<PAGE>

                                                                     Exhibit 4.1
                                                                     -----------

                                   Avici (R)

Number                                                                    Shares
MSPR

                               Avici Systems Inc.

              INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

                                  COMMON STOCK

                                                                 SEE REVERSE FOR
                                                             CERTAIN DEFINITIONS

THIS CERTIFIES THAT                                             CUSIP 05367 10 9

is the owner of

   FULLY PAID AND NON-ASSESSABLE SHARES OF THE PAR VALUE OF $.0001 EACH OF THE
                                COMMON STOCK OF

Avici Systems Inc. transferable upon the books of the Corporation in person or
by attorney upon surrender of this certificate duly endorsed or assigned.  This
certificate and the shares represented hereby are subject to the laws of the
State of Delaware and to the Certificate of Incorporation and By-laws of the
Corporation as from time to time amended.

     This certificate is not valid until countersigned by the Transfer Agent and
registered by the Registrar.

     IN WITNESS WHEREOF, Avici Systems Inc. has caused its facsimile corporate
seal and the facsimile signatures of its duly authorized officers to be hereunto
affixed.


Dated:


/s/ Surya R. Panditi                    /s/ Paul F. Brauneis
-------------------------------------   -------------------------------------
President and Chief Executive Officer   Chief Financial Officer and Treasurer
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COUNTERSIGNED AND REGISTERED
                    CHASEMELON SHAREHOLDER SERVICES, L.L.C.
                                                            TRANSFER AGENT
                                                             AND REGISTRAR

BY                                                    AUTHORIZED SIGNATURE

                                     [SEAL]

THE CORPORATION IS AUTHORIZED TO ISSUE MORE THAN ONE CLASS OR SERIES OF STOCK.
A COPY OF THE PREFERENCES, POWERS, QUALIFICATIONS AND RIGHTS OF EACH CLASS AND
SERIES WILL BE FURNISHED BY THE CORPORATION UPON WRITTEN REQUEST AND WITHOUT
CHARGE.

     The following abbreviations, when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable law or regulations:

TEN COM - as tenants in common
TEN ENT - as tenants by the entireties
JT TEN  - as joint tenants with right of
          survivorship and not as tenants
          in common

UNIF GIFT MIN ACT - __________ Custodian __________
                    (Cust)               (Minor)
                    under Uniform Gifts to Minors
                    Act ______________________
                        (State)

          Additional abbreviations may also be used though not in the above
list.  For value received _______________________ hereby sell, assign and
transfer unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE
___________________________________________

_____________________________________________________________________________
            PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS OF ASSIGNEE

_____________________________________________________________________________


_________________________________________________________________Shares of the
capital stock represented by the within Certificate, and do hereby irrevocably
constitute and appoint
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_________________________________________________________________ Attorney to
transfer the said stock on the books of the within-named Corporation with full
power of substitution in the premises.

Dated, ___________________________

                                       _______________________________________
                                       NOTICE: THE SIGNATURE TO THIS ASSIGNMENT
                                       MUST CORRESPOND WITH THE NAME AS WRITTEN
                                       UPON THE FACE OF THE CERTIFICATE IN EVERY
                                       PARTICULAR WITHOUT ALTERATION OR
                                       ENLARGEMENT OR ANY CHANGE WHATEVER

Signature(s) Guaranteed
_________________________________
THE SIGNATURE(S) SHOULD BE GUARANTEED
BY AN ELIGIBLE GUARANTOR INSTITUTION
(BANKS, STOCKBROKERS, SAVINGS AND LOAN
ASSOCIATIONS AND CREDIT UNIONS WITH
MEMBERSHIP IN AN APPROVED SIGNATURE
GUARANTEE MEDALLION PROGRAM), PURSUANT
TO S.E.C. RULE 17Ad-15.