Document:

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                                                                    Exhibit 10.8

                                 PROMISSORY NOTE

$252,500                                                       December 19, 2001

         FOR VALUE RECEIVED, David M. Lerten ("Borrower"), hereby promises to
pay to the order of USA BROADBAND, INC. (the "Company"), a Delaware corporation
at its offices at 921 Transport Way, Suite 4, Petaluma, California 94954, or at
such other place as may be designated in writing by the holder, from time to
time in immediately available lawful money of the United States of America, the
principal sum of TWO HUNDRED FIFTY-TWO THOUSAND FIVE HUNDRED DOLLARS ($252,500)
(which principal sum may be increased from time to time as provided below),
together with interest on the outstanding principal amount of this Note (as
modified and supplemented and in effect from time to time, the "Note") at the
rate of six percent (6.0%) per annum. Interest for each year shall be computed
on the basis of a year of 360 days for the actual number of days elapsed. This
Note evidences a loan made between the Borrower and Company.

         This Note is being executed and delivered by Borrower to the Company in
connection with Borrower's purchase of 166,667 shares of the Company's Series A
Convertible Preferred Stock ($0.001 par value) (the "Preferred Stock"). The
amount of principal represents the purchase price owed for the Preferred Stock;
provided that the principal sum of this Note shall be reduced on January 2, 2002
to ONE HUNDRED TWENTY-SEVEN THOUSAND FIVE HUNDRED DOLLARS ($127,500) (the
"Principal Reduction Event"). The Note shall mature on December 31, 2002 (the
"Maturity Date"), provided, however, that if on or prior to the Maturity Date,
Borrower has paid to Company an amount equal to fifty percent (50%) of the
principal sum of this Note together with interest accrued thereon, Borrower may
extend the Maturity Date to December 31, 2003 by providing written notice to the
Company of his desire to extend the maturity Date.

         Interest on the Note shall be payable quarterly within 60 days of the
last day of March, June, September and December of each year, solely from the
proceeds of any dividends or distributions payable to Borrower by the Company in
respect of the Preferred Stock whether such dividends or distributions are paid
in cash or in kind. If these dividends or distributions are not sufficient to
pay the interest when due and payable, then the unpaid interest shall be added
to the principal balance of this Note. If these dividends or distributions
exceed the interest then due and payable, the excess amount will be applied to
reduce the principal balance of this Note. If a dividend or distribution becomes
payable to Borrower by the Company or Borrower sells the Preferred Stock and
interest is not then due and payable, the Company shall be entitled to apply the
proceeds of any dividend, distribution or sale to pay first, accrued interest,
and second, the principal balance of this Note.

         Principal on this Note may be prepaid in whole or in part at any time
without premium or penalty, together with accrued interest on the principal
amount being prepaid.

         Borrower and every endorser now or hereafter appearing on this Note
waives presentment, demand for payment, protest, notice of protest and notice of
nonpayment of this Note.

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         This Note shall be binding upon and inure to the benefit of and be
enforceable by the respective successors and assigns of the Company and
Borrower.

         This Note may be changed or amended only by an instrument in writing
agreed upon by both parties, signed by the party against whom enforcement of the
change or amendment is sought.

         If any of the events specified below shall occur (herein individually
referred to as an "Event of Default"), the Company may, at its option, in
addition to any other rights the Company may have in equity or at law as to all
or any part of the principal and interest then due and owing, declare this Note
mature, and all sums owing hereon and under any instrument or agreement executed
in connection with this Note shall be due and payable immediately without
presentment, protest, demand, notice of intention to accelerate, notice of
acceleration, notice of non-payment, notice of protest, or other notice of any
kind, all of which are hereby expressly waived by Borrower. Each of the
following shall be an Event of Default:

         a. The failure of Borrower to pay all amounts when due and owing on
            this Note (whether at stated maturity, by acceleration or
            otherwise);

         b. The breach or default by Borrower of the terms and conditions of the
            Separation Agreement by and between Borrower and the Company
            dated December 19, 2001;

         c. A receiver, custodian, liquidator or trustee of Borrower or any of
            his property is appointed by the order or decree of any court or
            agency or supervisory authority having jurisdiction; or Borrower
            is adjudicated bankrupt or insolvent; or a petition is filed
            against Borrower under any state or federal bankruptcy,
            reorganization, arrangement, insolvency, readjustment of debt,
            dissolution, liquidation or debt receivership law of any
            jurisdiction, whether now or hereafter in effect;

         d. Borrower files a petition in voluntary bankruptcy or seeks relief
            under any provision of any bankruptcy, reorganization,
            arrangement, insolvency, readjustment or debt, dissolution or
            liquidation law of any jurisdiction, whether now or hereafter in
            effect, or consents to the filing of any petition against him
            under any such law;

         e. Borrower makes an assignment for the benefit of his creditors, or
            admits in writing his inability to pay his debts generally as
            they become due, or consents to the appointment of a receiver,
            trustee, or liquidator;

         f. The institution by Borrower of proceedings to be adjudicated as
            bankrupt or insolvent, or the consent by Borrower to the
            institution of bankruptcy or insolvency proceedings against
            Borrower or the filing by Borrower of a petition or answer or
            consent seeking release under any statute, law or regulation, or
            the consent by Borrower to the filing of any such petition or the
            appointment of a receiver, assignee, trustee or other similar
            official of Borrower, or of any substantial part of Borrower's

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            property, or the making by Borrower of an assignment for the
            benefit of creditors, or the taking of action by Borrower in
            furtherance of any such action; or

         g. If, within thirty (30) days after the commencement of an action
            against Borrower (and service of process in connection therewith
            on Borrower seeking any bankruptcy, insolvency, reorganization,
            liquidation, dissolution or similar relief under any present or
            future statute, law or regulation) and such action shall not have
            been resolved in favor of Borrower, or if, within sixty (60) days
            after the appointment without the consent or acquiescence of
            Borrower of any trustee or receiver of Borrower, such appointment
            shall not have been vacated.

         If any Event of Default shall occur and be continuing, the Company may
proceed to protect and enforce its rights under this Note by exercising such
remedies as are available to the Company in respect thereof under applicable
law, either by suit in equity or by action at law, or both, whether for specific
performance of any covenant or other agreement contained in this Note or in aid
of the exercise of any power granted in this Note. No remedy conferred in this
Note is intended to be exclusive of any other remedy, and each and every such
remedy shall be cumulative and shall be in addition to every other remedy
conferred herein or now or hereafter existing at law or in equity or by statute
or otherwise. No delay or omission of the Company to exercise any right or
remedy with respect to this Note will impair, or constitute a waiver of, any
such right or remedy.

         As security for the payment of this Note, Borrower has pledged the
Preferred Stock and all proceeds therefrom pursuant to the Stock Pledge and
Security Agreement between the Borrower and the Company executed and dated
herewith.

         THIS NOTE IS PAYABLE ON THE MATURITY DATE, HOWEVER BROUGHT ABOUT.
THE COMPANY IS UNDER NO OBLIGATION TO REFINANCE THE LOAN EVIDENCED HEREBY AT THE
MATURITY DATE.

         In case of suit, or if this obligation is placed in an attorney's hands
for collection, or to protect the security for its payment, Borrower shall pay
the costs of collection and litigation, including reasonable attorneys' fees.

         Borrower and all sureties, endorsers, guarantors and other parties
hereafter assuming or otherwise becoming liable for the payment of any sum of
money payable under this Note (a) jointly and severally waive grace, presentment
and demand for payment, protest and notice of protest, and non-payment, and all
other notice, including notice of intent to accelerate the Maturity Date and
notice of acceleration of the Maturity Date, filing of suit and diligence in
collecting this Note or enforcing any of the security herefor, (b) severally
agree to any substitution, exchange or release of any such security or the
release of any party primarily or secondarily liable hereon, (c) severally agree
that the Company shall not be required first to institute suit or exhaust its
remedies hereon against Borrower or others liable or to become liable hereon or
to enforce its rights against any security hereof in order to enforce payment of
this Note by it, and (d) consent to any extension or

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postponement of time of payment of this Note and in any other indulgence with
respect hereto without notice thereof.

         The invalidity or unenforceability in particular circumstances of any
provision of this Note shall not extend beyond such provision or such
circumstances and no other provision of this Note shall be affected thereby.

         Any check, draft, money order or other instrument given in payment of
all or any portion of this Note may be accepted by the Company and handled in
collection in the customary manner, but the same shall not constitute payment
hereunder or diminish any rights of the Company except to the extent that actual
cash proceeds of such instrument are unconditionally received by the Company and
applied to the indebtedness as herein provided.

         THIS NOTE HAS BEEN EXECUTED AND DELIVERED IN, AND SHALL BE GOVERNED BY
AND CONSTRUED ACCORDING TO THE LAWS OF THE STATE OF ILLINOIS EXCEPT TO THE
EXTENT PRE-EMPTED BY APPLICABLE LAWS OF THE UNITED STATES OF AMERICA.

         This note may not be changed or terminated without the prior written
approval of the Company and Borrower. No waiver of any term or provision hereof
shall be valid unless in writing signed by the Company.

         All notices and other communications in respect of this Note
(including, without limitation, any modifications of, or requests, waivers or
consents under, this Note) shall be given or made in writing (including, without
limitation, by telecopy) to the Borrower or the Company, as the case may be, at
the applicable address for notices specified in the Stock Pledge and Security
Agreement, or at such other address as shall be designated by either party in a
notice to the other party. Except as otherwise provided in this Note, all such
communications shall be deemed to have been duly given when transmitted by
telecopier or personally delivered or, in the case of a mailed notice, upon
receipt, in each case given or addressed as aforesaid.

         IN WITNESS WHEREOF, the undersigned has executed this Note on the day
and year first written above.

                                    BORROWER

                                    --------------------------------------------
                                    David M. Lerten

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                                                                     Exhibit 4.4

================================================================================

                    ALLIED RISER COMMUNICATIONS CORPORATION,

                                     ISSUER,

                       COGENT COMMUNICATIONS GROUP, INC.,

                                   CO-OBLIGOR,

                                       AND

                            WILMINGTON TRUST COMPANY,

                                     TRUSTEE

                                   ----------

                          FIRST SUPPLEMENTAL INDENTURE

                          Dated as of January ___, 2002

                                       TO

                                    INDENTURE

                            Dated as of June 28, 2000

                                   ----------

                  7.50% CONVERTIBLE SUBORDINATED NOTES DUE 2007

================================================================================

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         FIRST SUPPLEMENTAL INDENTURE, ("First Supplemental Indenture") dated as
of January ___, 2002, among ALLIED RISER COMMUNICATIONS CORPORATION, a
corporation duly organized and existing under the laws of the State of Delaware,
having its principal office at 1700 Pacific Avenue, Suite 400, Dallas, Texas
75201 (herein called the "Company"), COGENT COMMUNICATIONS GROUP, INC., a
corporation duly organized and existing under the laws of the State of Delaware,
having its principal office at 1015 Thirty-First Street, N.W., Washington, D.C.
20007 (herein called "Cogent"), and WILMINGTON TRUST COMPANY, a banking
corporation, not in its individual capacity but solely as Trustee hereunder and
under the Indenture (as defined below) (herein called the "Trustee").
Capitalized terms not otherwise defined herein shall have the meanings set forth
in the Indenture.

                                    RECITALS

         WHEREAS, the Company and the Trustee have heretofore executed and
delivered a certain Indenture, dated as of June 28, 2000 (the "Indenture"),
providing for the issuance of Securities and, subject to and upon compliance
with applicable provisions of the Indenture, the conversion of any Security at
the option of the Holder thereof into fully paid and nonassessible shares of
Common Stock of the Company at the Conversion Rate in effect at the time of
conversion;

         WHEREAS the Company, Cogent and Augustus Caesar Merger Sub, Inc., a
corporation duly organized and existing under the laws of the State of Delaware
and a wholly owned subsidiary of Cogent (the "Merger Sub"), have entered into an
Agreement and Plan of Merger, dated as of August 28, 2001, as amended by
Amendment No. 1, dated as of October 13, 2001 (as so amended, the "Merger
Agreement"), which contemplates the execution and filing of a certificate of
merger (the "Certificate of Merger") providing for the merger of the Merger Sub
with and into the Company (the "Merger"), with the Company continuing its
corporate existence under the laws of the State of Delaware;

         WHEREAS, Section 7.1 of the Indenture provides, among other things,
that upon the merger of any other Person with or into the Company, the due and
punctual payment of the principal of, premium, if any, and interest on all of
the Securities as applicable, and the performance or observance of every
covenant of the Indenture to be performed or observed by the Company shall be
expressly assumed, by an indenture supplemental to the Indenture, executed and
delivered to the Trustee, in form satisfactory to the Trustee by the Person with
or into which the Company shall have been merged;

         WHEREAS, under the Merger Agreement and at the effective time of the
Merger, each share of common stock of the Company, par value $0.0001 per share,
will be converted into the right to receive 0.0321679 shares of common stock of
Cogent, par value $0.001 per share, and the Company shall become a wholly owned
subsidiary of Cogent;

         WHEREAS, Section 7.1 of the Indenture further provides, among other
things, that any Person into or with which the Company is merged shall have
provided for conversion rights in accordance with Article XII;

         WHEREAS, Section 8.1 of the Indenture provides, among other things,
that, without the

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consent of any Holders of Securities, the Company, when authorized by a Board
Resolution of the Company, and the Trustee, at any time and from time to time,
may enter into one or more indentures supplemental to the Indenture to evidence
the succession of another Person to the Company and the assumption by any such
successor of the covenants and obligations of the Company in the Indenture and
in the Securities;

         WHEREAS, the execution and delivery of this First Supplemental
Indenture has been authorized by resolutions of the board of directors of the
Company and by resolutions of the board of directors of Cogent;

         NOW, THEREFORE, THIS FIRST SUPPLEMENTAL INDENTURE WITNESSETH:

         For and in consideration of the premises and intending to be legally
bound hereby, it is mutually covenanted and agreed, for the equal and
proportionate benefit of all Holders of the Securities, as follows:

                                    ARTICLE I
                    REPRESENTATIONS OF THE COMPANY AND COGENT

         Each of the Company and Cogent represents and warrants as of the date
hereof and as of the time the Merger becomes effective to the Trustee as
follows:

         SECTION 1.1. It is a corporation duly organized, validly existing and
in good standing under the laws of the State of Delaware.

         SECTION 1.2. The execution, delivery and performance by it of this
First Supplemental Indenture have been authorized and approved by all necessary
corporate action on its part.

                                   ARTICLE II
                   CONFIRMATION AND ASSUMPTION OF OBLIGATIONS

         SECTION 2.1. The Company hereby confirms its obligations in respect of
the due and punctual payment of the principal of, premium, if any, and interest
on all of the Securities as applicable, and the performance or observance of
every covenant of the Indenture to be performed or observed by the Company.

         SECTION 2.2. Cogent hereby assumes, as of the date hereof, pursuant to
this First Supplemental Indenture and as the co-obligor of the Company, all
obligations in respect of the due and punctual payment of the principal of,
premium, if any, and interest on all of the Securities as applicable, and the
performance or observance of every covenant of the Indenture to be performed or
observed by the Company.

         SECTION 2.3. The Holder of each Security that is Outstanding as of the
date hereof shall have the right, during the period such Security shall be
convertible as specified in Section 12.1 of the Indenture, to convert such
Security only into the kind and amount of common stock of Cogent receivable upon
the Merger by a holder of the number of shares of common stock of the Company
into which such Security might have been converted immediately prior to the
Merger, assuming such holder of common stock of the Company is not a Constituent
Person or

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an Affiliate of a Constituent Person. From and after the Effective Time, the
Conversion Rate shall be subject to adjustments from time to time which shall be
as nearly equivalent as may be practicable to the adjustments provided for in
Article XII of the Indenture.

         SECTION 2.4. Securities authenticated and delivered after the execution
of this First Supplemental Indenture may, and shall if required by the Trustee,
bear a notation in form approved by the Trustee as to any matter provided for in
this First Supplemental Indenture.

         SECTION 2.5. Upon the effective time of this First Supplemental
Indenture, Cogent may exercise every right and power of the Company under the
Indenture with the same effect as if Cogent had been named as a co-obligor of
the Company in the Indenture.

         SECTION 2.6. Pursuant to Section 14.5 of the Indenture, as of the date
hereof, the provisions of the Indenture relating to the right of Holders to
cause repurchase of the Securities following a Change in Control are hereby
modified to make such provisions in the Indenture apply, in the event of a
Change in Control subsequent to the effective time of this First Supplemental
Indenture, to the common stock of Cogent and to Cogent, as follows:

                  The first sentence of the definition of "Common Stock" in
         Section 1.1 of the Indenture is hereby modified and amended to read as
         follows:

                  "Common Stock" means the Common Stock, par value $0.001 per
         share, of Cogent Communications Group, Inc. authorized as of the
         effective date of the First Supplemental Indenture hereto.

                                   ARTICLE III
                                  MISCELLANEOUS

         SECTION 3.1. This First Supplemental Indenture constitutes an integral
part of the Indenture, which, as supplemented and amended by this First
Supplemental Indenture, is in all respects hereby adopted, ratified and
confirmed.

         SECTION 3.2. Notice of the execution of this First Supplemental
Indenture shall be given by the Company to the Holder of each Security as
provided in Sections 1.6 and 12.11 of the Indenture promptly upon such execution
hereof.

         SECTION 3.3. The recitals contained herein shall be taken as the
statements of the Company and Cogent, and the Trustee does not assume any
responsibility for their correctness. The Trustee makes no representations as to
the validity or sufficiency of this First Supplemental Indenture.

         SECTION 3.4. The Trustee acknowledges that the Company has delivered to
the Trustee, in accordance with Sections 7.1(3), 8.3 and 12.11 of the Indenture,
an Officers' Certificate and an Opinion of Counsel.

         SECTION 3.5. If any provision of this First Supplemental Indenture
limits, qualifies or conflicts with the duties imposed by any of Sections 310 to
317, inclusive, of the Trust Indenture Act through operation of Section 318(c)
thereof, such imposed duties shall control.

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         SECTION 3.6. Nothing in this First Supplemental Indenture, express or
implied, shall give to any Person, other than the parties hereto and their
successors and assigns, any benefit or any legal or equitable right, remedy or
claim under this First Supplemental Indenture.

         SECTION 3.7. This First Supplemental Indenture shall be governed by and
construed in accordance with the laws of the State of New York, and for all
purposes shall be governed by and construed in accordance with the laws of such
State, without regard to the conflicts of laws principles thereof.

         SECTION 3.8. This First Supplemental Indenture may be executed in any
number of counterparts, each of which so executed shall be deemed to be an
original, but all such counterparts shall together constitute but one and the
same instrument.

         SECTION 3.9. This First Supplemental Indenture shall become effective
as of 5:00 p.m. New York City time on the date hereof.

         IN WITNESS WHEREOF, the parties hereto have caused this First
Supplemental Indenture to be duly executed all as of the day and year first
above written.

                           ALLIED RISER COMMUNICATIONS CORPORATION

                           By:
                               -------------------------------------------------
                               Name:    [Gerald Dinsmore]
                               Title:   [Chief Executive Officer]

                           COGENT COMMUNICATIONS GROUP, INC.

                           By:
                               -------------------------------------------------
                               Name:    [David Schaeffer]
                               Title:   [Chairman and Chief Executive Officer]

                           WILMINGTON TRUST COMPANY,
                           not in its individual capacity but solely as Trustee
                           hereunder and under the Indenture

                           By:
                               -------------------------------------------------
                               Name:    [Michael W. Diaz]
                               Title:   [Authorized Signer]

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