding the foregoing, the Tenant shall
be responsible for future laws, by-laws, ordinances, codes, rules, regulations,
orders and other lawful requirements of governmental bodies having jurisdiction
that require a structural change to the Premises, Building or Property or other
capital outlay.

12.  Alterations by Tenant:
     ---------------------

     The Tenant shall erect no signs (other than one at the driveway to the
Building and on the facade of the Building, subject to approval of the Landlord
with respect to location [which approval shall not be unreasonably withheld,
conditioned or delayed] and subject to any applicable ordinances related to
signs as promulgated by the Town of Billerica) and shall make no alterations,
additions or improvements in or to any portion of the Premises without the
Landlord's prior written consent subject to the provisions of this Paragraph 12,
which consent shall not be unreasonably withheld, conditioned or delayed. In
connection therewith, the Landlord shall have ten days, from receipt of written
notice from the Tenant detailing the proposed alterations, to approve or reject
the same. The Landlord shall be deemed to have given his approval to any
proposed alterations unless he shall have objected to the same within the time
set forth herein. As part of any request for such consent, the Tenant shall
provide the Landlord with plans and specifications drawn in accordance with good
engineering practice (only if it would be usual and appropriate to prepare plans
and specifications given the nature and extent to the proposed alterations,
additions or improvements), reasonable evidence of suitable insurance and, lien
bonds or other suitable assurances of the Tenant's obligation and wherewithal to
complete the same at no expense to the Landlord and without failure to pay any
contractor engaged to do the work.

                                      -23-
<PAGE>

     The Landlord agrees that in the absence of a Terminable Default on the part
of the Tenant hereunder, its consent shall not be required for interior, non-
structural alterations to the Building from time to time constituting a part of
the Premises if the same are consistent with the use of the Premises as
contemplated hereby and do not affect the heating, ventilating and air
conditioning and other engineering and mechanical systems in the Building. At
the time Tenant requests Landlord's consent to any future alterations,
installations, removals, additions or improvements, Landlord agrees it will only
require Tenant to remove such alteration, installment, removal, addition or
improvement at the end of the Lease Term provided the Landlord, in his sole
discretion, determines such improvements will impair his ability to re-let the
Premises to another tenant.

13.  Landlord's Access:
     -----------------

     The Tenant agrees to permit the Landlord and any Mortgagees and their
authorized representatives to enter the Premises (i) at all reasonable times
during usual business hours after not less than two business days' prior oral or
written notice for the purposes of inspecting the same, but not more often than
annually, exercising such other rights as it or they may have hereunder or under
any mortgages (provided such rights are consistent with the Landlord's rights
hereunder) and, during the last six (6) months of the Term or after any
Terminable Default (inclusive of any applicable notice period) hereunder on the
part of the Tenant, to show the Premises to other prospective tenants,
purchasers or mortgagees and (iii) at any time and without notice in the event
of emergency (in which event the Landlord shall attempt to give oral notice by
telephone.) In connection therewith, Landlord shall use all commercially
reasonable efforts to protect Tenant's property and personnel from loss and
injury and to avoid interfering with conduct of Tenant's business.

                                      -24-
<PAGE>

14.  Indemnities:
     -----------

  The Tenant and Landlord agree as follows: each shall indemnify and save the
other harmless from and against any and all losses, claims, liability, expenses
and damages (other than consequential damages) which either directly or
indirectly, in whole or in part arise out of or result from (i) the negligence
or willful misconduct of either party, its agents, servants and employees (ii)
any act or occurrence in or about the Premises, unless caused by the negligence
or willful misconduct of the applicable party, its agents, servants, contractors
or employees (iii) the breach of any provision of this Lease by either party or
any of its agents, servants, employees or contractors (iv) judgments, citations,
fines or other penalties rendered or assessed against one or the other (with the
exception of any claims under the applicable party's workmen's compensation
insurance policy) as a result of one of the party's failure to comply with all
federal, state and local laws, safety and health regulations relating to either
party's use or occupation of the Premises. In connection therewith, both the
Landlord and Tenant agree to give the other prompt notice of any such violation
which may be asserted by a governmental agency.

     The Landlord and Tenant further agree to indemnify the other from and
against all costs, expenses (including reasonable attorneys' fees) and other
liabilities incurred in connection with any such indemnified claim or action
and/or proceeding brought thereon. If the Landlord shall breach this covenant
and fail to reimburse the Tenant for such costs and expenses after written
demand, the Tenant shall be allowed to offset the same against basic or
additional rent. If the Tenant shall breach this covenant and fail to reimburse
the Landlord for such costs and expenses after written demand, the Landlord may
declare a Default under this Lease based upon non-payment of rent.

                                      -25-
<PAGE>

     Nothing contained above is intended to require indemnification for any
property claim for which insurance is required to be maintained under the terms
this Lease. The rights and obligations of the Landlord and Tenant under this
Paragraph 14 shall survive the expiration and/or earlier termination of this
Lease.

15.  Casualty Damage:
     ---------------

     A.  Tenant's Right to Terminate:  If the Premises or the Building shall be
damaged or destroyed by fire, windstorm or any other insured casualty, the
Tenant shall immediately give written notice thereof to Landlord and unless this
Lease is terminated as hereinafter provided, the Landlord, at his own expense,
shall repair or rebuild the same so as to restore the Premises to substantially
the same condition they were in immediately prior to such damage or destruction,
subject however, to zoning and building laws then in existence, provided that
the Landlord shall not be responsible for any delay in such repair or
reconstruction which may result from any cause beyond its reasonable control,
and provided further that the Landlord shall not be required to expend more than
the net amount of insurance proceeds, if any received, by Landlord for such
purposes, plus the amount of any deductible, so long as the Landlord maintains
the insurance required of it under this Lease. Notwithstanding the foregoing,
the Landlord within 30 days after receipt of said written notice of damage,
shall inform the Tenant if said damage can be repaired within 180 days, based
upon a reasonable estimate of the Landlord's architect and Engineer. If the
Landlord notifies the Tenant it cannot repair or rebuild the Premises so as to
restore the same to substantially the same condition they were in immediately
prior to the destruction within said 180 days, or if in fact the Premises is not
restored within 180 days, then in either of such event, the Tenant may elect to
cancel this Lease upon notice to the Landlord.

                                      -26-
<PAGE>

     B.  Landlord's Right to Terminate: If the Premises or the Building are
substantially damaged by fire or casualty (the term "substantially damaged"
meaning damage of such character that the same cannot, in the ordinary course,
reasonably be expected to be repaired within 180 days from the time that repair
work would commence or if either the Premises or Building shall be damaged or
destroyed to the extend of 25% or more on a square footage basis by any cause,
(whether insured against by the Landlord or not) the Landlord may elect by
written notice to the Tenant, to be provided to Tenant within 30 days of
Landlord's receipt of said damage, either to terminate this Lease or to repair
or rebuild on the conditions set forth in the above clause. The Tenant shall
likewise be afforded the right to terminate the Lease if more than 25% of either
the Premises or Building shall be damaged or destroyed.

     C.  Tenant's Obligations to Repair: In the event that the Premises or
Building is damaged or destroyed by any cause, then, unless this Lease is
terminated as provided above, the Tenant at its own expense [but in no event
beyond the limits of its insurance less any applicable deductible] and
proceeding with all reasonable dispatch shall repair or replace all trade
fixtures, equipment, signs or other property installed by or belonging to the
Tenant which shall be damaged to destroyed.

     D. Abatement of Rent: If the Premises shall be damaged by fire or casualty,
the Basic Rent and Additional Rent payable by the Tenant shall abate or be
reduced proportionately for the period in which, by reason of such damage, there
is substantial interference with the operation of the Tenant's use of the
Premises, having regard to the extent to which Tenant may

                                      -27-
<PAGE>

be required to discontinue its use of the Premises, but such abatement or
reduction shall end if and when Landlord shall have substantially restored the
Premises (exclusive of any of Tenant's fixtures, furnishings, equipment and the
like or work performed therein by Tenant) to substantially the condition in
which the Premises were in prior to such damage, so long as Tenant has been
provided 180 days from the date it is afforded access to the Premises to
complete the Tenant's repairs. In no event shall Landlord be obligated in
connection with the restoration of the Premises, as stated herein, to expend an
amount in excess of the proceeds of insurance recovered with respect thereto. In
the event the Premises shall be damaged by fire or other casualty resulting from
negligence of the Tenant, its agents, contractors, employees or invitees, and
this Lease shall not be terminated as a result of such damage, Tenant shall not
be released from any of its obligations hereunder including, without limitation,
its duty to pay the Basic Rent and Additional Rent payable by Tenant without
abatement or reduction.

     E.  Award:  Landlord shall have and hereby reserves and accepts and Tenant
hereby grants and assigns to Landlord, all rights to recover damages to the
Premises, Building or Property and the leasehold interest hereby created, from
Landlord's insurance and to compensation accrued or hereafter to accrue by
reason of such damage or destruction, as aforesaid, and by way of confirming the
foregoing, Tenant hereby grants and assigns, and covenants with Landlord, to
grant and assign to Landlord all rights to such damages or compensation from
Landlord's insurance. Nothing contained herein shall be construed to prevent
Tenant from prosecuting in any proceedings a claim for the value of the Tenant's
inventory, furnishings, equipment or usual trade fixtures installed in the
Premises by Tenant and at Tenant's expense, provided that such action shall not
affect the amount of compensation otherwise recoverable by Landlord.

                                      -28-
<PAGE>

16.  Condemnation:
     ------------

     If a substantial portion of the useable floor areas of the building, or of
the parking area, from time to time constituting part of the Premises shall be
taken, such that the same shall materially and adversely interfere with the
Tenant's ability to conduct business on the Premises as contemplated under the
Lease, or if the Tenant's access to the Premises shall be deprived for one (1)
month or more, by eminent domain or appropriated by public authority, the Tenant
may terminate this Lease by giving written notice to the Landlord within thirty
(30) days after such taking or appropriation becomes final and Tenant is
informed of same by Landlord in writing, the Landlord enclosing with such notice
the applicable notifications of taking from the appropriating public authority.
In the event of any such termination, this Lease shall terminate as of the date
the Tenant must surrender possession or, if later, the date the Tenant actually
surrenders possession, or the date Tenant's access is deprived and the Basic
Rent and Additional Rent reserved shall be apportioned and paid to and as of
such date.

  If all or any part of the Improvements or any other improvements from time to
time constituting part of the Premises is taken or appropriated by public
authority as aforesaid and this Lease is not terminated as set forth above, the
Landlord shall, subject to the rights of any mortgagees, apply any such damages
and compensation awarded (net of the costs and expenses, including reasonable
attorneys' fees, incurred by the Landlord in obtaining the same) to secure,
close and restore so much of the Improvements or other improvements constituting
part of the Premises as remain to an architectural whole except that in no event
shall the Landlord be obligated to expend more for such replacement than the net
amount of any such damages, compensation or award which the Landlord may have
received as damages in respect to the Improvements and any other improvements
from time to time constituting part of the Premises as

                                      -29-
<PAGE>

they existed immediately prior to such taking or appropriation; in such event,
there shall be an equitable abatement of Basic Rent in proportion to the loss of
useable floor area in the Improvements after giving effect to such replacement,
from and after the date the Tenant must surrender possession (or access is
deprived) or, if later, the date the Tenant actually surrenders possession and
except further, that if, in such event, for any reason any Mortgagee retains any
portion of such damages or award and the Landlord tails within thirty (30) days
after a request by notice from the Tenant to agree to secure and close the
remaining improvements as aforesaid, then the Tenant shall have an additional
period of sixty (60) days after the expiration of such thirty (30) day period
within which to terminate this Lease, in which event, this Lease shall thereupon
be void and without recourse to the parties as to obligations thereafter
accruing. The Tenant agrees that the foregoing rights shall be its sole
remedies, both at law and in equity, for the failure of any Mortgagees and the
Landlord to fulfill its obligations to secure and close the remaining
improvements as aforesaid.

     The Landlord hereby reserves, and the Tenant hereby assigns to the
Landlord, any and all interest in and claims to the entirety of any damages or
other compensation by way of damages which may be awarded in connection with any
such taking or appropriation except so much of such damages or award as is
specifically and separately awarded to the Tenant and expressly attributable to
Improvements constructed and/or made by the Tenant as well as to the Tenant's
trade fixtures, personal property or moving expenses. Nothing contained herein
shall prohibit the Tenant from making claim in its own name against the
municipality for damages including but not limited to loss of business and an
amount equal to its unamortized costs of leasehold improvements on condition
such application does not interfere with the Landlord's paramount right to claim
damages in connection with any such taking. Notwithstanding anything to the
contrary set forth above, if the Tenant is deprived of access to the Property,
an abatement of rent, effective on the date of deprivation, shall take effect.
In addition thereto, in the event of Condemnation, all Basic and Additional Rent
shall be abated.

                                      -30-
<PAGE>

17.  Landlord's Covenant of Quiet Enjoyment; Title:
     ---------------------------------------------

     The Landlord covenants that the Tenant, upon haying the Basic Rent and
Additional Rent provided for hereunder and performing and observing all of the
other covenants and provisions hereof, may peaceably and quietly hold and enjoy
the Premises for the Term as aforesaid, subject, however, to all of the terms
and provisions of this Lease. The Landlord warrants that, as Trustee of Y-CEE
INVESTMENT TRUST, u/d/t dated January 12, 1979 and recorded with the Middlesex
(Northern District) Registry of Deeds at Book 2348, Page 327, that he has the
right and lawful authority to enter into this Lease and further, that the
Landlord is the owner of the fee in the Property. In connection herewith, the
Landlord, upon request, shall furnish to the Tenant evidence sufficient to
establish that it owns the Premises, including a list of encumbrances, if any,
perfected and recorded against the same.

18.  Tenant's Obligation to Quit:
     ---------------------------

     The Tenant shall, upon expiration of the Term or other termination of this
Lease, leave and peaceably and quietly surrender and deliver to the Landlord the
Premises and any alterations, additions and improvements which have been made by
the Tenant to the Premises, and any replacements thereof in the order, condition
and repair required by Paragraph 10 hereof and the other provisions of this
Lease, except, however, that the Tenant shall first remove any trade fixtures
and equipment and any alterations, additions and improvements which the Landlord
has required be removed pursuant to the terms of Paragraph 12 hereof and Tenant
if so requested by the Landlord pursuant to the terms of Paragraph 12 hereof,
restoring the Premises in each case to substantially their condition prior to
the installation of such fixtures or the undertaking of such alterations,
additions or improvements, as the case may be reasonable wear and tear and
casualty excepted. The provisions of this Paragraph 18 shall expressly survive
the termination or expiration of the Term of this Lease.

                                      -31-
<PAGE>

19.  Transfers of Tenant's Interest:
     ------------------------------

     A.  Subletting: Tenant shall have the right, subject to approval of the
Landlord which consent shall not be unreasonably withheld, conditioned or
delayed, to sublet all or any portion of the Premises or grant a license therein
provided: (i) Tenant is not in default of the Lease beyond any applicable cure
period (ii) Tenant provides Landlord with prior written notice of the Sublease
or license, at least thirty (30) days prior to the commencement date of the
sublease or license (iii) Tenant delivers to Landlord an executed copy of the
sublease or license by the commencement date of the sublease or license (iv)
Tenant remains liable under the Lease (v) if the Basic Rent and Additional Rent
charged by the Tenant to the Subtenant under the Sublease should exceed the
amount of Basic Rent and Additional Rent the Tenant is required to pay the
Landlord under the Lease, the Tenant agrees to share any excess proceeds equally
with the Landlord, 50%-50%, after deducting Tenant's reasonable costs (subject
to approval by the Landlord, such approval not be unreasonably withheld) for
attorney's fees, brokerage commissions and Tenant's improvements associated with
the Sublease of the Premises (vi) Tenant reimburses Landlord for all reasonable
legal fees incurred in reviewing all documents related to the Sublease.

     Notwithstanding the provisions of (S)26 hereafter, Landlord shall, at
Tenant's Request, provide Tenant with (i) an Estoppel Certificate stating
whether Landlord knows of any Default under the Lease at the time of the
proposed subletting (or assignment as hereinafter provided).

                                      -32-
<PAGE>

     B.  Assignment:  Should the Tenant desire to assign its interest in the
Lease to a third party, it shall be required to notify the Landlord of the same
in writing and to furnish the Landlord timely any and all materials it might
reasonably request concerning the proposed Assignee's financial condition and a
detailed description of such entity's business operations and proposed use of
the Premises. The Landlord shall be required to notify the Tenant in writing of
its decision whether to agree to or reject the proposed Assignment within
fourteen (14) days of its receipt of all documentation it might request be
supplied to it from either the Tenant and/or proposed assignee. In connection
therewith, Landlord shall not unreasonably withhold, condition or delay its
consent, provided that such Assignee assumes the terms of this Lease by
execution of a separate writing, the Tenant shall remain liable to the Landlord
for all covenants contained in the Lease, notwithstanding assignment of the same
to a third party, and provided Tenant shall reimburse Landlord for all
reasonable legal fees incurred in reviewing all documents related to such
assignment.

     Any attempted assignment without the consent of the Landlord and without an
executed Assumption Agreement between it, the Tenant and the Assignee, shall be
deemed void.

     If the Tenant shall (other than as a part of a sale of its entire business)
receive any consideration for the assignment as described aforesaid, the Tenant
shall share proceeds of the same equally with the Landlord, 50%-50%, after
deducting Tenant's reasonable costs (subject to approval by the Landlord, such
approval not to be unreasonably withheld) for attorney's fees and brokerage
commissions.

     Notwithstanding anything to the contrary, the Landlord shall not withhold,
its consent to an assignment by the Tenant, upon reasonable notification to the
Landlord, to a subsidiary, parent or affiliate of Tenant or to a successor to
Tenant by means of merger, consolidation,

                                      -33-
<PAGE>

corporate reorganization or the purchase of all or substantially all of the
Tenant's assets on condition such Assignee shall demonstrate its financial
ability to comply with all terms under the Lease including but not limited to
the fact that its net worth is not less than that of the Tenant and the
transferee assumes the terms of this Lease by execution of a separate writing.
In addition thereto, any transfer, pledge, sale or other disposition and/or
power to vote the outstanding shares of less than the majority of corporate
stock of the Tenant shall not be deemed an Assignment. In the event of any
Assignment under this clause, Tenant shall remain liable under the provisions of
the Lease jointly and severally with Assignee.

20.  Transfers of Landlord's Interest:
     --------------------------------

     The Landlord shall have the right from time to time to sell or mortgage its
interest in the Building and Property, to assign its interest in this Lease, or
to assign from time to time the whole on any portion of the Basic Rent,
Additional Rent or other sums and charges at any time paid or payable hereunder
by the Tenant to the Landlord, to any Mortgagees or other transferees designated
by the Landlord (or, in the case of any purchasers at foreclosure sales or
grantees under deeds in lieu of foreclosure, by any Mortgagee) by duly recorded
instruments and after receipt of copies of such instruments, the Tenant shall
pay the Basic Rent, Additional Rent and such other sums and charges so assigned,
subject to the terms of the Lease, upon demand to such Mortgagees and other
transferees, purchasers and grantees at the addresses mentioned in and in
accordance with the terms of such instruments.  Each foreclosing Mortgagee or
other transferee, purchaser or grantee shall, in writing, assume the obligations
of the Landlord under this Lease, subject, however, to the limitations upon
liability of the Landlord as set forth in Paragraph 27.

     Within ten (10) days following any transfer by Landlord of its ownership
interest in the property, Landlord shall provide Tenant with written notice of
such transfer and the name and

                                      -34-
<PAGE>

address of the successor Landlord to whom Tenant should send future rent
payments and notices (the "Transfer Notice"). In the event that the Landlord
fails to provide the Transfer Notice, (a) the Tenant shall not be liable to any
successor Landlord for rent payments paid to the former (predecessor) Landlord
and (b) any successor Landlord shall be bound by any notice sent to the former
(predecessor) Landlord by the Tenant.

21.  Mortgagees' Rights:
     ------------------

     The Tenant hereby agrees that this Lease is and shall be subject and
subordinate to any mortgage (and to any amendments, extensions, increases,
refinancings or restructurings thereof) of the Premises, whether or not such
mortgage is filed subsequent to the execution, delivery or the recording of this
Lease or any notice hereof (the holder from time to time of any such mortgage
being in this Lease sometimes called the "Mortgagee"). The foregoing
subordination shall be self-operative and automatically effective as to any
mortgage filed subsequent to the execution and delivery hereof but only if
either the Mortgagee agrees in a recordable writing or such mortgage provides
that, for so long as there exists no Terminable Default under this Lease on the
part of the Tenant, the Mortgagee, in foreclosing against or taking possession
of the Premises or otherwise exercising its rights under such mortgage, will not
join the Tenant in any foreclosure proceedings (except to the extent required by
law) and will not terminate this Lease (except as provided herein) or disturb
the Tenant's possession of the Premises hereunder in customary form or words of
similar import and will make insurance proceeds available as and to the extent
provided in Paragraph 8. The Tenant hereby agrees to execute, acknowledge and
deliver in recordable form such instruments confirming and evidencing the
foregoing subordination as the Landlord or any such Mortgagee may from time to
time reasonably require. Failure by the Tenant to comply with the provisions
contained in the preceding sentence shall be deemed an event of Terminable
Default under this Lease.

                                      -35-
<PAGE>

     No notice from the Tenant of any default by the Landlord in its obligations
shall be valid, and the Tenant shall not attempt to terminate this Lease,
withhold Basic Rent or Additional Rent or exercise any other remedy which may
arise by reason of any such default, unless the Tenant first gives such notice
to all Mortgagees of whom Tenant has been given notice and provides such
Mortgagees with the same period(s) for cure as are available to the Landlord
after such notice within which to cure such default.  The Tenant shall and does
hereby agree, upon default by the Landlord under any mortgage, to attorn to and
recognize the Mortgagee or anyone else claiming under such mortgage, including a
purchaser at a foreclosure sale, at it request as successor to the interest of
the Landlord under this Lease, to execute, acknowledge and deliver in recordable
form such evidence of this attornment, which shall nevertheless be self-
operative and automatically effective, as the Mortgagee or such successor may
request and to make payments of Basic Rent and Additional Rent hereunder
directly to the Mortgagee or any such successor, as the case may be, upon
request. In such event, the Tenant shall not be liable to the Landlord for any
payment made to such Mortgagee.  By any such request, such Mortgagee or
successor shall be deemed and construed without further agreement to have
assumed and agreed to carry out and perform all covenants and obligations of the
Landlord under this Lease thereafter arising, subject, however, to the
provisions of Paragraph 27. Any Mortgagee may, at any time, by giving written
notice to and without any further consent from the Tenant, subordinate its
mortgage to this Lease, and thereupon the interest of the Tenant under this
Lease shall automatically be deemed to be prior to the lien of such mortgage
without regard to the relative dates of execution, delivery or recording thereof
or otherwise.

                                      -36-
<PAGE>

     In connection herewith, the Tenant acknowledges that any Mortgagee reserves
the right to use whatever reasonable format of a non-disturbance agreement it
might elect to employ and therefore the Landlord does not warrant or otherwise
represent what the precise provisions of that instrument might be or what the
same might provide. Notwithstanding the foregoing, any Subordination and
Nondisturbance Agreement prepared by the Mortgagee and presented to the Tenant
for execution shall include a provision that the Tenant's possession shall not
be disturbed and the Mortgagee shall abide by the terms and conditions of this
Lease if the Tenant is then in compliance with the terms of this instrument [to
wit: not in default beyond applicable notice and cure periods] and continues to
abide by the terms and conditions of the same. The costs of recording any such
non-disturbance agreement shall be borne by the Tenant.

22.  Tenant's Default; Landlord's Remedies:
     -------------------------------------

     If (a) the Tenant shall default in the payment of any Basic Rent or
Additional Rent and such default shall continue for fourteen (14) days after
notice from the Landlord of such default without cure; or (b) the Tenant shall
default in the performance or observance of any of the other covenants contained
in this Lease and on the Tenant's part to be performed or observed and shall
fail, within thirty (30) days after notice from the Landlord of such default to
cure such default, or if it is beyond the reasonable control of the Tenant to
cure such default within thirty (30) days, promptly to commence such cure and
thereafter to pursue such cure diligently to completion; or (c) if the estate
hereby created shall be taken on execution, or by other process of law, or if
the Tenant shall be involved in financial difficulties as evidenced:

     1.  by its commencement of a voluntary case under Title 11 of the United
States Code as from time to time in effect, or by its authorizing, by
appropriate proceedings of trustees or other governing body, the commencement of
such voluntary case;

                                      -37-
<PAGE>

     2.  by its filing an answer or other pleading admitting or failing to deny
the material allegations of a petition filed against it commencing an
involuntary case under said Title 11, or seeking, consenting to or acquiescing
in the relief therein provided, or by its failing to controvert timely the
material allegations of any such petition;

     3.  by the entry of an order for relief in any involuntary case commenced
under said Title 11;

     4.  by its seeking relief as a debtor under any applicable law, other than
said Title 11, of any jurisdiction relating to the liquidation or reorganization
of debtors or to the modification or alteration of the rights of creditors, or
by its consenting to or acquiescing in such relief;

     5.  by the entry of an order by a court of competent jurisdiction (i)
finding it to be bankrupt or insolvent, (ii) ordering or approving its
liquidation, reorganization or any modification or alteration of the rights of
its creditors, or (iii) assuming custody of, or appointing a receiver or other
custodian for, all or a substantial part of its property; or

     6.  by its making an assignment for the benefit of, or entering into a
composition with, its creditors, or appointing or consenting to the appointment
of a receiver other custodian or all or a substantial part of its property;

     then, and in any of said cases (S)(S)(a), (b), or (c), each of which,
subject to the following sentence is herein sometimes called a ("Terminable
Default"), the Landlord may, to the extent permitted by law, immediately or at
any time thereafter and without demand or notice, terminate this Lease and enter
onto and upon the Premises, or any part thereof, repossess the same as the
Landlord's former estate, and expel the Tenant and those claiming through or
under the Tenant and remove its effects without being deemed guilty of any
manner of trespass, and without prejudice to any remedies which might otherwise
be used for arrears of rent or preceding breach of covenant.

                                      -38-
<PAGE>

     No termination or repossession provided for in this Paragraph 22 shall
relieve the Tenant, of its obligations and liabilities under this Lease, all of
which shall survive any such termination or repossession. In the event of any
such termination or repossession, the Tenant shall pay to the Landlord either
(i) in advance on the first day of each month, for what would have been the
entire balance of the Term, one-twelfth (1/12) (and a pro rata portion thereof
for any fraction of a month) of the Basic Rent, Additional Rent and all other
amounts for which the Tenant is obligated hereunder, less, in each case, the
actual net receipts by the Landlord by reason of any reletting of the Premises
after deducting the Landlord's reasonable expenses in connection with such
reletting, including, without limitation, removal, storage and repair costs and
reasonable broker's and attorneys' fees, or (ii) at the option of the Landlord
exercisable by the Landlord's giving notice to the Tenant within thirty (30)
days after any such termination, the present value of the amount by which the
payments of Basic Rent and the Additional Rent reasonably estimated to be
payable for the balance of the Term after the date of the exercise of said
option would exceed the payments reasonably estimated to be the fair rental
value of the Premises on the terms and conditions of this Lease over such
period, determined as of such date. In connection with reletting the Premises as
provided for herein, Landlord agrees to use all reasonable efforts in connection
with the same.

     Without thereby affecting any other right or remedy of the Landlord
hereunder, in the event of (i) any default on the part of the Tenant in the
performance or observance of any non-monetary covenant contained in this Lease
and on the Tenant's part to be performed or observed and the failure of the
Tenant, within thirty (30) days after notice from the Landlord of

                                      -39-
<PAGE>

such default, to cure such default or if it is beyond the reasonable control of
the Tenant to cure such default within thirty (30) days or promptly to commence
such cure and thereafter to pursue such cure diligently to completion or (ii)
any default on the part of the Tenant which results in jeopardy to the
Landlord's title to the Premises or to the Landlord's interest under any
mortgage of the Premises and which remains uncured for two (2) business days
after notice of such default from the Landlord to the Tenant (or if it is beyond
the reasonable control of the Tenant to cure such default within two (2)
business days, if the Tenant shall not promptly commence such cure and
thereafter diligently pursue such cure to completion) or (iii) imminent danger
of injury to persons or damage to property as a result of any default on the
part of the Tenant as to which the Landlord has given telephone notice to the
Tenant, or (iv) any Terminable Default on the part of the Tenant hereunder, then
in any of such events the Landlord may, at its option, cure such default or
Terminable Default for the Tenant's account and the cost to the Landlord of such
cure, together with interest thereon at the Lease Interest Rate, shall be deemed
to be Additional Rent and shall be paid to the Landlord by the Tenant with the
installment of Basic Rent next accruing.

  If the Landlord shall be required to commence proceedings to enforce its
remedies as provided for herein, it shall, in addition to damages, be entitled
to receive all its costs and its reasonable attorney's fees incurred by it in
any such action.

23.  Remedies Cumulative; Waivers:
     ----------------------------

     The specific remedies to which the Landlord or Tenant may resort under the
terms of this Lease are cumulative and are not intended to be exclusive of any
other remedies of means of redress to which the Landlord or Tenant may be
lawfully entitled under any provision of this Lease or otherwise.  The failure
of the Landlord or the Tenant to insist in any one or more cases upon the strict
performance of any of the covenants of this Lease shall not be construed as a

                                      -40-
<PAGE>

waiver or relinquishment for the future of such covenant. A receipt by the
Landlord, or payment by the Tenant, of Basic Rent or Additional Rent with
knowledge of the breach of any covenant hereof shall not be deemed a waiver of
such breach, and no waiver, change, modification or discharge by the Landlord of
the Tenant of any provision in this Lease shall be deemed to have been made or
shall be effective unless expressed in writing and signed by an authorized
representative of the Landlord or the Tenant as appropriate.  In addition to the
other remedies provided for in this Lease, the Landlord and the Tenant shall be
entitled to the restraint by injunction of the covenants, conditions or
provisions of this Lease, or to a decree compelling performance of or compliance
with any of such covenants, conditions or provisions.  In connection herewith,
the prevailing party, should litigation be initiated, shall be entitled to an
award of its costs and its reasonable attorney's fees.

24.  Notices:
     -------

     Any notices, requests, approvals, specifications, demands or consents
required or permitted hereunder shall be in writing and mailed, postage prepaid,
by registered or certified mail, return receipt requested, if to the Landlord or
to the Tenant at the address set forth herein for each, with copies of such
notice(s) sent to respective counsel for both the Landlord and Tenant, as
follows: if to the Landlord, Robert L. Marder, Esquire, 23 Central Avenue, Suite
408, Lynn, MA 01901; if to the Tenant to: Avici Systems, Inc., 101 Billerica
Avenue, Building 3, North Billerica, MA 01862, Attention: Contracts Manager; if
to any Mortgagee, at such address as it may specify by such notice to the
Landlord and the Tenant, or at such other address as any of them may from time
to time specify by like notice to the others.  When this Lease provides for any
period to commence after notice, such period shall be deemed to commence one day
after postal records indicate   delivery of such notice was first sent as
attested to by Post Office Stamped Receipt. When this Lease requires that notice
be given on or before a certain date or within a certain period, such notice
shall be deemed given on the date mailed as in this Paragraph 24 provided.  Time
shall be of the essence of all notice provisions of this Lease.

                                      -41-
<PAGE>

25.  Recording:
     ---------

     The Landlord and the Tenant hereby agree, each at the request of the other
or of any Mortgagee, promptly to execute, acknowledge and deliver a recordable
form or notice of lease, notices of any assignments of rents and profits,
notices of any amendments to this Lease and of such other information as may
from time to time be necessary under G.L. (Ter. ed.) Chapter 183, Section 4 or
under similar additional or successor statutes for the protection of any
interest of the Landlord, the Tenant or any Mortgagee having a perfected
interest in the Premises. The party which seeks preparation of any such
document(s) listed herein shall bear the cost(s) of recording the same.

26.  Estoppel Certificates:
     ---------------------

     The Landlord and the Tenant hereby agree from time to time, but not more
than twice during any calendar year, each after not less than twenty (20) days
prior written notice from the other or any Mortgagee, to execute, acknowledge
and deliver, without charge, to the other party, the Mortgagee or any other
person designated by the other party, a statement in writing certifying: that
this Lease is unmodified and in full force and effect (or if there have been
modifications, identifying the same by the date thereof and specifying the
nature thereof); that to the knowledge of such party there exist no defaults (or
if there be any defaults, specifying the same); the amount of the Basic Rent,
the dates to which the Basic Rent, Additional Rent and other sums and charges
payable hereunder have been paid; and that such party to its knowledge has no
claims against the other party hereunder except for the continuing obligations
under this Lease (or if such party has any such claims, specifying the same).

                                      -42-
<PAGE>

27.  Hazardous Materials and Compliance with Environmental Laws:
     ----------------------------------------------------------

     A.  Landlord and Tenant shall at their own expense promptly observe and
comply with all preset and future Environmental Laws including without
limitation the Clean Air Act Amendments of 1990 and any regulations (as amended)
and all regulations and standards as are or may be promulgated thereunder.

     B.  Without limiting the foregoing, Landlord represents that it, its
agents, servants, contractors and employees have (i) operated the Property
Premises and Building and have at all times received, handled, used, stored,
treated, transported and disposed of any chemical, material or substance,
exposure to which is prohibited, limited or regulated by any federal, state,
country, regional, or local authority or which even if not so prohibited,
limited or regulated, poses a hazard to the health and safety of the occupants
of the Property, Premises and Building (collectively "Hazardous Materials") in
strict compliance with Environmental Laws and (ii) removed (or will remove prior
to the Commencement Date) from the Property all Hazardous Materials.

     C.  Landlord represents it has received no notices of any violation or
claimed violation of any of the matters referred to above including notices
related to any pending investigation, lawsuit or other action relating thereto.
In addition thereto, Landlord hereby agrees to indemnify and hold Tenant
harmless from and against any and all "Remediation Costs" (as hereafter defined)
sustained or incurred by Tenant in the event that Tenant is required by any
state or federal agency to affect a Remediation of any Hazardous Substances
which may be located on the Property and the Building as of the date of this
Lease, unless such Hazardous Substances are present or released as the result of
the acts or omissions of Tenant or any of Tenant's agents, servants, employees,
contractors or invitees.  If such Hazardous Substances are present or release as
a direct result of the Tenant, or Tenant's agents, servants, employees,
contractors or invitees, Tenant shall indemnify and hold Landlord harmless from
and against any and all Remediation Costs.

                                      -43-
<PAGE>

     As used herein, the term "Remediation Costs" shall mean the cost of
remediation and clean-up of the Hazardous Substances which Tenant may incur as
the result of any order of the DEP, the U.S. Environmental Protection Agency or
any State or Federal Court of competent jurisdiction requiring that Tenant
affect a remediation of any Hazardous Substances.

     D.  Landlord and Tenant shall defend, indemnify and hold one another
harmless, including the respective employees, agents, officers, and directors
from and against any claims, demands, penalties, fines, liabilities;
settlements, damages, costs or expenses (including attorney's fees) arising out
of or in any way related to the breach of their respective representations,
warranties and covenants contained in this Section 27 including without
limitation liability arising out of or in any way related to hazardous materials
permitted or suffered by either or by any of their respective employees, agents,
officers, directors or invitees at the Premises or the soil, water, vegetation,
buildings, personal property,   persons, animals or otherwise and any personal
injury (including wrongful death) or property damage arising out of or related
to such hazardous materials.

     E.  The provisions of this S27 shall survive the expiration or earlier
termination of this Lease.

                                      -44-
<PAGE>

28.  Other Utilities:
     ---------------

     Subject to approval from the Landlord, which approval shall not be
unreasonably withheld, conditioned or delayed, the Tenant, at its sole cost and
expense, shall have right to introduce into the Premises such other utilities as
it may be deem necessary for the conduct of its business. The Tenant shall pay
the cost of all such other utilities directly to the applicable utility
supplier.

29.  Option to Extend Term:
     ---------------------

     Provided that Tenant is not in default under this Lease after notice
thereof and beyond any applicable grace/cure period, then, in such event, the
Tenant shall have the option to extend the term for two (2) five (5) year
periods ("Option 1 and Option 2") by notifying the Landlord not later than two
hundred seventy (270) days prior to the expiration of the Term, as defined in
(S)4 above, or, if applicable, 270 days prior to the expiration of Option 1 of
its intent to do so.

     During the term of Option 1, the Tenant shall pay to the Landlord, as
Annual Basic Rent, the then fair market rental value of the Premises, but in no
event shall the Basic Rent for Option 1 be less than Thirteen and 00/100
($13.00) Dollars per square foot.

     During the term of Option 2, the Tenant shall pay to the Landlord, as
Annual Basic Rent, the then fair market rental value of the Premises, but in no
event shall the Basic Rent for Option 2 be less than the Basic Rent paid for
Option 1.

     Notwithstanding anything to the contrary, all obligations of the Tenant
with respect to Additional Rental as defined in (S)6 above and in other
applicable portions of this instrument, as well as all terms, conditions and
covenants of this Lease, shall be applicable to the Option Period without
necessity that any separate writing setting forth the same be executed by the
parties.

     If within thirty (30) days after receipt of written notice by the Tenant of
its intent to extend the term of the Lease or to exercise the right for Option 2
the Landlord and Tenant shall

                                      -45-
<PAGE>

be unable to agree upon the amount of the ten fair market rental for the
Premises to wit: the annual rate of Basic Rent which the Landlord proposes be
charged for either Option 1 or Option 2, then, in such event, (i) the Tenant may
notify the Landlord in writing within said thirty (30) day period that it is
withdrawing its intent or extend the term of the Lease, in which event, the
Lease shall end either at the time set forth in (S)4 above or at the end of
Option 1, if applicable, (ii) the Landlord and Tenant may elect arbitration as
follows: within ten (10) days the Landlord and Tenant shall each select an
appraiser and those two shall within ten (10) days select a third. The third
appraiser shall thereupon, within thirty (30) days of his selection, issue a
written report to the Landlord and Tenant informing each of the annual rate of
Basic Rent as determined by two of the three appraisers for the Option Period.
The decision of the appraisers referred to in the preceding sentence shall be
binding upon the parties (subject to the other terms of the Lease and payment of
Additional Rent as described aforesaid).

     Each party shall pay fees and costs for its own appraiser and shall pay one
half (1/2) of those fees and costs charged by the third appraiser.

     For purposes of this clause, an "appraiser" shall be an individual who has
an "MAI" (Member of the Appraisal Institute) designation for a minimum of five
years and who is independent of both the Landlord and Tenant and the brokers,
advisors and mortgagees or either of them.  Such appraiser shall also be an
individual with at least five years experience in appraising commercial and
industrial property in the Greater Boston Area.

     Within five days after decision of the Appraisers related to Basic Rent to
be applicable for either Option Period, or, in the alternative, within five days
of Agreement between the Landlord and Tenant of Basic Rent to be charged for the
applicable Option Period, whichever first occur, the Landlord and Tenant shall
execute an Addendum to the Lease incorporating thereto the amount of Basic Rent
to be paid for the applicable Option Period.

                                      -46-
<PAGE>

30.  Security Deposit:
     ----------------

     Within fifteen (15) days from the execution of this Lease, the Tenant shall
pay the Landlord the sum of Two Hundred Forty Three Thousand Two Hundred Fifty
Eight and 00/100 ($243,258.00) (the same equal to three (3) months of Basic
Rent) as a security deposit.  The security deposit shall be an Irrevocable
Letter of Credit, subject to approval of the Landlord as to form and content,
issued in favor the Landlord from a recognized banking institution and shall be
in the sum set forth in the preceding paragraph. In connection therewith, the
Landlord agrees to furnish to the bank against which the Letter of Credit is
drawn its Tax Identification Number, should the same be required.

     Upon termination of the Lease, and after inspection of the Demised Premises
by the Landlord to assure the Tenant has caused no damage (reasonable wear and
tear excepted as provided for aforesaid) to the leased area, the Landlord shall
notify the bank against which the Letter of Credit is drawn that it is negating
the same. If damage to the Premises shall have been caused by the Tenant, the
Landlord shall be entitled to exercise its rights under the aforesaid Letter of
Credit in order to receive proceeds from which it might repair the same. If the
security deposit shall be insufficient to reimburse the Landlord for affecting
such repairs, the Tenant shall be liable for any excess expenses incurred by the
Landlord to complete said repairs.

31.  Brokerage:
     ---------

  The Landlord warrants and represents that it has dealt with no broker in
connection with the consummation of this lease except for Meredith & Grew
Incorporated (the "Broker"), and in

                                      -47-
<PAGE>

the event of any brokerage claims against the Tenant predicated upon prior
dealings with the Landlord named herein, the Landlord agrees to defend the same
and indemnify the Tenant against any such claim. The Tenant warrants and
represents that it has dealt with no broker in connection with the consummation
of this Lease except for Meredith & Grew Incorporated, and in the event of any
other brokerage claims against the Landlord predicated upon prior dealings with
the Tenant named herein, the Tenant agrees to defined the same and indemnify the
Landlord against any such claim. Landlord shall be responsible for any
commission due to the Broker in connection with this Lease.

32.   Additional Provisions:
      ---------------------

     A.  The Tenant shall have 24 hour access to the Premises and parking
facilities each day of the year.

     B.  Landlord covenants and agrees that in exercising any of its rights and
performing its obligations, set forth herein, Landlord shall use commercially
reasonable efforts to minimize its inconvenience and interference with Tenant's
business and operations and use and enjoyment of the Premises and shall not
cause or permit any obstruction or diminution of the Premises or access or
egress thereto or therefrom.

     C.  Landlord shall hold in confidence all financial information obtained
from Tenant or Tenant's records, except in respect of the normal kind of
disclosure which in the regular course would be made to prospective bona fide
mortgagees an purchasers of the Building.

     D.  All of Tenant's signs and directory listings shall be installed by the
Tenant in the foyer/lobby, but not the exterior of the Building without prior
approval of the Landlord. Installation of any exterior sign shall be subject to
compliance by the Tenant to any and all applicable zoning ordinance or by law
promulgated by the Town of Tewksbury related to the same, if any there may be.

                                      -48-
<PAGE>

     E.  If requested by Landlord in connection with either financing or sale of
the Premises, the Tenant shall furnish to Landlord or to any holder of a
mortgage (or perspective mortgagee) on the Premises, as Landlord may designate
by notice to Tenant, a copy of Tenant's and Guarantor's most recent annual
report(s) and/or quarterly reports.

33.  Bind and Inure; Limited Liability of Landlord:
     ---------------------------------------------

     All of the covenants, agreements, stipulations, provisions, conditions and
obligations herein expressed and set forth shall be considered as running with
the land and shall extend to, bind and inure to the benefit of the Landlord and
the Tenant, which terms as used in this Lease shall include their respective
successors and assigns where the context hereof so admits.

     The Landlord shall not have any individual or personal liability for the
fulfillment of the covenants, agreements and obligations of the Landlord
hereunder, the Tenant's recourse and the Landlord's liability hereunder being
limited to the Premises, Property and Building. In connection therewith, the
Landlord agrees it shall not cause the Premises, Property or Building to be
mortgaged beyond 85% of its fair market value determined by an Appraisal to be
performed by an applicable lender.  The term "Landlord" as used in this Lease
shall refer only to the owner or owners from time to time of the Premises who
shall be deemed and construed without further agreement to have assumed and
agreed to carry out and perform all covenants and obligations of the Landlord
arising under this Lease during the period of such ownership (and, except in the
case of foreclosing Mortgagees, purchasers at foreclosure sales, grantees under
deeds in lieu of foreclosure and their successors and assigns, prior to such
period if not carried out and performed by the prior owner), subject, however,
to the provisions of this Paragraph 31, it being understood

                                      -49-
<PAGE>

that no such owner shall have any liability hereunder for matters arising from
and after the date such owner ceases to have any interest in the Premises. In no
even shall the Landlord be liable to the Tenant for any special consequential or
indirect damages suffered by the Tenant or any other person or entity by reason
of a default by the Landlord under any provisions of this Lease. Likewise, the
Tenant shall not be liable to the Landlord for any special, consequential or
indirect damages suffered by the Landlord or any other person or entity by
reason of a default by the Tenant under any provisions of this Lease.

34.   Authority:
      ---------

     The Landlord represents he has been specifically authorized by the
Beneficiaries of Y-CEE INVESTMENT TRUST to execute this instrument.
     The Tenant represents its duly authorized officer, identified hereafter,
has been authorized to execute this instrument by the Board of Directors of
Avici Systems, Inc.

35.  Captions:
     --------

     The captions for the numbered Paragraphs of this Lease are provided for
reference only and they do not constitute a part of this agreement or any
indication of the intentions of the parties hereto.

36.  Integration:
     -----------

     The parties acknowledge that all prior written and oral agreements between
them and all prior representations made by either party to the other have been
incorporated in this instrument or otherwise satisfied prior to the execution
hereof.

37.  Severability; Choice of Law, and Forum  :
     --------------------------------------

     If any provision of this Lease shall be declared to be void or
unenforceable either by law or by a court of competent jurisdiction, the
validity or enforceability of remaining provisions shall not thereby be
affected.

                                      -50-
<PAGE>

     This Lease is made under, and shall be construed in accordance with, the
laws of The Commonwealth of Massachusetts.

     In addition thereto, and should either the Landlord or Tenant initiate
litigation to seek enforcement of the terms, conditions and/or covenants of the
within Lease, then in such event the parties agree that Jurisdiction, other than
for Summary Process proceedings, shall vest solely and exclusively with Essex
County Superior Court Department for the Commonwealth of Massachusetts, each
signatory hereafter having agreed to the within forum selection clause.

     IN WITNESS WHEREOF, the parties hereto have caused this instrument to be
executed in multiple counterparts under seal as of the date first above written.

                              LANDLORD
                              YVON CORMIER
                              TRUSTEE OF Y-CEE INVESTMENT TRUST

                              /s/ Yvon Cormier
                              ------------------------------------------
                              Yvon Cormier, as Trustee as
                              aforesaid, and not individually

                              TENANT
                              AVICI SYSTEMS, INC.
                              By:

                              /s/ EPL Surya Panditi
                              ----------------------------------------------

                                      -51-

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