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Exhibit 10.1

COGENT COMMUNICATIONS GROUP, INC.

THIRD AMENDED AND RESTATED
STOCKHOLDERS AGREEMENT

        THIS STOCKHOLDERS AGREEMENT (this "Agreement") is made as of the 30th
day of March, 2004 by and among (i) Cogent Communications Group, Inc., a
Delaware corporation (the "Company"), (ii) David Schaeffer (the "Founder") and
(iii) those persons whose names are set forth under the heading "Purchasers" on
Schedule I hereto and any person who later becomes a party to this Agreement by
executing and delivering to the Company an Instrument of Accession in the form
of Schedule II hereto (the "Purchasers").

WITNESSETH:

        WHEREAS, simultaneously herewith, the Company and certain of the
Purchasers (the "Series I Purchasers" and the "Series J Purchasers") have
consummated the following transactions in contemplation of their respective
merger agreements (collectively, the "Merger Agreements"): (i) pursuant to the
merger agreement between the Company, the Company's subsidiary Lux Merger
Sub, Inc. ("Lux Merger Sub") and Symposium Gamma, Inc. ("Symposium Gamma"),
dated January 5, 2004 (the "Gamma Merger Agreement"), whereby the Series I
Purchasers, subject to the terms and conditions set forth therein, received
2,575 shares of the Company's Series I Participating Convertible Preferred
Stock, par value $.001 per share ("Series I Preferred Stock"), and (ii) pursuant
to the merger agreement between the Company, the Company's subsidiary DE Merger
Sub Inc. ("DE Merger Sub") and Symposium Omega, Inc. ("Symposium Omega"), dated
March 30, 2004 (the "Omega Merger Agreement"), whereby the Series J Purchasers,
subject to the terms and conditions set forth therein, received 3,891 shares of
the Company's Series J Participating Convertible Preferred Stock, par value
$.001 per share ("Series J Preferred Stock");

        WHEREAS, the Founder and certain of the Purchasers who purchased the
Series F Participating Convertible Preferred Stock, par value $.001 per share
("Series F Preferred Stock"), of the Company, and the various sub-series of
Series G Participating Convertible Preferred Stock, par value $.001 per share
(collectively, the "Series G Preferred Stock"), of the Company (collectively,
the Series F and Series G Preferred Stock shall be known as the "Preferred
Stock"), are parties to that certain Second Amended and Restated Stockholders
Agreement, dated as of July 31, 2003 (the "Second A&R Stockholders Agreement"),
and in connection with the consummation of the transactions contemplated by the
Merger Agreements, the Company, the Founder and such Purchasers, constituting
signatories sufficient under Section 15 of the Second A&R Stockholders Agreement
to amend the Second A&R Stockholders Agreement, desire to amend and restate the
Second A&R Stockholders Agreement as set forth herein and execute and deliver
this Agreement, setting forth herein certain terms and conditions governing
their relative ownership of the Shares (as hereinafter defined);

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

        1.    Prohibited Transfers.    The Founder shall not sell, assign,
transfer, pledge, hypothecate, mortgage or dispose of, by gift or otherwise, or
in any way encumber, all or any part of the Shares (as hereinafter defined)
owned by him except in compliance with the terms of this Agreement. For purposes
of this Agreement, the term "Shares" shall mean and include all shares of Common
Stock of the Company and all shares of any class or series of equity securities
or equity-backed securities of the Company or any subsidiary, including without
limitation, capital stock (including any shares of treasury stock) or rights,
options, warrants or other securities convertible into or exercisable or
exchangeable for capital stock or any debt security which by its terms is
convertible into or exchangeable for any equity

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security or has any other equity feature or any security that is a combination
of debt and, in any event that is owned by the Founder, whether presently held
or hereafter acquired.

        2.    Purchasers' Right of Refusal on Dispositions made by the
Founder.    Except as set forth in Section 4, if the Founder wishes to sell,
assign, transfer or otherwise dispose of any or all Shares owned by him pursuant
to the terms of a bona fide offer received from a third party at any time prior
to the consummation of a firmly underwritten public offering pursuant to an
effective registration statement under the Securities Act of 1933, as amended,
covering the offer and sale of Common Stock of the Company in which (a) the
pre-money valuation of the Company is at least $500,000,000 and (b) the gross
cash proceeds (before underwriting discounts, commissions and fees) are at least
$50,000,000 (a "Qualified Offering"), the Founder shall submit a written offer
to sell such Shares to the Purchasers (with a copy to the Company) on terms and
conditions, including price, not less favorable to the Purchasers than those on
which the Founder proposes to sell such Shares to such third party (the
"Offer"). The Offer shall disclose the identity of the proposed purchaser or
transferee, the Shares proposed to be sold or transferred, the agreed terms of
the sale or transfer and any other material facts relating to the sale or
transfer. Within thirty (30) days after receipt of the Offer, each Purchaser and
each Qualified Transferee, if any, shall give notice to the Founder of its
intent to purchase all or any portion of the offered Shares on the same terms
and conditions as set forth in the Offer. Each Purchaser and Qualified
Transferee shall have the right to purchase that number of the Shares as to
which the Offer applies as shall be equal to the aggregate number of such Shares
multiplied by a fraction, the numerator of which is the number of shares of
Common Stock of the Company then owned by such Purchaser or Qualified Transferee
(as applicable) (calculated on an as converted basis, and including any shares
of Common Stock deemed to be beneficially owned by such Purchaser pursuant to
Rule 13d-3 promulgated under the Securities Exchange Act of 1934 ("Rule 13d-3"))
and the denominator of which is the aggregate number of shares of said Common
Stock then issued and outstanding and held by (and deemed to be beneficially
owned by) all the Purchasers (calculated on an as converted basis). The amount
of Shares each Purchaser or Qualified Transferee, as that term is defined below,
is entitled to purchase under this Section 2 shall be referred to as such
Purchaser's "Pro Rata Fraction." Each Purchaser shall have the right to transfer
his right to any Pro Rata Fraction or part thereof to any Qualified Transferee.
If any Purchaser or Qualified Transferee does not wish to purchase or to
transfer his right to purchase his Pro Rata Fraction (such shares, the
"Remaining Offered Shares"), then any Purchasers or Qualified Transferees who so
elect shall have the right to purchase, on a pro rata basis with any other
Purchasers or Qualified Transferees who so elect, any Pro Rata Fraction not
purchased by a Purchaser or Qualified Transferee. Each Purchaser or Qualified
Transferee shall act upon the Offer as soon as practicable after receipt from
the Company of notice that a Purchaser or Qualified Transferee has not elected
to purchase all of the offered Shares, and in all events within fifteen
(15) days after receipt thereof. Each Purchaser and Qualified Transferee shall
have the right to accept the Offer as to all or part of the Remaining Offered
Shares offered thereby. If a Purchaser or Qualified Transferee shall elect to
purchase all or part of such Purchaser's or Qualified Transferee's Pro Rata
Fraction, said Purchaser or Qualified Transferee shall individually communicate
in writing such election to purchase to whichever of the Purchasers has made the
Offer, which communication shall be delivered by hand or delivered to such
Purchaser at the address set forth in Section 8 below and shall, when taken in
conjunction with the Offer be deemed to constitute a valid, legally binding and
enforceable agreement for the sale and purchase of the Shares covered thereby.

        If the Purchasers, taken together, do not agree to purchase all of the
Shares offered by the Founder pursuant to the Offer, and consummate such
purchase within the later of forty-five (45) days after receipt of the Offer and
twenty (20) days after the Company shall have notified each Purchaser and
Qualified Transferee of Remaining Offered Shares, such Shares subject to the
Offer as shall not have been purchased may be sold by the Founder at any time
within 90 days after the expiration of the Offer, but subject to the provisions
of Section 3 below. Any such sale shall be at not less than the price and upon
other terms and conditions, if any, not more favorable to the purchaser than
those specified

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in the Offer. Any Shares not sold within such 90-day period shall continue to be
subject to the requirements of a prior offer and re-sale pursuant to this
Section.

        For purposes hereof, a "Qualified Transferee" shall mean any person
(i) who is a Purchaser, (ii) who is an "affiliated person" of a Purchaser, as
that term is defined in the Investment Company Act of 1940, (iii) who is a
partner, member or stockholder of a Purchaser that is a partnership, limited
liability company or corporation, as applicable, and who is offered a pro rata
right, based on his, her or its interest in the Purchaser, to acquire the Shares
offered by a Purchaser pursuant to this Section 2, or (iv) who acquires at least
twenty five percent (25%) of the shares of Preferred Stock issued by the Company
to any Purchaser (as adjusted for stock splits, stock dividends,
reclassifications, recapitalizations or other similar events).

        3.    Purchasers' Right of Participation in Sales made by the
Founder.    Except as set forth in Section 4, if at any time the Founder wishes
to sell, transfer or otherwise dispose of any Shares owned by him to any person
(the "Acquiror") in a transaction which is subject to the provisions of
Section 2 hereof, and such sale, transfer or other disposition would, when
combined with all prior sales, transfers and other dispositions by the Founder,
result in the transfer by the Founder of Shares representing more than
twenty-five percent (25%) of the total number of shares held by the Founder (the
"Founder's Stock"), each Purchaser shall have the right to require, as a
condition to such sale or disposition, that the Acquiror purchase from said
Purchaser at the same price per Share and on the same terms and conditions as
involved in such sale or disposition by the Founder the same percentage of
shares of Common Stock owned (and deemed to be beneficially owned under
Rule 13d-3) by such Purchaser as such sale or disposition represents with
respect to the number of shares of Founder's Stock (calculated on an as
converted, fully diluted basis) owned by the Founder immediately prior to such
sale. Each Purchaser wishing so to participate in any such sale or disposition
shall notify the Founder of such intention as soon as practicable after receipt
of the Offer made pursuant to Section 2, and in all events within fifteen
(15) days after receipt thereof. If a Purchaser shall elect to participate in
such sale or disposition, said Purchaser shall individually communicate such
election to the Founder, which communication shall be delivered by hand or
mailed to the Founder at the address set forth in Section 8 below. The Founder
and/or each participating Purchaser shall sell to the Acquiror all, or at the
option of the Acquiror, any part of the Stock (as defined in Section 5 below)
proposed to be sold by them at not less than the price and upon other terms and
conditions, if any, not more favorable to the Acquiror than those set forth in
the Offer; provided, however, that any purchase of less than all of such Stock
by the Acquiror shall be made from the Founder and/or each participating
Purchaser based upon a fraction, the numerator of which is the number of shares
of Stock of the Company then owned by the Founder or such participating
Purchaser (including any shares of Common Stock deemed to be owned under
Rule 13d-3) and the denominator of which is the aggregate number of shares of
Stock held by (and deemed to be held by) the Founder and all of the
participating Purchasers. The Founder shall use his commercially reasonable
efforts to obtain the agreement of the Acquiror to the participation of the
participating Purchasers in the contemplated sale, and shall not sell any Stock
to such Acquiror if such Acquiror declines to permit the participating
Purchasers to participate pursuant to the terms of this Section 3. The
provisions of this Section 3 shall not apply to the sale of any Shares by the
Founder (i) to a Purchaser pursuant to an Offer under Section 2 or (ii) made
upon or after the occurrence of a Qualified Offering, the sale of the Company or
control thereof, whether by merger, sale, recapitalization or similar corporate
event or the transfer of more than a majority of its capital stock (calculated
on an as converted, fully diluted basis) or assets, or the conversion into
Common Stock of all then outstanding shares of Preferred Stock (each such event
in this clause (ii), a "Corporate Event").

        4.    Permitted Transfers.    Anything herein to the contrary
notwithstanding, the provisions of Sections 1, 2 and 3 shall not apply to:
(a) any transfer of Shares by the Founder by gift or bequest or through
inheritance to, or for the benefit of, any member or members of his immediate
family; (b) any

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transfer of Shares by the Founder to a trust in respect of which he serves as
trustee, provided that the trust instrument governing said trust shall provide
that Founder, as trustee, shall retain sole and exclusive control over the
voting and disposition of said Shares until the termination of the applicable
restrictions on transfer under this Agreement; (c) any sale of Common Stock in a
public offering pursuant to a registration statement filed by the Company with
the Securities and Exchange Commission; (d) any repurchase of Shares from
officers, employees, directors or consultants of the Company which are subject
to restrictive stock purchase agreements under which the Company has the option
to repurchase such shares upon the occurrence of certain events, including
termination of employment; and (e) any pledge, hypothecation or other similar
financing transaction in which the Founder continues to have the sole and
exclusive authority and right to vote the shares subject to such pledge,
hypothecation or other financing transaction. In the event of any such transfer,
other than pursuant to subsection (c) of this Section 4, the transferee of the
Shares shall hold the Shares so acquired with all the rights conferred by, and
subject to all the restrictions imposed by, this Agreement on the party from
whom the transferee received the Shares, and as a condition to such transfer,
each such transferee shall execute and deliver a written agreement agreeing to
be bound by the provisions of this Agreement.

        5.    Election of Directors; Issuance of Options; Protective
Provisions.    

        (a) Election of Directors.    Each of the parties hereto agrees to vote
all of the Stock (as hereinafter defined and that entitles the holder thereof to
vote in the election of the Board of Directors) now owned or hereafter acquired
by such party (and attend, in person or by proxy, all meetings of stockholders
called for the purpose of electing directors), and the Company agrees to take
all actions (including, but not limited to the nomination of specified persons)
to cause and maintain the election to the Board of Directors of the Company, to
the extent permitted pursuant to the Company's certificate of incorporation, of
the following:

        (i) two (2) individuals designated by the Founder (the designees under
this subsection shall initially be David Schaeffer and H. Helen Lee);

        (ii) two (2) individuals designated by the holders of a majority in
interest of the Shares owned as of the date hereof by Jerusalem Venture Partners
and certain of its affiliates (the designees under this subsection shall
initially be Erel Margalit and Michael Carus);

        (iii) one (1) individual designated by the holders of a majority in
interest of the Shares owned as of the date hereof by Worldview Technology
Partners and certain of its affiliates (the designee under this subsection shall
initially be Tim Weingarten);

        (iv) one (1) individual designated by the holders of a majority in
interest of the Shares owned as of the date hereof by Oak Investment Partners
and certain of its affiliates (the designee under this subsection shall
initially be Edward Glassmeyer);

        (v) one (1) individual designated by the holders of a majority in
interest of the Shares owned as of the date hereof by Broadview Capital Partners
and certain of its affiliates (the designee under this subsection shall
initially be Steven Brooks); and

        (vi) one (1) individual designated by BNP Europe Telecom & Media Fund
II, LP and Natio vie Developpement 3, FCPR (the designee under this subsection
shall initially be Jean-Jacques Bertrand); and

        (vii) a three (3) member Compensation Committee, one of the members of
which shall be nominated by the directors elected pursuant to subparagraph
(i) above and who shall not be the Founder and two of the members of which shall
be nominated by the directors elected pursuant to subparagraphs (ii), (iii),
(iv) and (v) above.

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        Each of the parties further covenants and agrees to vote, to the extent
possible, all Stock of the Company now owned or hereafter acquired by such party
so that (i) the Company's Board of Directors shall consist of nine (9) members,
eight (8) of whom shall be nominated or designated as set forth above and the
ninth of whom shall be an independent director nominated by the Company's Board
of Directors, (ii) the Compensation Committee thereof shall consist of three
(3) members, each of whom shall be nominated as set forth above, and (iii) the
holders of the Series G Preferred Stock, the Series I Preferred Stock, the
Series J Preferred Stock and the Founder's Stock shall be entitled, in the
aggregate, to have up to four persons as observers at each meeting of the Board
of Directors and any committee (other than the Compensation Committee) thereof,
provided that at least one of such observers shall be selected by the holders of
the Series I Preferred Stock and the Series J Preferred Stock. For the purposes
of this Agreement, "Stock" shall mean and include all Preferred Stock and Common
Stock and all other securities of the Company which may be exchangeable for or
issued in exchange for or in respect of shares of Common Stock (whether by way
of stock split, stock dividends, combination, reclassification, reorganization
or any other means).

        In the absence of any designation from the persons or groups so
designating directors as specified above, the director previously designated by
them and then serving shall be reelected if still eligible to serve as provided
herein.

        No party hereto shall vote to remove any member of the Board of
Directors or the Compensation Committee thereof designated in accordance with
the aforesaid procedure unless the persons or groups so designating directors as
specified above so vote, and, if such persons or groups so vote then the
non-designating party or parties shall likewise so vote.

        Any vacancy on the Board of Directors or the Compensation Committee
thereof created by the resignation, removal, incapacity or death of any person
designated under this Section 5 shall be filled by another person designated in
a manner so as to preserve the constituency of the Board or such Committee as
provided above.

        (b) Issuance of Options to Holders of Preferred Stock. From and after
the date hereof, each of the parties hereto agrees that the Company shall not,
and each agrees to vote all of their Shares of the Company now owned or
hereafter acquired by such party (and attend, in person or by proxy, all
meetings of stockholders called for the purpose of electing directors) to
preclude the Company from issuing any options under the Company's stock option
plans as in effect from time to time to any holder of Preferred Stock or any
person employed by or controlled by or under common control with any such holder
or any affiliate thereof without first obtaining the unanimous approval of the
Board of Directors of the Company.

        (c) Protective Provisions. For so long as at least 9,500 shares of
Series G Preferred Stock, Series I Preferred Stock and Series J Preferred Stock
collectively remain outstanding, the affirmative vote or consent of the holders
of two-thirds (2/3) of the issued and outstanding shares of Series G Preferred
Stock, Series I Preferred Stock and Series J Preferred Stock, voting together as
a single class, shall be required to take any of the following actions
(including by way of merger, consolidation or otherwise):

        (i) designate, authorize, create, issue, sell, redeem or repurchase any
class or series of equity securities or equity-backed securities of the Company
or any subsidiary thereof, including without limitation, capital stock
(including any shares of treasury stock) or rights, options, warrants or other
securities convertible into or exercisable or exchangeable for capital stock or
any debt security which by its terms is convertible into or exchangeable for any
equity security or has any other equity feature or any security that is a
combination of debt and equity (collectively, "Equity Securities"), other than
pursuant to (i) employee stock option and similar incentive plans approved by
the Board of Directors, (ii) the issuance of Common Stock upon the conversion of
the 7.5% Convertible Subordinated Notes due 2007 of Allied Riser Communications
Corporation (the "Notes") in accordance with the terms thereof or the issuance
of additional convertible debt or

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equity as a paid-in-kind interest payment on the Notes in accordance with the
terms thereof approved by the Board of Directors or (iii) a conversion or
exchange right set forth in Company's certificate of incorporation;

        (ii) except as otherwise expressly provided as of the date hereof in the
Company's certificate of incorporation or in a certificate of designations
thereto, declare or pay any dividends or make any distributions of any kind with
respect to any outstanding Equity Securities of the Company or any subsidiary
thereof;

        (iii) approve the merger, consolidation, dissolution or liquidation of
the Company or any subsidiary thereof, or any transaction having the same
effect;

        (iv) increase or decrease the aggregate number of authorized shares of
Common Stock or Preferred Stock of the Company;

        (v) sell all or substantially all of the assets of the Company and its
subsidiaries taken as a whole, whether directly through a sale of the Company's
interests in its subsidiaries or other assets, or indirectly through a sale of
the assets of its subsidiaries, in one transaction or any series of
transactions, or approve any transaction or series of transactions having the
same effect;

        (vi) cause, directly or indirectly, a material change in the nature of
the business or strategic direction of the Company and its subsidiaries, taken
as a whole;

        (vii) approve the filing for bankruptcy of or any decision not to take
action to prevent a filing for bankruptcy or not to oppose an involuntary filing
for bankruptcy or other winding up of the Company or any subsidiary thereof;

        (viii) approve the establishment and maintenance of an Executive
Committee of the Board of Directors or increase or decrease the number of
directors composing the Board of Directors; or

        (ix) amend, repeal or modify any provision of the Company's certificate
of incorporation in a manner that adversely affects the rights, powers or
preferences of the Preferred Stock.

        6.    Right of Participation in Sales by the Company.    

        (a) Right of Participation. Except as provided in Section 6(f) of this
Agreement, the Company shall not issue, sell or exchange, agree or obligate
itself to issue, sell or exchange, or reserve or set aside for issuance, sale or
exchange, any (i) shares of Common Stock, (ii) any other equity security of the
Company, including without limitation, shares of Preferred Stock, (iii) any debt
security of the Company (other than debt with no equity feature) including
without limitation, any debt security which by its terms is convertible into or
exchangeable for any equity security of the Company, (iv) any security of the
Company that is a combination of debt and equity, or (v) any option, warrant or
other right to subscribe for, purchase or otherwise acquire any such equity
security or any such debt security of the Company, unless in each case the
Company shall have first offered to sell such securities (the "Offered
Securities") to the Purchasers who hold individually or together with their
affiliates at least 2,500,000 Shares of the Common Stock on an as converted
basis then outstanding and, for so long as the Founder holds not less than fifty
percent (50%) of the number of shares of Founder's Stock held by him on the date
hereof (in both cases, as adjusted for stock splits, stock dividends,
reclassifications, recapitalizations or other similar events), the holders of
the Founder's Stock (such Purchasers and, if applicable, the holders of the
Founder's Stock being referred to as the "Participating Stockholders") as
follows: The Company shall offer to sell to each Participating Stockholder
(a) that portion of the Offered Securities as the number of shares of Common
Stock (including all shares of capital stock convertible into Common Stock, on a
fully-diluted basis) then held by such Participating Stockholder, as the case
may be, bears to the total number of shares of Common Stock (including all
shares of capital stock convertible into Common Stock, on a fully-diluted basis)
of the Company then outstanding (the "Basic Amount," and the aggregate of the
Basic Amounts of all Participating Stockholders being

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referred to as the "Aggregate Basic Amount"), and (b) such additional portion of
the Aggregate Basic Amount as such Participating Stockholder shall indicate it
will purchase should the other Participating Stockholders subscribe for less
than their Basic Amounts (the "Undersubscription Amount"), at a price and on
such other terms as shall have been specified by the Company in writing
delivered to such Participating Stockholder (the "Offer"), which Offer by its
terms shall remain open and irrevocable for a period of twenty (20) days from
receipt of the offer.

        (b) Notice of Acceptance. Notice of each Participating Stockholder's
intention to accept, in whole or in part, any Offer made pursuant to
Section 6(a) shall be evidenced by a writing signed by such Participating
Stockholder and delivered to the Company prior to the end of the 20-day period
of such offer, setting forth such of the Participating Stockholder's Basic
Amount as such Participating Stockholder elects to purchase and, if such
Participating Stockholder shall elect to purchase all of its Basic Amount, such
Undersubscription Amount as such Participating Stockholder shall elect to
purchase (the "Notice of Acceptance"). If the Basic Amounts subscribed for by
all Participating Stockholders are less than the total Aggregate Basic Amount,
then each Participating Stockholder who has set forth Undersubscription Amounts
in its Notice of Acceptance shall be entitled to purchase, in addition to the
Basic Amounts subscribed for, all Undersubscription Amounts it has subscribed
for; provided, however, that should the Undersubscription Amounts subscribed for
exceed the difference between the Aggregate Basic Amount and the Basic Amounts
subscribed for (the "Available Undersubscription Amount"), each Participating
Stockholder who has subscribed for any Undersubscription Amount shall be
entitled to purchase only that portion of the Available Undersubscription Amount
as the Undersubscription Amount subscribed for by such Participating Stockholder
bears to the total Undersubscription Amounts subscribed for by all Participating
Stockholders, subject to rounding by the Board of Directors to the extent it
reasonably deems necessary.

        (c)    Conditions to Acceptances and Purchase.    

        (i) Permitted Sales of Refused Securities. If Notices of Acceptance are
not given by the Participating Stockholders in respect of all the Aggregate
Basic Amount, the Company shall have ninety (90) days from the expiration of the
period set forth in Section 6(a) to close the sale of all or any part of such
Aggregate Basic Amount as to which a Notice of Acceptance has not been given by
the Participating Stockholders (the "Refused Securities") to the person or
persons specified in the Offer, but only for cash and otherwise in all respects
upon terms and conditions, including, without limitation, unit price and
interest rates, which are no more favorable, in the aggregate, to such other
person or persons or less favorable to the Company than those set forth in the
Offer.

        (ii) Reduction in Amount of Offered Securities. If the Company shall
propose to sell less than all the Refused Securities (any such sale to be in the
manner and on the terms specified in Section 6(c)(i) above), then each
Participating Stockholder may, at its sole option and in its sole discretion,
reduce the number of, or other units of the Offered Securities specified in its
respective Notices of Acceptance to an amount which shall be not less than the
amount of the Offered Securities which the Participating Stockholder elected to
purchase pursuant to Section 6(b) multiplied by a fraction, (A) the numerator of
which shall be the amount of Offered Securities which the Company actually
proposes to sell, and (B) the denominator of which shall be the amount of all
Offered Securities. In the event that any Participating Stockholder so elects to
reduce the number or amount of Offered Securities specified in its respective
Notices of Acceptance, the Company may not sell or otherwise dispose of more
than the reduced amount of the Offered Securities until such securities have
again been offered to the Participating Stockholders in accordance with
Section 6(a).

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        (iii) Closing. Upon the closing, which shall include full payment to the
Company, of the sale to such other person or persons of all or less than all the
Refused Securities, the Participating Stockholders shall purchase from the
Company, and the Company shall sell to the Participating Stockholders, the
number of Offered Securities specified in the Notices of Acceptance, as reduced
pursuant to Section 6(c)(ii) if the Participating Stockholders have so elected,
upon the terms and conditions specified in the Offer. The purchase by the
Participating Stockholders of any Offered Securities is subject in all cases to
the preparation, execution and delivery by the Company and the Participating
Stockholders of a purchase agreement relating to such Offered Securities
reasonably satisfactory in form and substance to the Participating Stockholders
and their respective counsel.

        (d) Further Sale. In each case, any Offered Securities not purchased by
the Participating Stockholders or other person or persons in accordance with
Section 6(c) may not be sold or otherwise disposed of until they are again
offered to the Participating Stockholders under the procedures specified in
Sections 6(a), 6(b) and 6(c).

        (e) Termination of Right of Participation. The rights of the
Participating Stockholders under this Section 6 shall terminate immediately
prior to the consummation of a Qualified Offering. In addition, the rights of
holders of the Founder's Stock under this Section 6 shall terminate when the
Founder owns less than 50% of the shares of Founder's Stock held by the Founder
on the date first above written (as adjusted for stock splits, stock dividends,
reclassifications, recapitalizations or other similar events). Upon the
termination of the rights of an individual or entity pursuant to this subsection
(e), such individual or entity shall no longer be deemed to be a "Participating
Stockholder" under this Section 6.

        (f) Exception. The rights of the Participating Stockholders under this
Section 6 shall not apply to:

        (i) Common Stock issued as a stock dividend to holders of Common Stock
or upon any subdivision or combination of shares of Common Stock,

        (ii) shares of any series of Preferred Stock issued as a dividend to
holders of such series of Preferred Stock upon any subdivision or combination of
shares of such series of Preferred Stock,

        (iii) shares of Common Stock issued or issuable upon conversion of the
Preferred Stock,

        (iv) up to 10,000 shares of Common Stock issued or issuable pursuant to
options, warrants or other rights (as adjusted for any stock dividends,
combinations, splits, recapitalizations and the like) issued to employees,
officers or directors of, or consultants or advisors to the Company or any
subsidiary pursuant to stock purchase or stock option plans or other
arrangements that are approved by the Board of Directors,

        (v) up to 54,001 shares of Series H Preferred Stock issued to employees,
officers or directors of, or consultants or advisors to the Company or any
subsidiary pursuant to that certain 2003 Incentive Award Plan of the Company,

        (vi) Common Stock issued pursuant to the acquisition of another
corporation by the Company by merger (whereby the Company owns no less than 51%
of the voting power of such corporation) or purchase of substantially all of its
stock or assets, if such acquisition is approved by a majority of the Directors
nominated by the holders of the Preferred Stock in the manner set forth in
Section 5 hereof,

        (vii) Common Stock offered to the public pursuant to a registration
statement filed under the Securities Act,

        (viii) Common Stock, or options or warrants to purchase Common Stock,
issued to financial institutions or lessors in connection with commercial credit
arrangements, equipment financings or

8

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similar transactions, as approved by the two-thirds (2/3rds) of the then sitting
members of the Board of Directors,

        (ix) shares of Series F Preferred Stock issued pursuant to that certain
Exchange Agreement by and among the Company, Cisco Systems Capital Corporation
("Cisco Capital"), and the other parties thereto, dated June 26, 2003 (the
"Exchange Agreement"),

        (x) shares of Series G Preferred Stock issued pursuant to that certain
Participating Convertible Preferred Stock Purchase Agreement by and among the
Company and the Series G Preferred Stock Purchasers ("Series G Purchasers"),
dated June 26, 2003 (the "Purchase Agreement"),

        (xi) shares of Series I Preferred Stock issued pursuant to that certain
Gamma Merger Agreement, dated January 5, 2004,

        (xii) shares of Series J Preferred Stock issued pursuant to that certain
Omega Merger Agreement, dated March 30, 2004, and

        (xiii) the issuance of Common Stock upon the conversion of the Notes or
the issuance of additional convertible debt or equity as a paid-in-kind interest
payment the Notes approved by the Board of Directors.

        (g) Waiver. The rights of the Purchasers under this Section 6 may be
waived in any instance, on behalf of all of the Purchasers, prospectively or
retroactively, by the written agreement of the holders of two-thirds in interest
of the Preferred Stock owned beneficially or of record by the Purchasers. The
rights of holders of the Founder's Stock under this Section 6 may be waived in
any instance, on behalf of all such holders, prospectively or retroactively, by
the written agreement of the holders of a majority of the Founder's Stock then
outstanding. Upon waiver of the rights of the Purchasers or holders of Founder's
Stock in accordance with this subsection (g) with respect to a particular
issuance, sale or exchange of Offered Securities, the Purchasers or such holder,
as the case may be, shall be excluded from the definition of "Participating
Stockholders" for purposes of this Section 6 with respect to such issuance, sale
or exchange.

        7.    Termination.    This Agreement, and the respective rights and
obligations of the parties hereto, shall terminate upon the earliest to occur of
the following: (i) the completion of a Qualified Offering, provided that the
obligations of Section 9 shall survive such termination; or (ii) the sale of the
Company, whether by merger, sale, or transfer of more than ninety percent (90%)
of its capital stock, or sale of substantially all of its assets. In addition,
any Purchaser or Qualified Transferee may elect to terminate its rights and
obligations with respect to any or all of Sections 2, 3, 5 or 6 by providing
written notice of such election to the Company at any time.

        8.    Notices.    All notices and other communications hereunder shall
be in writing and shall be deemed to have been given when delivered or mailed by
first class, registered or certified mail (air mail if to or from outside the
United States), return receipt requested, postage prepaid, or by
internationally, recognized overnight courier service (two business days after
deposit with such overnight courier service in the case of deliveries to
non-U.S. residents), if to each Purchaser at his respective address set forth on
Schedule I hereto or on the Instrument of Accession pursuant to which he became
a party to this Agreement, and if to the Founder, at his address set forth on
Schedule I hereto or to such other address as the addressee shall have furnished
to the other parties hereto in the manner prescribed by this Section 8.

        9.    Lock-up Agreement.    Each of the Purchasers and holders of
Founder's Stock hereby agrees in connection with the Company's Qualified
Offering, upon the request of the principal underwriter managing the Qualified
Offering of the Company, not to sell publicly any Shares now owned or hereafter
acquired by him, her or it and subject to this Agreement (other than Shares
being registered in such offering or any shares purchased in the open market
after the Company's initial public offering)

9

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without the prior written consent of such underwriter for a period of time (not
to exceed one hundred eighty (180) days) from the consummation of such Qualified
Offering as the underwriter may specify, in all events subject to the provisions
of Section 13(f) of a certain Fourth Amended and Restated Registration Rights
Agreement dated as of the date hereof.

        10.    Failure to Deliver Shares.    If the Founder becomes obligated to
sell any Shares owned by, or held for the benefit of, such Purchaser to the
Founder, another Purchaser or a Qualified Transferee under this Agreement and
fails to deliver such shares in accordance with the terms of this Agreement, the
Founder or such Purchaser, as applicable, may, at his or its option, in addition
to all other remedies it may have, send to the Company for the benefit of such
selling Purchaser the purchase price for such Shares as is herein specified.
Thereupon, the Company upon written notice to said Purchaser, (a) shall cancel
on its books the certificate(s) representing the Shares to be sold and (b) shall
issue, in lieu thereof, in the name of the Founder or such Purchaser, as
applicable, a new certificate(s) representing such Shares, and thereupon all of
said Purchaser's rights in and to such shares shall terminate. The Company may
exercise a similar remedy in enforcing its rights under Section 2. If the
Founder transfers any shares to a Purchaser in violation of this Agreement, the
Company may, at the election of a majority of the disinterested members of the
Board of Directors, cancel on the books of the Company any shares of capital
stock then held by the Founder, and compel the Founder to purchase from any
transferee a number of shares of capital stock equal to the amount so
transferred in violation of this Agreement.

        11.    Specific Performance.    The rights of the parties under this
Agreement are unique and, accordingly, the parties shall, in addition to such
other remedies as may be available to any of them at law or in equity, have the
right to enforce their rights hereunder by actions for specific performance to
the extent permitted by law.

        12.    Legend.    Until this Agreement terminates in full, the
certificates representing the Shares shall bear on their face a legend
indicating the existence of the restrictions imposed hereby. After the Qualified
Offering, the Company shall not issue or deliver to any transfer agent a stop
transfer notice with respect to any Shares, the transfer of which is permitted
pursuant to Rule 144(k) and the Securities Act of 1933.

        13.    Entire Agreement.    This Agreement (including any and all
exhibits, schedules and other instruments contemplated thereby) constitutes the
entire agreement among the parties with respect to the subject matter hereof and
supersedes all prior agreements and understandings between them or any of them
as to such subject matter.

        14.    Waivers and Further Agreements.    Except as otherwise expressly
set forth herein, the rights of the Purchasers and holders of Founder's Stock
under this Agreement may be waived by an instrument in writing executed and
delivered by Purchasers holding at least two-thirds in interest of the Common
Stock (including shares of Common Stock into which any shares of Preferred Stock
are convertible) then held or deemed to be held by all Purchasers and holders of
Founder's Stock; provided, however, that the rights set forth in Section 5 with
respect to the designation of the Board of Directors of the Company may not be
waived without the prior written consent of the constituency affected by such
waiver, which waiver shall be obtained in a manner consistent with, and shall
require the same percentages prescribed in, Section 5. Any waiver by any party
of a breach of any provision of this Agreement shall not operate or be construed
as a waiver of any subsequent breach of that provision or of any other provision
hereof. Each of the parties hereto agrees to execute all such further
instruments and documents and to take all such further action as any other party
may reasonably require in order to effectuate the terms and purposes of this
Agreement. Notwithstanding the foregoing, no waiver approved in accordance
herewith shall be effective if and to the extent that such waiver grants to any
one or more Purchasers or holders of Founder's Stock any rights more favorable

10

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than any rights granted to all other Purchasers and holders of Founder's Stock
or otherwise treats any one or more of such parties differently than all other
such parties.

        15.    Amendments.    Except as otherwise expressly provided herein,
this Agreement may not be amended except by an instrument in writing executed by
(i) holders of at least two-thirds in interest of the shares of Common Stock
issued or issuable to the Purchasers (including shares of Common Stock into
which any shares of Preferred Stock are convertible) and (ii) the Company.
Notwithstanding the foregoing, no amendment approved in accordance with this
Section 15 shall be effective if and to the extent that such amendment
(i) creates any additional affirmative obligations to be complied with by any or
all of the Purchasers and holders of Founder's Stock unless approved by holders
of all of the Preferred Stock then outstanding and/or (ii) adversely affects any
of the Founder's rights existing under this Agreement prior to such amendment in
a manner that is inconsistent with, or disproportionate to, the effect of such
amendment on the other parties hereto. In addition, the rights set forth in
Section 5 with respect to the designation of the Board of Directors of the
Company may not be amended without the prior written consent of the constituency
affected by such amendment, which consent shall be obtained in a manner
consistent with Section 5.

        16.    Assignment; Successors and Assigns.    This Agreement shall be
binding upon and shall inure to the benefit of the parties hereto and their
respective heirs, executors, legal representatives, successors and permitted
transferees, except as may be expressly provided otherwise herein, and provided,
further, that no Purchaser may transfer its rights or obligations hereunder
except to a Qualified Transferee. Notwithstanding anything contained herein to
the contrary, until the first to occur of (i) termination of this Agreement and
(ii) a Qualified Offering, any transferee of Preferred Stock shall, as a
condition to such transfer, deliver to the Company a written instrument by which
such transferee agrees to be bound by the obligations imposed hereunder on
holders of Preferred Stock to the same extent as if such transferee had signed
this Agreement.

        17.    Severability.    In case any one or more of the provisions
contained in this Agreement shall for any reason be held to be invalid, illegal
or unenforceable in any respect, such invalidity, illegality or unenforceability
shall not affect any other provision of this Agreement and such invalid, illegal
and unenforceable provision shall be reformed and construed so that it will be
valid, legal, and enforceable to the maximum extent permitted by law.

        18.    Counterparts.    This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

        20.    Section Headings.    The headings contained in this Agreement are
for reference purposes only and shall not in any way affect the meaning or
interpretation of this Agreement.

        21.    Governing Law.    This Agreement shall be construed and enforced
in accordance with and governed by the laws of the State of New York.

        22.    Second A&R Stockholders Agreement.    Upon and after the Closing
(as defined in the Merger Agreement), the Second A&R Stockholders Agreement
shall be terminated and of no further force and effect.

11

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        IN WITNESS WHEREOF, the parties hereto have executed this THIRD AMENDED
AND RESTATED STOCKHOLDERS AGREEMENT as of the date first above written.

[Signature Page to Third Amended and Restated Stockholders' Agreement]

BNP EUROPE TELECOM & MEDIA FUND II, LP    
By:
 
/s/  SHAWNA MOREHOUSE AND JENNIFER FISCHETTI      

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

 
  Name: Shawna Morehouse and Jennifer Fischetti     Title: Authorized
Signatories    
By:
 
General Business, Finance and Investment Ltd., its General Partner
 
  By:   Commerce Advisory Services Ltd, as Director and Partnership Secretary  
 
NATIO VIE DEVELOPPEMENT 3, FCPR
 
 
By:
 
/s/  JEAN-JACQUES BERTRAND      

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

 
  Name: Jean-Jacques Bertrand     Title: Authorized Signatory    

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[Signature Page to Third Amended and Restated Stockholders' Agreement]

OAK INVESTMENT PARTNERS IX, LIMITED PARTNERSHIP    
By:
 
Oak Associates IX, LLC, its General Partner
 
 
By:
 
/s/  EDWARD GLASSMEYER      

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  Name: Edward Glassmeyer     Title: Managing Member    
OAK IX AFFILIATES FUND, LIMITED PARTNERSHIP
 
 
By:
 
Oak IX Affiliates, LLC, its General Partner
 
 
By:
 
/s/  EDWARD GLASSMEYER      

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

 
  Name: Edward Glassmeyer     Title: Managing Member    
OAK IX AFFILIATES FUND-A, LIMITED PARTNERSHIP
 
 
By:
 
Oak Associates IX, LLC, its General Partner
 
 
By:
 
/s/  EDWARD GLASSMEYER      

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  Name: Edward Glassmeyer     Title: Managing Member    

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

[Signature Page to Third Amended and Restated Stockholders' Agreement]

JERUSALEM VENTURE PARTNERS III, L.P.    
By:
 
Jerusalem Partners III, L.P., its General Partner
 
  By:   Jerusalem Venture Partners Corporation, its General Partner    
By:
 
/s/  EREL MARGALIT      

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

 
  Name: Erel Margalit    
JERUSALEM VENTURE PARTNERS III (ISRAEL), L.P.
 
 
By:
 
Jerusalem Venture Partners III (Israel) Management Company Ltd.,
its General Partner
 
  By:   Jerusalem Venture Partners Corporation, its General Partner    
By:
 
/s/  EREL MARGALIT      

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

 
  Name: Erel Margalit    
JERUSALEM VENTURE PARTNERS ENTREPRENEURS FUND III, L.P.
 
 
By:
 
Jerusalem Partners III, L.P., its General Partner
 
  By:   Jerusalem Venture Partners Corporation, its General Partner    
By:
 
/s/  EREL MARGALIT      

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

 
  Name: Erel Margalit    

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

[Signature Page to Third Amended and Restated Stockholders' Agrement]

JERUSALEM VENTURE PARTNERS IV, L.P.    
By:
 
Jerusalem Partners IV, L.P., its General Partner
 
  By:   JVP Corp IV, its General Partner    
By:
 
/s/  EREL MARGALIT      

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

 
  Name: Erel Margalit    
JERUSALEM VENTURE PARTNERS IV (Israel), L.P.
 
 
By:
 
Jerusalem Partners IV—Venture Capital, L.P., its General Partner
 
  By:   JVP Corp IV, its General Partner    
By:
 
/s/  EREL MARGALIT      

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

 
  Name: Erel Margalit    
JERUSALEM VENTURE PARTNERS IV-A, L.P.
 
 
By:
 
Jerusalem Partners IV, L.P., its General Partner
 
  By:   JVP Corp IV, its General Partner    
By:
 
/s/  EREL MARGALIT      

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

 
  Name: Erel Margalit    
JERUSALEM VENTURE PARTNERS ENTREPRENEURS FUND IV, L.P.
 
 
By:
 
Jerusalem Partners IV, L.P., its General Partner
 
  By:   JVP Corp IV, its General Partner    
By:
 
/s/  EREL MARGALIT      

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

 
  Name: Erel Margalit    

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

[Signature Page to Third Amended and Restated Stockholders' Agreement]

WORLDVIEW TECHNOLOGY PARTNERS III, L.P.    
WORLDVIEW TECHNOLOGY INTERNATIONAL III, L.P.
 
 
WORLDVIEW STRATEGIC PARTNERS III, L.P.
 
 
WORLDVIEW III CARRIER FUND, L.P.
 
 
By:
 
Worldview Capital III, L.P., its General Partner
 
  By:   Worldview Equity I, L.L.C., its General Partner    
By:
 
/s/  COLIN SAVAGE      

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

 
  Name: Colin Savage    
WORLDVIEW TECHNOLOGY PARTNERS IV, L.P.
 
 
WORLDVIEW TECHNOLOGY INTERNATIONAL IV, L.P.
 
 
WORLDVIEW STRATEGIC PARTNERS IV, L.P.
 
 
By:
 
Worldview Capital IV, L.P., its General Partner
 
  By:   Worldview Equity I, L.L.C., its General Partner    
By:
 
/s/  COLIN SAVAGE      

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

 
  Name: Colin Savage    

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

[Signature Page to Third Amended and Restated Stockholders' Agreement]

BROADVIEW CAPITAL PARTNERS L.P.    
By:
 
Broadview Capital Partners Management LLC, its General Partner
 
 
By:
 
/s/  STEVEN D. BROOKS      

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

 
  Name: Stephen D. Brooks     Title: Managing Director    
BROADVIEW CAPITAL PARTNERS QUALIFIED PURCHASER FUND L.P.
 
 
By:
 
Broadview Capital Partners Management LLC, its General Partner
 
 
By:
 
/s/  STEVEN D. BROOKS      

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  Name: Stephen D. Brooks     Title: Managing Director    

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[Signature Page to Third Amended and Restated Stockholders' Agreement]

BOULDER VENTURES IV, L.P.    
By:
 
/s/  ANDREW E. JONES      

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

 
  Name: Andrew E. Jones     Title: General Partner    
BOULDER VENTURES IV (ANNEX), L.P.
 
 
By:
 
/s/  ANDREW E. JONES      

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

 
  Name: Andrew E. Jones     Title: General Partner    

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[Signature Page to Third Amended and Restated Stockholders' Agreement]

NAS PARTNERS I L.L.C.    
By:
 
Nassau Capital LLC, its General Partner
 
 
By:
 
/s/  RANDALL A. HACK      

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  Name: Randall A. Hack     Title: Sr. Managing Partner    
NASSAU CAPITAL PARTNERS IV L.P.
 
 
By:
 
Nassau Capital LLC, its General Partner
 
 
By:
 
/s/  RANDALL A. HACK      

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  Name: Randall A. Hack     Title: Sr. Managing Partner    

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[Signature Page to Third Amended and Restated Stockholders' Agreement]

By:   /s/  DAVID SCHAEFFER      

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David Schaeffer    

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