EXECUTION COPY

AGREEMENT

This AGREEMENT (this “Agreement”), dated as of June 24, 2007, is by and between
NEON Communications Group, Inc., a Delaware corporation (“Company”), and the
undersigned holder (the “Holder”) of shares or options or warrants to acquire
shares of capital stock of Company. Capitalized terms used and not defined
herein shall have the respective meanings ascribed to them in the Merger
Agreement defined below.

RECITALS

      A.   Concurrently with the execution of this Agreement, RCN Corporation, a
Delaware corporation (“Parent”), Company and Raven Acquisition Corporation, a
Delaware corporation and a direct and wholly-owned subsidiary of Parent (“Merger
Sub”), have entered into an Agreement and Plan of Merger, dated as of the date
hereof (the “Merger Agreement”), pursuant to which Merger Sub will merge (the
“Merger”) with and into Company, with Company surviving the Merger, on the terms
and subject to the conditions set forth in the Merger Agreement;

      B.   Concurrently with the execution of this Agreement, Holder, as the
“beneficial owner” (as such term is defined in Rule 13d-3 under the Securities
Exchange Act of 1934, as amended (the “Exchange Act”)) of shares of common
stock, par value $0.01 per share, of Company (the “Common Stock”) and of shares
of 6% Series A Cumulative Convertible Preferred Stock, par value $0.01 per
share, of Company (the “Preferred Stock”, and together with the Common Stock,
the “Capital Stock”) has entered into a Support Agreement, dated as of the date
hereof (the “Support Agreement”) pursuant to which Holder agrees to vote
Holder’s Subject Shares on the terms and subject to the conditions set forth in
the Support Agreement.

         NOW, THEREFORE, intending to be legally bound, the parties agree as
follows:

1.    Amendment to Support Agreement.

     (a)   Amendment. Holder hereby agrees that during the period commencing on
the date hereof and continuing until the termination of the Support Agreement in
accordance with the provisions of the Support Agreement, Holder will not amend
the Support Agreement in any manner that would interfere with Company’s ability
to consummate an Acquisition Proposal that constitutes a Superior Proposal
without obtaining the prior written consent of Company.

     (b)   Agreement with Parent. Holder hereby agrees that it will not enter
into any agreement (other than the Support Agreement) with either Parent or
Merger Sub regarding the voting or transfer of Holder’s shares without obtaining
the prior written consent of the Company

     (c)   Reliance by the Company. Holder understands and acknowledges that the
Company is entering into the Merger Agreement in reliance upon the execution and
delivery of this Agreement by Holder.

2.    Miscellaneous.

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     (a)   Further Actions. Each of the parties hereto agrees that it will use
its reasonable efforts to do all things necessary to effectuate this Agreement.

     (b)   Entire Agreement. This Agreement contains the entire understanding of
the parties hereto with respect to the subject matter contained herein and
supersedes all prior agreements and understandings, oral and written, with
respect thereto.

     (c)   Binding Effect; Benefit; Assignment. This Agreement shall inure to
the benefit of and be binding upon the parties hereto and their Permitted
Transferees and successors. Neither this Agreement nor any of the rights,
interests or obligations hereunder shall be assigned by any of the parties
hereto without the prior written consent of each of the other parties. Nothing
in this Agreement, expressed or implied, is intended to confer on any person,
other than the parties hereto, any rights or remedies.

     (d)   Amendments, Waivers, etc. This Agreement may not be amended, changed,
supplemented, waived or otherwise modified or terminated, except upon the
execution and delivery of a written agreement executed by all of the parties
hereto.

     (e)   Specific Enforcement. The parties agree that irreparable damage would
occur in the event that any of the provisions of this Agreement were not
performed in accordance with their specific terms or were otherwise breached.
Accordingly, the parties shall be entitled to an injunction or injunctions to
prevent breaches of this Agreement and to enforce specifically the terms and
provisions of this Agreement, this being in addition to any other remedy to
which they are entitled at law or in equity.

     (f)   Remedies Cumulative. All rights, powers and remedies provided under
this Agreement or otherwise available in respect hereof at law or in equity
shall be cumulative and not alternative, and the exercise of any thereof by any
party shall not preclude the simultaneous or later exercise of any other such
right, power or remedy by such party.

     (g)   No Waiver. The failure of any party hereto to exercise any right,
power or remedy provided under this Agreement or otherwise available in respect
hereof at law or in equity, or to insist upon compliance by any other party
hereto with its obligations hereunder, and any custom or practice of the parties
at variance with the terms hereof shall not constitute a waiver by such party of
its right to exercise any such or other right, power or remedy or to demand such
compliance.

     (h)   Governing Law; Waiver of Jury Trial. THIS AGREEMENT SHALL BE GOVERNED
BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF DELAWARE WITHOUT
REGARD TO THE PRINCIPLES OF CONFLICTS OF LAW OF SUCH JURISDICTION. EACH PARTY
ACKNOWLEDGES AND AGREES THAT ANY CONTROVERSY WHICH MAY ARISE UNDER THIS
AGREEMENT IS LIKELY TO INVOLVE COMPLICATED AND DIFFICULT ISSUES, AND THEREFORE
EACH SUCH PARTY HEREBY IRREVOCABLY AND UNCONDITIONALLY WAIVES ANY RIGHT SUCH
PARTY MAY HAVE TO A TRIAL BY JURY IN RESPECT OF ANY LITIGATION DIRECTLY OR
INDIRECTLY ARISING OUT OF OR RELATING TO THIS AGREEMENT, OR THE TRANSACTIONS
CONTEMPLATED BY THIS AGREEMENT.

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     (i)   Headings. The descriptive headings of this Agreement are inserted for
convenience only, do not constitute a part of this Agreement and shall not
affect in any way the meaning or interpretation of this Agreement.

     (j)   Counterparts; Facsimiles. This Agreement may be executed in several
counterparts, each of which shall be deemed to be an original, and all of which
together shall be deemed to be one and the same instrument. A signature
transmitted by facsimile shall be treated for all purposes by the parties hereto
as an original, shall be binding upon the party transmitting such signature
without limitation.

     (k)   Termination. This Agreement shall terminate, and neither Company nor
Holder shall have any rights or obligations hereunder, immediately upon the
termination of the Support Agreement.

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IN WITNESS WHEREOF, this Agreement is executed as of the date first stated
above.

    NEON COMMUNICATIONS GROUP, INC.                By:       

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  Name:       

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  Title:       

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Signature Page to Agreement

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  HOLDER                Signature:        Printed
Name:______________________________                Number of Shares of Common
Stock owned:       

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      Number of Shares of Common Stock issuable upon    exercise of options or
warrants held:       

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      Number of Shares of Preferred Stock owned:       

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      Number of Shares of Preferred Stock issuable upon    exercise of options
or warrants held:       

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Signature Page to Agreement

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