Document:

<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                                                   Exhibit 10.22

                   SECOND AMENDMENT TO COST SHARING NATIONAL
                                 IRU AGREEMENT

     THIS SECOND AMENDMENT TO COST SHARING NATIONAL IRU AGREEMENT ("Amendment")
is made and entered into as of the 30th day of March, 2000 (the "Amendment
Effective Date"), by and between LEVEL 3 COMMUNICATIONS, LLC, a Delaware limited
liability company ("Grantor") and SPLITROCK SERVICES, INC., a Delaware
corporation ("Grantee").  This Amendment modifies and amends that certain Cost
Sharing National IRU Agreement dated April 26, 1999 between Grantor and Grantee
(the  "Agreement").  Capitalized terms used but not defined herein shall have
the meanings set forth in the Agreement.

                                    RECITALS
                                    --------

     A.  Grantee desires to obtain an IRU in fibers within and along new
Segments and additional fibers within and along existing Segments of the Grantor
System.

     B.  Grantor desires to convey to Grantee an IRU in the fibers specified
herein, subject to and in accordance with the terms and provisions set forth in
this Amendment and the Agreement.

     C.  Grantee desires to revise the pricing and space amounts that will be
allocated to Grantee by Grantor for the placement of Grantee's electronic and
optronic equipment in the Facilities.

     D.  Grantor desires to revise the segregated space amounts for the
Facilities as specified herein, subject to and in accordance with the terms and
provisions set forth in this Amendment.

     E.  Grantor and Grantee desire to revise certain Exhibits of the Agreement.
                                                      --------

                               TERMS OF AMENDMENT
                               ------------------

     Accordingly, in consideration of the foregoing and other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged,
Grantor and Grantee hereby agree as follows:

                                   ARTICLE 1
                                  NEW SEGMENTS
                                  ------------

     1.01  The following Segments are hereby added to the Grantor System, and
Grantor shall deliver an IRU in, and Grantee shall accept and pay for, an IRU in
between (***) Grantee Fibers within and along the following additional Segments
(the estimated Route Miles and Scheduled Completion Dates for each Segment are
also set forth below):

<TABLE>
<CAPTION>
Route Segment        City Pairs                  Estimated    Fiber Count   Price per      Estimated IRU        IRU Scheduled
                                                Route Miles                  mile          Contribution        Completion Date

-----------------------------------------------------------------------------------------------------------------------------------
<S>             <C>                           <C>                <C>          <C>              <C>             <C>
3b              Pittsburgh to Washington D.C      272              4         (***)            (***)                 (***)
-----------------------------------------------------------------------------------------------------------------------------------
3d              Cleveland to Pittsburgh           162              4         (***)            (***)                 (***)
-----------------------------------------------------------------------------------------------------------------------------------
11b             Houston to New Orleans            366              4         (***)            (***)                 (***)
-----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

CONFIDENTIAL DRAFT                                                        Page 1
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

      1.02  Grantee shall make the following payments with respect to the
additional Initial Fibers set forth in Section 1.01:

      a.    The IRU Contribution for the additional Initial Fibers shall be
(***) per Route Mile. The IRU Contribution shall be due from Grantee to Grantor
with respect to the additional Initial Fibers in each Segment as follows: (***).

      b.    The IRU Contribution for the additional Initial Fibers set forth in
Section 1.02(a) is unique to the additional Initial Fibers described in Section
1.01, and the Parties recognize that the IRU Contribution for Fibers requested
by Grantee in other Segments of Grantor's System where there are no Grantee
Fibers shall be subject to negotiation by the Parties.

      c.    The additional Initial Fibers designated above shall be considered
to be Grantee Fibers under the Agreement.

                                   ARTICLE 2
                               ADDITIONAL FIBERS
                               -----------------

      2.01  Grantee desires to add fiber to certain Segments ("Additional
Fibers") where Grantee initially was to acquire an IRU in only (***) Initial
Fibers.  Grantor shall provide Grantee with an IRU in the Additional Fibers on
the Scheduled Completion Dates specified below, and Grantee shall pay the
"Additional IRU Contribution" for each Segment of Additional Fibers:

<TABLE>
<CAPTION>
Route Segment       City Pairs             Estimated       Additional     Additional         Estimated          IRU Scheduled
                                          Route Miles       Fibers          IRU            Additional IRU      Completion Date
                                                                         Contribution       Contribution
                                                                          per Mile
-----------------------------------------------------------------------------------------------------------------------------------
<S>             <C>                      <C>                <C>             <C>              <C>                  <C>
N/A             Salt Lake City to Ogden        35              4            (***)              (***)                 (***)
-----------------------------------------------------------------------------------------------------------------------------------
12b             Ft. Worth to Dallas            30              8            (***)              (***)                 (***)
-----------------------------------------------------------------------------------------------------------------------------------
12d             Ft. Worth to Stratford        424              4            (***)              (***)                 (***)
-----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

      2.02  The Additional IRU Contribution shall be due from Grantee to Grantor
with respect to the Additional Fibers in each Segment as follows: (***). The IRU
Contribution for all other Initial Fibers shall remain the same.

      2.03   The Additional Fibers designated above shall be considered to be
Grantee Fibers under the Agreement.

      2.04   The Additional IRU Contribution set forth in Section 2.01 is unique
to the Additional Fibers described in Section 2.01, and the Parties recognize
that any IRU Contribution for Fibers requested by Grantee in other Segments
shall be subject to negotiation by the Parties (subject to the pricing for (***)
along the entire System Route as described in Article 4 of the Agreement).

                                   ARTICLE 3
                                FACILITIES SPACE
                                ----------------
      3.01  Section 7.03 of the Agreement shall be deleted and replaced as
            follows:

      In the event that Grantee elects to obtain segregated space within
      Grantor's Facilities, Grantor will, upon the written request of Grantee
      (which request must be delivered to Grantor within sixty (60) days after
      the Effective Date, or, if new Segments are added to this Agreement
      through amendment, within sixty (60) days after the Amendment Effective
      Date), provide Grantee during the Term with segregated space in the
      Facilities

CONFIDENTIAL DRAFT                                                        Page 2
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

      along the System Route for placement of Grantee's electronic and optronic
      equipment. Grantee shall be provided either approximately 144 square feet
      of space, 96 square feet of space, or 48 square feet of space in any
      Regeneration Facility or Opamp Facility and approximately 200 square feet
      in any Terminal Facility. Grantee shall have two (2) payment options with
      respect to segregated space, nonrecurring and recurring.

      Nonrecurring Payment.  Grantee may elect to pay to Grantor, the following
      one-time fees for each Facility to be occupied by Grantee along such
      Segment based upon the number of square feet of space to be provided to
      Grantee (collectively, the "Facilities Contribution"):

         Approximately 200 square feet (Terminal Facilities only):  (***)
         Approximately 144 square feet:                             (***)
         Approximately 96 square feet:                              (***)
         Approximately 48 square feet:                              (***)

      The Parties recognize that the above-referenced Facilities Contributions
      are approximations, and in the event that Grantee obtains segregated space
      that has a different actual square footage than such approximations then
      Grantee shall pay the exact amount to Grantor based on the following
      formulae: (***) per square foot for Regeneration Facilities and Opamp
      Facilities; (***) per square foot for Terminal Facilities. The Parties
      also recognize that the Facilities Contributions are for raw square
      footage only, and do not include the cost of any racks to be installed by
      Grantor for Grantee, if any.

      Recurring Payment. Grantee may elect to pay the Facilities Contribution on
      a monthly basis. The monthly fees shall be paid over (***) for designated
      space to be occupied by Grantee throughout the Term, and shall be
      calculated as follows:

             Regen Facilities             (***) per month/per square foot
             Opamp Facilities             (***) per month/per square foot
             Terminal Facilities          (***) per month/per square foot

      The monthly fees shall be due and payable in advance on the first day of
      each month during the (***) term.  Grantee shall give written notice to
      Grantor of its election to pay the Facilities Contribution.

      3.02  The first sentence of Section 7.04 of the Agreement shall be deleted
and replaced as follows:

     In the event that Grantee elects to obtain shared space in lieu of the
     segregated space described in Section 7.03, Grantor will, upon the written
     request of Grantee within sixty (60) days of the Effective Date, or, if new
     Segments are added to this Agreement through amendment, within sixty (60)
     days after the Amendment Effective Date), provide Grantee during the Term
     with shared space in Regeneration Facilities, Opamp Facilities and Terminal
     Facilities along the System Route for the placement of Grantee's electronic
     and optronic equipment; such space shall be sufficient for the placement of
     up to four (4) equipment racks or cabinets (depending upon configuration)
     in the Regeneration Facilities and Opamp Facilities and up to eight (8)
     equipment racks or cabinets in the Terminal Facilities.

CONFIDENTIAL DRAFT                                                        Page 3
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                   ARTICLE 4
                         REVISION TO GRANTOR SYSTEM MAP
                         ------------------------------

     The Exhibit "A" attached to the Agreement (Grantor System Map) is hereby
         -----------
deleted and in lieu thereof the Exhibit "A" attached hereto (Grantor System Map)
                                -----------
is hereby substituted and may be further amended by letter agreement between the
Parties, pursuant to Article 25.

                                   ARTICLE 5
                                   ---------
                  REVISION TO GRANTOR INTERCITY ROUTE SCHEDULE
                  --------------------------------------------

     The Exhibit "B" attached to the Agreement (Grantor Intercity Route
         -----------
Schedule) is hereby deleted and in lieu thereof the Exhibit "B" attached hereto
                                                    -----------
(Grantor Intercity Route Schedule) is hereby substituted and may be further
amended by letter agreement between the Parties, pursuant to Article 25.

                                   ARTICLE 6
                    REVISION TO CONSTRUCTION SPECIFICATIONS
                    ---------------------------------------

     The Exhibit "F" attached to the Agreement (Construction Specifications) is
         -----------
hereby deleted and in lieu thereof the Exhibit "F" attached hereto (Construction
                                       -----------
Specifications) is hereby substituted and may be further amended by letter
agreement between the Parties, pursuant to Article 25..

     IN WITNESS WHEREOF, Grantor and Grantee have executed this Amendment as of
the date first above written.

                                LEVEL 3 COMMUNICATIONS, LLC, a
                                        Delaware limited liability company

                                By     /s/ Jon Yount
                                  --------------------------------------------
                                Title:  Vice President
                                      ----------------------------------------
                                Date:    March 28, 2000
                                      ----------------------------------------

                                SPLITROCK SERVICES, INC.,
                                        a Delaware corporation

                                By /s/ Patrick J. McGettigan, Jr.
                                  --------------------------------------------
                                Title:  Senior Vice President
                                      ----------------------------------------
                                Date:     February 29, 2000
                                      ----------------------------------------

CONFIDENTIAL DRAFT                                                        Page 4
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                  EXHIBIT "B"
                              SEGMENTS/CITY PAIRS
                              -------------------
                        GRANTOR INTERCITY ROUTE SCHEDULE
                        --------------------------------

<TABLE>
<CAPTION>
Route Segment          City Pairs               Estimated    Fiber Count  Price per    Estimated IRU      IRU Scheduled
                                               Route Miles                   mile        fee/1/          Completion Date
-----------------------------------------------------------------------------------------------------------------------------------
             NORTHWEST DISTRICT
-----------------------------------------------------------------------------------------------------------------------------------
<S>             <C>                                  <C>                <C>          <C>              <C>                  <C>
4e           Las Vegas to Ogden                  476            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
             Salt Lake City to Ogden/2/           35            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
13a          Sacramento to Portland              643            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
13b          Portland to Seattle                 193            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
18a          Seattle to Boise                    550            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
18b          Boise to Ogden                      395            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
                             Regional Total    2,257                                      (***)
-----------------------------------------------------------------------------------------------------------------------------------
             N. CALIFORNIA DISTRICT
-----------------------------------------------------------------------------------------------------------------------------------
2a           San Jose to San Francisco            45            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
2b           San Jose to San Luis Obispo         198            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
2c           San Luis Obispo to Los Angeles      244            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
2d           San Francisco to Oakland             10            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
2e           Oakland to Sacramento                85            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
                             Regional Total      582                                      (***)
-----------------------------------------------------------------------------------------------------------------------------------
             PHOENIX DISTRICT
-----------------------------------------------------------------------------------------------------------------------------------
4a1          LA to San Bernardino                 67            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
4a2          San Bernardino to Las Vegas         229            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
8            LA to San Diego/3/                  165            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
12a          San Diego to Phoenix                355            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
12c          Phoenix to Santa Teresa (El Paso)   417            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
                             Regional Total    1,233                                      (***)
-----------------------------------------------------------------------------------------------------------------------------------
             DENVER DISTRICT
-----------------------------------------------------------------------------------------------------------------------------------
4b           St. Louis to Chicago                299            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
4c           Kansas City to St. Louis            302            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
4d           Omaha to Kansas City                215            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
4f           Ogden to Denver                     603            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
4g           Denver to Omaha                     554            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
12e          Santa Teresa (El Paso) to Stratford 471            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
14a          Denver to Stratford                 358            4          (***)          (***)               (***)
-----------------------------------------------------------------------------------------------------------------------------------
                             Regional Total    2,802                                      (***)
-----------------------------------------------------------------------------------------------------------------------------------
</TABLE>
/1/ Estimated IRU Contribution:  IRU fee to be determined by actual route miles
/2/ This segment only broken out due to change in fiber count
/3/ In the original IRU, this segment is broken down into LA to Orange and
    Orange to San Diego

CONFIDENTIAL DRAFT                                                        Page 5
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

<TABLE>
<CAPTION>

------------------------------------------------------------------------------------------------------------------------
<S>             <C>                          <C>      <C>            <C>       <C>    <C>
5c           Dallas to Memphis            514            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
5e           Memphis to Nashville         251            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
7            Dallas to Houston            247            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
11b          Houston to New Orleans       366            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
11c1         New Orleans to Tallahassee   481            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
12b          Ft. Worth to Dallas           30           12          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
12d          Stratford to Ft. Worth       424            8          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
19a          San Antonio to Houston       218            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
19b          San Antonio to Austin         79            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
19c          Austin to Ft. Worth          208            4          (***)         (***)                (***)
------------------------------------------------------------------------------------------------------------------------
                       Regional Tot al  2,818                       (***)
------------------------------------------------------------------------------------------------------------------------
             SOUTHEAST DISTRICT
------------------------------------------------------------------------------------------------------------------------
5d           Louisville to Cincinnati     126            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
5f           Nashville to Louisville      230            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
9b           Atlanta to Charlotte         253            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
9d           Charlotte to Raleigh         200            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
10a          Atlanta to Jacksonville      350            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
10b          Orlando to Miami             252            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
10c          Jacksonville to Orlando      141            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
11a          Tampa to Miami               300            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
11c2         Tallahassee to Tampa/5/      244            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
17           Nashville to Atlanta         381            8          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
                       Regional Total   2,477                                    (***)
------------------------------------------------------------------------------------------------------------------------
             NORTHEAST DISTRICT
------------------------------------------------------------------------------------------------------------------------
1a           Newark to New York            16            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
1b           Princeton to Newark           42            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
1c           Philadelphia to Princeton     61            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
1d           Wilmington to Philadelphia    37            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
6a           New York to Stamford/6/       45            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
6b           Providence to Boston          49            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
6d           Stamford to Hartford          96            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
6e           Hartford to Providence        81            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
15a          Cleveland to Buffalo         192            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
15b          Albany to Boston             175            4          (***)        (***)               (***)
------------------------------------------------------------------------------------------------------------------------
                       Regional Total     794                                    (***)
------------------------------------------------------------------------------------------------------------------------
</TABLE>
/4/ In the original IRU, this segment is combined with Tallahassee to Tampa to
    form N.O. to Tampa
/5/ In the original IRU, this segment is combined with New Orleans to
    Tallahassee to form N.O. to Tampa
/6/ In the original IRU, this segment is broken down into New York to White
    Plains and White Plains to Stamford

CONFIDENTIAL DRAFT                                                        Page 6
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

<TABLE>
<CAPTION>
             NEW YORK DISTRICT
---------------------------------------------------------------------------------------
<S>             <C>                          <C>    <C>      <C>        <C>      <C>
1e           Baltimore to Wilmington        79        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
1f           Washington DC to Baltimore     78        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
3a           Chicago to Detroit            324        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
3b           Pittsburgh to Washington DC   272        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
3c           Detroit to Cleveland          190        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
3d           Cleveland to Pittsburgh       162        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
5a           Chicago to Indianapolis       239        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
5b           Indianapolis to Cincinnati    114        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
9a           Richmond to Washington DC     120        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
9c           Raleigh to Richmond           175        4       (***)     (***)    (***)
---------------------------------------------------------------------------------------
                         Regional Total  1,727                          (***)
---------------------------------------------------------------------------------------
             E. CANADA DISTRICT
---------------------------------------------------------------------------------------
15c          Toronto to Buffalo            111        4       (***)     (***)   (***)
---------------------------------------------------------------------------------------
15d          Montreal to Albany            240        4       (***)     (***)   (***)
---------------------------------------------------------------------------------------
15e          Toronto to Ottawa             260        4       (***)     (***)   (***)
---------------------------------------------------------------------------------------
15f          Ottawa to Montreal            103        4       (***)     (***)   (***)
---------------------------------------------------------------------------------------
                       Regional Total      714                          (***)
---------------------------------------------------------------------------------------
---------------------------------------------------------------------------------------

                     Nationwide Total=  15,467
---------------------------------------------------------------------------------------
</TABLE>

CONFIDENTIAL DRAFT                                                        Page 7
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                  EXHIBIT "F"
                          CONSTRUCTION SPECIFICATIONS
                          ---------------------------

                                 1.1.  General
                                 -------------

The intent of this Exhibit is to delineate the specifications and standards for
construction of the Grantor System.  In the event any federal, state, local or
private agency having jurisdiction shall impose higher standards, Grantor will
comply and conform with such higher standards.

                                1.1.1.  Material
                                ----------------
 .  Steel casings shall be minimum 35,000 PSI.
 .  Any exposed steel conduit, brackets or hardware (i.e., bridge attachments)
   shall be galvanized.
 .  Hand holes shall have a minimum 20,000 pound loading rating with six to
   twelve inches (6 to 12") of cover.
 .  Manholes shall have a minimum H-20 loading rating.
 .  Fiber optic cable shall be single-armored.
 .  HDPE SDR 11 conduits shall be used for plowing and HDPE SDR 9 conduits shall
   be installed in rock areas.

                             1.1.2.  Minimum Depths
                             ----------------------
 .  Minimum cover required in the placement of conduit shall be forty-two inches
   (42"), except in the following instances:

 .  In drainage ditches, the Grantor System shall be placed with a minimum cover
   of forty-two inches (42") below the clean out elevation; unless the
   controlling authority requires additional depth in which case the greatest
   depth will be maintained.

 .  Where the Grantor System crosses railroads, the Grantor System shall be
   placed at a minimum depth of sixty inches (60") below the base of rail or
   sixty inches (60") below the paralleling drainage ditches, whichever is
   greater.

 .  Unless otherwise specified, where the Grantor System crosses gullies,
   ditches, streams, rivers, creeks, canals, and washes, the Grantor System will
   be placed at a minimum cover of forty-eight inches (48") below the bottom of
   the waterway unless the controlling authority requires additional depth in
   which case the greatest depth will be maintained. In navigable waterways, the
   Grantor System shall be placed a minimum of twenty feet (20') below the
   bottom of the waterway, or as detailed on the drawings or directed by the
   contractor.

 .  Where the Grantor System crosses existing facilities, including pipes,
   cables, or other structures, the Grantor System will be placed to maintain a
   minimum of twelve inches (12") clearance under the existing facility or a
   minimum clearance as required by the governing authorities or facility owner,
   whichever is greater. In the event that the top of sub-surface existing
   facility is greater than seventy-two inches (72") below grade, the Grantor
   System may be placed over the object while maintaining the twelve-inch (12")
   clearance if such action is not in conflict with permits and in accordance
   with the requirements of governing authorities and ROW owners. In all cases,
   the Grantor System shall be placed a minimum of forty-two inches (42") below
   grade.

        When rock is encountered at grade, the rock shall be excavated to
        ---------------------------------
        provide duct system cover of eighteen inches (18"). The Grantor System
        shall be installed by one of the following methods:

CONFIDENTIAL DRAFT                                                        Page 8
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

 .  The Grantor System shall be installed on a two-inch (2") bed of select
   backfill and covered in a one and one-half (1-1/2) sack concrete mix eight
   inches (8") thick. The remainder of the trench shall be backfilled per the
   requirements of Section 13.

 .  The Grantor System shall be installed on a two-inch (2") bed of select
   backfill and then covered by select backfill to two inches (2") above the
   Grantor System. A one-quarter-inch (1/4") steel plate will cap the select
   backfill and the remainder of the trench backfilled per the requirements of
   Section 13.

 .  The Grantor System shall be installed in a steel casing. The remainder of the
   trench shall be backfilled per the requirements of Section 13.

        When rock is encountered at six inches (6") or less below grade, the
        ---------------------------------------------------------------
        rock shall be excavated a minimum of eight inches (8") deep from the top
        of the rock, maintaining a minimum cover of eighteen inches (18") below
        grade. The Grantor System shall be installed by one of the following
        three (3) methods:

 .  The Grantor System shall be installed on a two-inch (2") bed of select
   backfill and covered in a one and one half (1-1/2) sack concrete mix eight
   inches (8") thick. The remainder of the trench shall be backfilled per the
   requirements of Section 13.

 .  The Grantor System shall by installed on a two-inch (2") bed of select
   backfill and then covered by select backfill to two inches (2") above the
   Grantor System. A one-quarter-inch (1/4") steel plate will cap the backfill.
   The remainder of the trench shall be backfilled per the requirements of
   Section 13.

 .  The Grantor System shall be installed in a steel casing. The remainder of the
   trench shall be backfilled per the requirements of Section 13.

        When rock is encountered more than six inches (6") but less than thirty
        -----------------------------------------------------------------------
        inches (30") below grade, the rock shall be excavated a minimum of eight
        ------------------------
        inches (8") deep from the top of the rock, maintaining a minimum cover
        of eighteen inches (18") below grade. The Grantor System shall be
        installed by one of the following four (4) methods:

 .  The Grantor System shall be installed on a two-inch (2") bed of select
   backfill and covered in a one and one half (1-1/2) sack concrete mix eight
   inches (8") thick. The remainder of the trench shall be backfilled per the
   requirements of Section 13.

 .  The Grantor System shall by installed on a two-inch (2") bed of select
   backfill and then covered by select backfill to two inches (2") above the
   Grantor System. A one-quarter-inch (1/4") steel plate will cap the backfill.
   The remainder of the trench shall be backfilled per the requirements of
   Section 13.

 .  The Grantor System shall be installed in a steel casing. The remainder of the
   trench shall be backfilled per the requirements of Section 13.

 .  The rock shall be excavated eighteen inches (18") deep from the surface of
   the rock or to forty-two inches (42") of cover whichever requires less
   excavation. No further protection will be required other than a two-inch (2")
   bed of select backfill and then covered by select backfill to four inches
   (4") above the Grantor System.

        When rock is encountered thirty inches (30") or more below grade, the
        ----------------------------------------------------------------
        rock shall be excavated eight inches (8") deep from the top of the rock,
        or forty- two inches (42") of cover whichever requires less excavation.
        The Grantor System shall be installed by the following method:

CONFIDENTIAL DRAFT                                                        Page 9
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

 .  The Grantor System shall be installed on a two-inch (2") bed of select
   backfill and then covered by select backfill to two inches (2") above the
   Grantor System. The remainder of the trench shall be backfilled per the
   requirements of Section 13.

 .  In the case of the use/conversion of existing steel pipelines or salvaged
   conduit systems, the existing depths shall be considered adequate.

                       1.1.3.  Buried Cable Warning Tape
                       ---------------------------------

All conduit will be installed with buried cable warning tape except where
existing steel pipelines or salvaged conduit systems are used.  The warning tape
shall generally be placed at a depth of 12 inches (12") below grade and directly
above the conduit.

                          1.1.4.  Conduit Construction
                          ----------------------------

 .  Conduits may be placed by means of trenching, plowing, jack and bore, or
   directional bore. Conduit will generally be placed on a level grade parallel
   to the surface, with only gradual changes in grade elevation.

 .  Steel conduit will be joined with threaded collars, Zap-Lok or welding.

 .  When directional bore methods are used, the bore depth with steel casing
   shall be a minimum of five feet (5') below base of rail and five feet (5')
   below natural ground. For depths greater than ten feet (10') below base of
   rail and five feet (5') below natural ground, a single HDPE SDR 11 casing may
   be used in lieu of a steel casing. Approved mini-directional wet bores shall
   be installed a minimum depth of ten feet (10') below the toe of ballast
   section and a minimum of five feet (5') feet below natural ground line,
   pursuant to the following parameters:

 .  All bores are to be either Duct or HDPE-casing bores with the exception of
   those where the ROW owner specifies steel.

  All Duct bores (bores without casing) with the exception of common driveway
  ---------------------------------------------------------------------------
 crossings shall use SDR 9 Duct. Driveway bores and cased bores may use SDR 11
 -----------------------------------------------------------------------------
                                     Duct.
                                     -----

 .  All conduits placed on DOT bridges will be bullet-proof fiberglass where
   allowed by the authority and all other bridges galvanized steel conduit shall
   be installed.

 .  All conduits placed on bridges shall have expansion joint placed at each
   structural (bridge) expansion joint or at least every one hundred feet
   (100'), whichever is the shorter distance.

                         1.1.5.  Innerduct Installation
                         ------------------------------

 .  HDPE innerducts, where utilized, shall be one and one-quarter inches (1-
   1/4").
 .  HDPE innerduct(s), where utilized, shall be encased by a HDPE or steel
   conduit.
 .  HDPE innerduct(s) shall extend beyond the end of all conduits a minimum of 18
   inches (18").

                           1.1.6.  Cable Installation
                           --------------------------

 .  The fiber optic cable shall be installed using a powered pulling winch and
   hydraulic powered assist pulling wheels.  The maximum pulling force to be
   applied to the fiber optic cable shall be 600 pounds.

 .  Bends of small radii (less then 20 times the outside diameter of the cable)
   and twists that may damage the cable shall be avoided during cable placement.

CONFIDENTIAL DRAFT                                                       Page 10
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

 .  The cable shall be lubricated and placed in accordance with the cable
   manufacturer specifications.
 .  A pulling swivel break-away rated at 600 pounds shall be used at all times.
 .  All splices will be contained in a hand hole or manhole.
 .  A minimum of thirteen feet (13') of slack cable will be left in all
   intermediate hand holes or manholes.
 .  A minimum of fifty feet (50') of slack cable from each cable end will be left
   in all splice locations.
 .  A minimum of one hundred feet (100') of slack cable will be left in all
   Regeneration and ILA Facilities.

                         1.1.7  Manholes and Hand holes
                         ------------------------------
 .  Hand holes and Manholes placed in traveled surface streets be HS-20 loading
   rated and shall have locking lids.
 .  Hand holes shall be placed in all other areas and be installed with a minimum
   of six inches (6") of soil covering the lid.

                     1.1.8.  Cable Markers (Warning Signs)
                     -------------------------------------

Cable markers (with the same information as buried cable warning tape) shall be
installed at all changes in cable running line directions, waterways, subsurface
utilities, hand holes and at both sides of street, highway, bridge or railroad
crossings.  At no time shall any markers be spaced more than one thousand feet
(1000') feet apart.  Markers shall be positioned so that they can be seen from
the location of the cable and generally set facing perpendicular to the cable
running line.

                               1.1.9.  Compliance
                               ------------------

All work will be done in strict accordance with federal, state, local and
applicable private rules and laws regarding safety and environmental issues,
including those set forth by OSHA and the EPA.  In addition, all work and the
resulting fiber system will comply with the current requirements of all
governing entities (FCC, NEC, DEC, and other national, state, and local codes).

                           1.1.10.  As-Built Drawings
                           --------------------------

 .  As-built drawings will contain a minimum of the following:

 .  Information showing the location of running line, relative to permanent land
   marks, including but not limited to, railroad mileposts, boundary crossings
   and utility crossings.

 .  Manhole and hand hole locations.
 .  Conduit information (type, length, expansion joints, etc.).
 .  Notation of all deviations from specifications (depth, etc.).
 .  ROW detail (type, centerline distances, boundaries, waterways, road
   crossings, known utilities and obstacles).
 .  Cable marker locations and stationing.
 .  Fiber Optic Cable Data (type, manufacturer, reel Ids, sequentials, slack
   coils, splice points, etc.).
 .  Regeneration and Optical Amplifier Facility locations and floor plans.
 .  Drawings will be updated with actual field data during and after
   construction.
 .  Metro area scale shall not exceed 1 inch = 200 feet.
 .  Rural area scale shall not exceed 1 inch = 500 feet.

CONFIDENTIAL DRAFT                                                       Page 11
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

 .  As-built drawings will be provided within 90 days after the Acceptance Date,
   in both hard copy and electronic format. Updates to the as-built drawings
   will be provided within 60 days of completion of change.

                          1.1.11.  Aerial Construction
                          ----------------------------

 .  Subject to Section 7.01 of the IRU Agreement, aerial construction methods
   will only be in local loop construction and in those circumstances where
   buried construction techniques are impractical due to environmental
   conditions, schedule or economic considerations, right-of-way issues or code
   restrictions. Aerial construction on utility structures using optical
   groundwire or all dielectric self-support methods may be used.

 .  Aerial design standards and construction techniques will conform with
   industry-accepted practices for aerial fiber option cable systems. All aerial
   plant must comply with applicable national (i.e., NEC, NESC, etc.), state and
   local codes.The fiber optic cable placed on an aerial system shall be armored
   and designed for aerial applications.

 .  The cable will be placed in accordance with manufacturer specifications.
   Cable tension will be monitored during placement. Cable rollers will be
   placed at a maximum interval of thirty-five feet (35'). Cable expansion loops
   will be placed at ever pole. Cable identification/warning tags will be placed
   at every pole. All cable splices will be buried in hand holes or manholes.

 .  Cable sheath to suspension strand bonds and grounding will be performed at
   the first and last pole of the system and at .25 mile intervals.

 .  Fiber optic cable at all riser poles will be protected with galvanized steel
   U-guard from twelve inches (12") below grade to a point twenty four inches
   (24") below the suspension strand. Conduit sweeps will be used to transition
   from the U-guard to either a hand hole or manhole.

 .  All aerial plant will be designed and constructed with 10M EHS (Class A
   galvanized) suspension strand unless otherwise dictated by the pole owners or
   field conditions. The fiber optic cable will be double lashed to the
   suspension strand using 45 mil stainless lashing wire.

 .  Span length shall account for storm loading (wind and ice) in accordance with
   zones outlined in NESC code. Sags and tensions will be calculated in
   accordance with industry accepted practices and account for strand size, span
   length, ambient temperature at placement, and loading. The suspension strand
   will be transformed with a strand dynamometer. A catenary suspension system
   may be used if the system exceeds maximum span length specifications.

 .  Prior to attachment to any existing pole line, the system will be inspected
   for compliance with applicable codes and standards, as well as the physical
   condition of the poles and existing hardware. Any make-ready work will be
   reviewed with the pole owner and specifically addressed prior to
   construction.

 .  If a pole line needs to be constructed, the preferred poles will be Class 4
   (40 feet) and Class 5 (35 feet). Use of the preferred poles will make it
   unnecessary to calculate pole loading (horizontal, vertical and bending
   moments) in most field conditions. Some unusual conditions may require the
   use of a stronger class pole. Depth of placement will be dictated by soil
   conditions, slope of terrain and length of pole. Poles will be guyed in
   accordance with industry-accepted standards. All pole attachment hardware
   will be galvanized steel.

 .  Aerial cable will be placed below power attachments and above all other
   attachments unless otherwise dictated by the pole owner. Pole contact
   clearances and locations will be dictated by current NESC code and the
   presence of existing attachments; however, the following minimum objective
   clearances will apply:

CONFIDENTIAL DRAFT                                                       Page 12
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

 .  Power line-forty inches (40") (below).
 .  Non-current carrying power line-thirty inches (30").
 .  Telephone, CATV and other signal lines-twelve inches (12") (below).

 .  Vertical clearances for crossings or parallel lines will be dictated by
   current NESC code; however, the objective clearance for most objects (roads,
   alleys, etc.) is eighteen feet (18') (at 100 degree F) with the exception of
   railroad tracks and waterways which have an objective of 27 feet (27') (at
   100 degree F).

CONFIDENTIAL DRAFT                                                       Page 13
SECOND  AMENDMENT TO NATIONAL COST SHARING IRU AGREEMENT<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission

                                                        Exhibit 10.23

                    THIRD AMENDMENT TO COST SHARING NATIONAL
                                 IRU AGREEMENT

     THIS THIRD AMENDMENT TO COST SHARING NATIONAL IRU AGREEMENT ("Amendment")
is made and entered into as of the 27th day of March 2000, by and between LEVEL
3 COMMUNICATIONS, LLC, a Delaware limited liability company ("Grantor") and
SPLITROCK SERVICES, INC., a Delaware corporation ("Grantee").  This Amendment
modifies and amends that certain Cost Sharing National IRU Agreement dated April
26, 1999 between Grantor and Grantee (the "Agreement").  Capitalized terms used
but not defined herein shall have the meanings set forth in the Agreement.

                                    RECITALS
                                    --------

     A.  Grantee desires to obtain an IRU in fibers within and along segments of
Grantor's lateral segments and metropolitan fiber-optic networks to connect
certain Segment End Points to Grantee Points of Presence.

     B.  Grantor desires to convey to Grantee an IRU in the additional fibers
specified herein, subject to and in accordance with the terms and provisions set
forth in this Amendment and the Agreement.

                               TERMS OF AMENDMENT
                               ------------------

     Accordingly, in consideration of the foregoing and other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged,
Grantor and Grantee hereby agree as follows:

                                   ARTICLE 1
                             METROPOLITAN SEGMENTS
                             ---------------------

     1.01  The Segments described in Exhibit "A," attached hereto, are hereby
                                     ----------
added to the Grantor System, and Grantor shall deliver an IRU in, and Grantee
shall accept and pay for, an IRU in the Grantee Fibers within and along such
Segments (the "Metropolitan Grantee Fibers").  Maps of the Segments are provided
in Exhibit "A-1," attached hereto.
   ------------

     1.02  Grantee shall make the following payments with respect to the
Metropolitan Grantee Fibers.

     a.  The IRU Contribution for the Metropolitan Grantee Fibers of (***) shall
be allocated on a Segment by Segment basis, as identified in Exhibit "A".  The
                                                             ----------
IRU Contribution shall be due from Grantee to Grantor with respect to the
Metropolitan Grantee Fibers in each Segment as follows:   (***).

     b.  The IRU Contribution for the Metropolitan Grantee Fibers set forth in
Section 1.02(a) is unique to the Segments described herein, and the Parties
recognize that any IRU

                                                                           PAGE1
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission

Contribution for additional Fibers requested by Grantee
in other metropolitan Segments shall be subject to negotiation by the Parties.

     1.03  Unless otherwise provided herein, for the purposes of the Agreement,
the Metropolitan Grantee Fibers shall be treated as Grantee Fibers.
Specifically, the Parties recognize that the Monthly Charge shall apply to the
Metropolitan Grantee Fibers.

     1.04  Grantee shall have the Option for the Metropolitan Grantee Fibers to
extend the IRU in additional fibers as provided in Section 3.02 of the
Agreement.

     1.05  The construction specifications for the Metropolitan Grantee Fibers
are attached hereto as Exhibit "C."
                       ----------

     1.06  The fiber, conduit, and cable vault specifications for the
Metropolitan Grantee Fibers are attached hereto as Exhibit "D."
                                                   ----------

     1.07  The fiber testing procedures for the Metropolitan Grantee Fibers are
attached hereto as Exhibit "E."
                   ----------

                                   ARTICLE 2
                                LATERAL SEGMENTS
                                ----------------

     2.01  Grantor shall construct and install the lateral segments described in

Exhibit "B," attached hereto, for Grantee in the metropolitan markets containing
----------
the Metropolitan Grantee Fibers (the "Lateral Segments"). The estimated costs
for constructing and installing the Lateral Segments are summarized in the
tables contained in Exhibit "B."  Grantee shall be solely responsible for
                    ----------
reimbursement of all of Grantor's Costs, subject to the agreed upon split, in
constructing and installing the Lateral Segments, regardless of whether the
actual Costs are above or below the estimates contained in Exhibit "B."
                                                           ----------
Grantor's Costs shall be documented by Grantor in commercially reasonable detail
and shall be submitted to Grantee in the form of invoices with supporting
documentation.

     2.02  Grantee shall reimburse Grantor for the Costs of the construction and
installation of the Lateral Segments within fifteen (15) days after receipt of
an invoice therefor, subject to Grantee's rights to dispute such Costs.

     2.03  The Parties may add Lateral Segments by letter agreement between the
Parties, pursuant to Article 25.

                                                                           PAGE2
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission

          IN WITNESS WHEREOF, Grantor and Grantee have executed this Amendment
as of the date first above written.

                          LEVEL 3 COMMUNICATIONS, LLC, a
                               Delaware limited liability company

                          By/s/Jon Yount
                          ------------------------------------------
                          Title: Vice President
                          ------------------------------------------
                          Date: March 29, 2000
                          ------------------------------------------

                          SPLITROCK SERVICES, INC.,
                               a Delaware corporation

                          By /s/ Patrick J. McGettigan, Jr.
                          ------------------------------------------
                          Title: Senior Vice President
                          ------------------------------------------
                          Date: February 29, 2000
                          ------------------------------------------

                                                                           PAGE3
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission

                                  Exhibit "A"
                          Metropolitan Grantee Fibers
                          ---------------------------

<TABLE>
<CAPTION>
   Route Segment                                              Estimated    Fiber   Price Per
                                                             Route Miles   Count     Mile
----------------------------------------------------------------------------------------------
   #            From Point              To Point
----------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------

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

<S>             <C>                  <C>                        <C>         <C>        <C>
Cambridge Loop                                                   11.35       4      (***)
----------------------------------------------------------------------------------------------
A-1       Gateway                     Bent & 3rd                             4
----------------------------------------------------------------------------------------------
B-8       Bent & 3rd                  Cambridge and 3rd                      4
----------------------------------------------------------------------------------------------
B-7       Cambridge & 3rd             Cambridge & Fulkerson                  4
----------------------------------------------------------------------------------------------
B1-1      Cambridge & Fulkerson       Bent & Fulkerson                       4
----------------------------------------------------------------------------------------------
A-18      Bent & Fulkerson            Gateway                                4
----------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------

LOOP A & C - NYC                                                  9.4        4      (***)
----------------------------------------------------------------------------------------------
A-1       17th Street & 3rd Ave    Park Row & St James                       4
----------------------------------------------------------------------------------------------
A-2       Park Row & St James      Fulton & Water                            4
-----------------------------------------------------------------------------------------------
A-3       Fulton & Water           Fulton & Gold                             4
-----------------------------------------------------------------------------------------------
A-6       Barclay & Church         15th St alley & 9th Ave                   4
-----------------------------------------------------------------------------------------------
A-7       15th St alley & 9th      Gateway (111 8th)                         4
          Ave
-----------------------------------------------------------------------------------------------
B-8       Gateway (111 8th)        17th St & 3rd Ave                         4
-----------------------------------------------------------------------------------------------
C-1       Gateway (100 William)    Maiden & Water                            4
-----------------------------------------------------------------------------------------------
C-2       Maiden & Water           West & Battery Place                      4
-----------------------------------------------------------------------------------------------
C-3       West & Battery Place     Barclay & Church                          4
------------------------------------------------------------------------------------------------
C-4       Fulton & Gold            Gateway (100 William)                     4
------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------

LOOP A - Dallas - Inner Loop                                     21.7       10      (***)
------------------------------------------------------------------------------------------------
A-1       Gateway                  Viceroy & I-35                           10
------------------------------------------------------------------------------------------------
A-2       Viceroy & I-35           Hwy 12 & Hillcrest                       10
------------------------------------------------------------------------------------------------
A-3       Hwy 12 & Hillcrest       Irving & Oak Lawn                        10
------------------------------------------------------------------------------------------------
A-4       Irving & Oak Lawn        Oak Lawn & Lemmon                        10
------------------------------------------------------------------------------------------------
A-5       Oak Lawn & Lemmon        Oak Lawn & Maple                         10
------------------------------------------------------------------------------------------------
A-6       Oak Lawn & Maple         Irving & Inwood                          10
------------------------------------------------------------------------------------------------
A-7       Irving & Inwood          Gateway                                  10
------------------------------------------------------------------------------------------------

------------------------------------------------------------------------------------------------
TOTALS                                                           32.4
------------------------------------------------------------------------------------------------
</TABLE>

NOTE:  The Grantee Fibers shall terminate in the nearest underground utility box
at each of the sub-segment end points identified above.

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission

                                  Exhibit "A"
                          Metropolitan Grantee Fibers
                          ---------------------------

<TABLE>
<CAPTION>
          Route Segment                                              (***) IRU      (***) IRU       Total IRU         Scheduled
                                                                    Contribution   Contribution    Contribution    Completion Date
----------------------------------------------------------------------------------------------------------------------------------
   #       From Point                To Point
----------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------------------

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

<S>       <C>                      <C>                          <C>               <C>          <C>             <C>
Cambridge Loop                                                          (***)          (***)           (***)              (***)
----------------------------------------------------------------------------------------------------------------------------------
A-1       Gateway                  Bent & 3rd
----------------------------------------------------------------------------------------------------------------------------------
B-8       Bent & 3rd               Cambridge and 3rd
----------------------------------------------------------------------------------------------------------------------------------
B-7       Cambridge & 3rd          Cambridge & Fulkerson
----------------------------------------------------------------------------------------------------------------------------------
B1-1      Cambridge & Fulkerson    Bent & Fulkerson
----------------------------------------------------------------------------------------------------------------------------------
A-18      Bent & Fulkerson         Gateway
----------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------------------

LOOP A & C - NYC                                                        (***)          (***)           (***)              (***)
----------------------------------------------------------------------------------------------------------------------------------
A-1       17th Street & 3rd Ave    Park Row & St James
----------------------------------------------------------------------------------------------------------------------------------
A-2       Park Row & St James      Fulton & Water
----------------------------------------------------------------------------------------------------------------------------------
A-3       Fulton & Water           Fulton & Gold
----------------------------------------------------------------------------------------------------------------------------------
A-6       Barclay & Church         15th St alley & 9th Ave
----------------------------------------------------------------------------------------------------------------------------------
A-7       15th St alley & 9th      Gateway (111 8th)
          Ave
----------------------------------------------------------------------------------------------------------------------------------
B-8       Gateway (111 8th)        17th St & 3rd Ave
----------------------------------------------------------------------------------------------------------------------------------
C-1       Gateway (100 William)    Maiden & Water
----------------------------------------------------------------------------------------------------------------------------------
C-2       Maiden & Water           West & Battery Place
----------------------------------------------------------------------------------------------------------------------------------
C-3       West & Battery Place     Barclay & Church
----------------------------------------------------------------------------------------------------------------------------------
C-4       Fulton & Gold            Gateway (100 William)
----------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------------------

LOOP A - Dallas - Inner Loop                                            (***)          (***)           (***)              (***)
----------------------------------------------------------------------------------------------------------------------------------
A-1       Gateway                  Viceroy & I-35
----------------------------------------------------------------------------------------------------------------------------------
A-2       Viceroy & I-35           Hwy 12 & Hillcrest
----------------------------------------------------------------------------------------------------------------------------------
A-3       Hwy 12 & Hillcrest       Irving & Oak Lawn
----------------------------------------------------------------------------------------------------------------------------------
A-4       Irving & Oak Lawn        Oak Lawn & Lemmon
----------------------------------------------------------------------------------------------------------------------------------
A-5       Oak Lawn & Lemmon        Oak Lawn & Maple
----------------------------------------------------------------------------------------------------------------------------------
A-6       Oak Lawn & Maple         Irving & Inwood
----------------------------------------------------------------------------------------------------------------------------------
A-7       Irving & Inwood          Gateway
----------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------------------
TOTALS                                                                                 (***)           (***)              (***)
----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

NOTE: The Grantee Fibers shall terminate in the nearest underground utility box
at each of the sub-segment end points identified above.

                                                                           PAGE4
<PAGE>

NOTE: Redacted portions have been marked with (***). The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission

                                  Exhibit "B"
                                Lateral Segments
                                ----------------

175 Bent Street, Cambridge, MA
  Estimated total cost:  (***)
  Estimated split cost:  (***)

345 Courtland Street, Atlanta, GA
  Estimated total cost:  (***)
  Estimated split cost:  (***)

Assumes Grantor is on floor 2 and Grantee is on floor 3.  Assumes 40 LF of new
riser to be built by Grantor and 440 LF total distance from Grantor OSX to
Grantee FTP.

601 E. 18th Street, Suite 1900, Denver, CO
  Estimated total cost:  (***)
  Estimated split cost:  (***)

Assumes Grantor is on floor 1 and Grantee is on floor 1.  Assumes 200 LF of new
riser to be built by Grantor and 250 LF total distance from Grantor OSX to
Grantee FTP.

818 West 7th Street, Los Angeles, CA
  Estimated total cost:  (***)
  Estimated split cost:  (***)

Assumes Grantor is on floor 11 and Grantee is on floor 4.  Assumes 0 LF of new
riser to be built by Grantor and 440 LF total distance from Grantor OSX to
Grantee FTP.

1950 Stemmons Freeway, Suite 2015, Dallas, TX
  Estimated total cost:  (***)
  Estimated split cost:  (***)

Assumes Grantor is on floor 5 and Grantee is on floor 2.  Assumes 1380 LF of new
riser to be built by Grantor and average of 690 LF total distance from Grantor
OSX to Grantee FTP.

General Comments on Lateral Segments:

1.  The estimated costs above are for the following items as noted in respective
    sections; Single Laterals, Diverse Laterals, Building Risers, Building
    Distribution, Fiber Installation, POP Splicing, Miscellaneous Items. Items
    not listed above have not been costed.
2.  Unless otherwise noted, costs do not include any costs associated with
    leasing backbone duct or fiber leased or sold in the backbone.
3.  If provided, costs for racks, power and alarms do not include the cost of
    any electronics.
4.  Building laterals have been estimated to the closest point(s) on the local
    loop backbone.  Creation of logical loops or fiber rings has not been
    considered.
5.  These costs assume the shortest; most direct route from the current or
    planned backbone at this time. Changes in lateral distances due to
    subsequent and necessary backbone or lateral reroutes for any reason have
    not been factored.
6.  These costs assume there are no moratoriums, extraordinary restrictions,
    route changes, franchise requirements or out of the ordinary construction
    conditions. Any of these factors will impact costs. It is also assumed there
    will be no right-of-way restrictions or associated costs.
7.  Building access will need to be gained to validate estimated costs.

                                                                           PAGE5
<PAGE>

NOTE: Redacted portions have been marked with (***).  The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                  Exhibit "C"
         Metropolitan (Local Loop) Network Construction Specifications
         -------------------------------------------------------------

The intent of this Exhibit is to delineate the general specifications and
standards for construction of the Grantor System within the metropolitan
networks.  In the event any federal, state, local or private agency having
jurisdiction shall impose higher standards, Grantor will comply and conform to
such higher standards. Grantor may deviate from the specifications and standards
described below in those instances where either (i) strict compliance is
impractical due to physical (including environmental) conditions, right-of-way
issues or code restrictions, or (ii) Grantor has acquired a portion of the
Grantor System from a third party.

                               1.  Minimum Depths
                               ------------------
 .  Minimum cover required in the placement of conduit shall be 42 inches, except
   in the following instances:
        .  The minimum cover in borrow ditches adjacent to roads, highways,
           railroads, and interstate shall be 48 inches below the clean-out line
           or existing grade, whichever is greater.
        .  The minimum cover across streams, river washes and other waterways
           shall be 48 inches below the clean-out line or existing grade,
           whichever is greater.
        .  Where conduit crosses railroads, the conduit shall be placed at a
           minimum depth of 60 inches below the base of rail or below the
           paralleling drainage ditch, whichever is greater.
        .  At locations where conduit crosses other subsurface utilities or
           other structures, the conduit shall be installed to provide a minimum
           of 12 inches of vertical clearance under the foreign object while
           maintaining a minimum cover of 42 inches.
        .  In rock, the conduit shall be placed either: (a) 8 inches deep in
           rock with 36 inches minimum cover; (b) 8 inches deep in rock with a
           concrete cap and 30 inches minimum cover; (c) 8 inches deep in rock
           with a 1/4 inch steel plate cover and 24 inches minimum cover or (d)
           in steel pipe placed on top of the rock with 18 inches minimum cover,
           whichever requires the least rock excavation. HDPE conduit will be
           back-filled with 2 inches of select materials (bedding) and 4 inches
           of select cover in rock areas.
        .  In the case of the use/conversion of conduit systems purchased or
           leased from third parties or salvaged conduit systems, the existing
           depths shall be considered adequate.

                             2.  Cable Marking Tape
                             ----------------------

 .  Cable marking tape shall be installed above all direct buried conduit,
   generally at a minimum of 12 inches below top of rough grade. Conduit
   installed by means other than direct burial as well as salvaged conduit
   systems or conduit systems purchased from third parties will not have cable
   marking tape installed.

                                                                           PAGE6
<PAGE>

NOTE: Redacted portions have been marked with (***).  The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                            3.  Conduit Construction
                            ------------------------

 .  Conduits may be placed by means of trenching, plowing, jack and bore, or
   directional bore. Conduit will generally be placed on a level grade parallel
   to the surface, with only gradual changes in grade elevation.

 .  Steel conduit will be joined with threaded collars, welding or other industry
   accepted methods.

 .  Railroad crossings will be encased in HDPE or steel conduit.

 .  All underground crossings of navigable waterways will be placed in either
   HDPE or steel conduit at a minimum depth of 20 feet below the bottom of the
   waterway.

 .  All conduits placed on DOT bridges will be bulletproof fiberglass or
   galvanized steel.

 .  All conduits placed on bridges shall have expansion joints placed at each
   structural (bridge) expansion joint or at least every 100 feet, whichever is
   the shorter distance.

                             4.  Cable Installation
                             ----------------------
 .  The maximum pulling force to be applied to the fiber optic cable shall be 600
   pounds.

 .  A pulling swivel breakaway rated at 600 pounds shall be used at all times
   when pulling fiber optic cable.

 .  Bends of small radii (less then 20 times the outside diameter of the cable)
   and twists that may damage the cable shall be avoided during cable placement.

 .  The cable shall be lubricated and placed in accordance with the cable
   manufacturer specifications.

 .  All splices will be contained in a cable vault or hand hole.

 .  A minimum of 60 feet of slack cable will be left in all intermediate cable
   vaults or hand holes.

 .  A minimum of 50 feet of slack cable from each cable end (100 feet total) will
   be left in all splice locations.

                        5.  Cable Vaults and Hand Holes
                        -------------------------------
 .  Cable vaults will be placed as required by local conditions and at
   approximately every 3000 feet in rural areas and 500 feet in urban areas.

 .  Cable vaults and hand holes placed in traveled surface streets shall be a
   minimum HS-20 loading rated.

 .  Cable vaults and hand holes not installed in traveled surface streets shall
   be a minimum HS-10 loading rated.

                            6.  Proofing of Conduits
                            ------------------------
 .  Upon completion of conduit installed, conduit shall be proofed to verify
   continuity and integrity of the conduit system.

 .  Proofing shall be accomplished by pulling or blowing a mandrel.  The outside
   diameter of the mandrel or pig shall be a minimum of 80 percent of the inside
   diameter of the conduit.

                                                                           PAGE7
<PAGE>

NOTE: Redacted portions have been marked with (***).  The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                 7.  Compliance
                                 --------------

 .  All work will be done in accordance with federal, state, local and applicable
   private rules and laws regarding safety and environmental issues, including
   those set forth by OSHA and the EPA. In addition, all work and the resulting
   fiber system will comply with the current requirements of all governing
   entities (FCC, NEC, DEC, and other national, state, and local codes).

                             8.  As-Built Drawings
                             ---------------------
 .  As-built drawings for conduit sales will contain a minimum of the following:
        .  Information showing the location of running line, relative to
           permanent landmarks.

        .  Cable vault and hand hole locations.

        .  Conduit information (type, length, etc.).

        .  Notation of all deviations from specifications (depth, etc.).

        .  ROW detail (type, centerline distances, boundaries, waterways, road
           crossings, known utilities and obstacles).

        .  Fiber optic cable data (type, manufacturer, reel IDs, sequentials,
           slack coils, splice points, etc.).

        .  Drawings will be updated with actual field data during and after
           construction.

        .  Metro area scale shall not exceed 1 inch = 200 feet.

        .  Rural area scale shall not exceed 1 inch = 500 feet.

        .  All deviations from specification (depth, etc.).

        .  As-built drawings will be provided within 90 days after the
           Acceptance Date in electronic format.

 .  As-built drawings for fiber sales will contain a minimum of the following:

        .  Maps depicting which streets the Grantee fiber route travels.

        .  Grantee logical loop configurations and fiber assignments per loop.

        .  Fiber assignments to all buildings Grantor provides connectivity for
           Grantee.

        .  As-built drawings will be provided within 90 days after the
           Acceptance Date in electronic format.

                                                                           PAGE8
<PAGE>

NOTE: Redacted portions have been marked with (***).  The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                  Exhibit "D"
                                  -----------
                        Fiber and Conduit Specifications
                        --------------------------------

The intent of this Exhibit is to delineate the specifications for the Grantee
Fibers and the conduit housing the Cable.  Deviations from these specifications
may occur if Grantor acquires a portion of the Grantor System from a third party
pursuant to the Agreement.

Fiber
-----

In the metropolitan segments, single mode fiber will be used.  In any case where
the routes for Grantor's long-haul and metropolitan networks converge, Grantor
shall have the right to elect to install either non-zero dispersion-shifted
optical fiber or single mode fiber.  Grantor may substitute alternative fibers
if and only if such alternative fibers have performance specifications which are
at least equal to the specifications set forth below.

Non-Zero Dispersion -Shifted Optical Fiber
------------------------------------------
 . Attenuation at 1550 nm = 0.25 dB/km max
 . Attenuation at 1625 nm = 0.25 dB/km max
 . Total Dispersion = 2.0 - 6.0 ps/nm-km for 1530 nm to 1565nm
               4.5 - 11.2 ps/nm-km for 1565nm to 1625nm
 . Effective Area (Aeff) = 72 /\m2, typical
 . Polarization Mode Dispersion = 0.20 ps max per fiber, typical 0.08 ps link
  value
 . Mode Field Diameter = 9.20 to 10.00 m at 1550 nm
 . Cladding Diameter = 125.0 +- 1.0 m
 . Core/Clad Concentricity less than = 0.5 m

Single Mode Fiber
-----------------

 . Attenuation at 1310 nm = 0.40 dB/km max
 . Attenuation at 1550 nm = 0.30 dB/km max
 . Zero Dispersion wavelength = 1312nm typical
 . Wavelength cutoff less than 1260nm typical
 . Dispersion slope = less than .0902ps typical
 . Polarization Mode dispersion = 0.20 ps max per fiber, 0.10 ps link value
 . Mode Field Diameter = 9.2+- 0.4 m at 1310 nm & 10.4 +-0.8 at 1550 nm
 . Cladding Diameter = 125.0 +- 1.0 m
 . Core/Clad Concentricity less than =0.5 m

Conduit
-------

 . OSP Conduit SDR 11
 . 1-1/4" (inside diameter)
 . High density polyethylene (HDPE) duct
 . Tensile yield 3200 psi
 . Flexural modulus 110,000 psi
 . Smoothwall inside & outer
 . Empty (no rope or tape)
 . Unlubricated

                                                                           PAGE9
<PAGE>

NOTE: Redacted portions have been marked with (***).  The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

                                  Exhibit "E"
   METROPOLITAN (LOCAL LOOP) NETWORK FIBER ACCEPTANCE TESTING PROCEDURES
   ----------------------------------------------------------------------
                                   AND STANDARDS
                                   -------------

The intent of this Exhibit is to identify the fiber acceptances testing

procedures and standards used within the Grantor's metropolitan networks.

Deviations from these specifications may occur if Grantor acquires a portion of

the Grantor System from a third party pursuant to the Agreement.

1. All splices shall be fusion spliced.  Mechanical splices are only allowed
   during temporary restoration and will be replaced with fusion splices.
2. Fibers shall be terminated with Ultra SC-PC connectors (typical return loss
   of 0.50 dB).
3. After end-to-end connectivity on the fibers has been completed, bi-
   directional OTDR span and power meter testing will be completed. Grantor
   shall perform tests after the fiber cable is installed and the splicing
   enclosures have been completed and are in their final resting configuration
   with the cable vault or hand hole covers closed. This ensures that no micro
   or macro bending problems with the cable or fiber strands will contribute to
   the loss/attenuation measurements.
4. Power meter tests shall be completed to verify and insure that no fibers have
   been crossed at any of the splice points within the network. Grantor shall
   test and record power level readings on all fiber strands in both directions
   of transmission (bi-directionally) using the 1310 & 1550 nm wavelengths.
5. All OTDR and power meter tests shall be completed as follows:
   a. All OTDR traces shall be taken from both ends of a section (between
      adjacent Locations) and recorded using the 1310 & 1550 nm wavelength.
      Loss/attenuation measurements for each splice point from both directions
      shall be taken and recorded.
   b. The end-to-end loss value as measured with an industry-accepted laser
      source and power meter should have an attenuation rating of less than or
      equal to the following:
      (1)  At 1310 nm: (0.40 dB/km x km of cable) + (number of connectors x
           0.50) + (0.10 x number of splices).
      (2)  At 1550 nm: (0.30 dB/km x km of cable) + (number of connectors x
           0.50) + (0.10 x number of splices).
   c. Grantor's loss/attenuation objective for each fiber optic splice is 0.10
      dB when measured in one direction with an OTDR test set (excluding
      connector loss, which is typically 0.50 dB per mated connector pair). If
      after three attempts this parameter is not met, the splice will be marked
      as Out-Of-Spec (OOS) and the splice will remain provided the average
      loss/attenuation value of all splices on an individual fiber basis shall
      not exceed 0.10 dB for the entire ring or subsystem.
   d. For bi-directional OTDR testing, the distance from Location "A" and
      Location "Z" shall be recorded for each splice point. The loss/attenuation
      at each splice point shall be recorded at both wavelengths (1310 nm & 1550
      nm) in each direction. Grantor shall then average the two readings to
      obtain the final average splice loss/attenuation for each splice point of
      each fiber strand within the fiber optic cable.
   e. Each fiber strand color must be recorded along with its buffer tube color
      or the ribbon color. The laser source transmit power level using the 1310
      & 1550 nm wavelengths will always be recorded together with the receive
      power level reading at the receiving end of the test.

                                                                          PAGE10
<PAGE>

NOTE: Redacted portions have been marked with (***).  The redacted portions are
subject to a request for confidential treatment that has been filed with the
Securities and Exchange Commission.

6.   OTDR traces will be taken and splice loss measurements recorded. Grantor
     will store OTDR traces on electronic media. Loss measurements will be
     recorded using an industry-accepted laser source and a power meter. Copies
     of all data sheets and tables and one set of diskettes with all traces will
     be available to Grantee.
7.   Following emergency restoral, Grantor personnel shall perform span test
     documenting end-to-end attenuation measurement of each fiber and will be
     completed in both directions at 1310 & 1550 nm wavelengths. Upon permanent
     repair, new splice loss readings should be no greater than the original
     splice loss specifications.

                                                                          PAGE11

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