Exhibit 4.1

AVICI

SYSTEMS



AVICI SYSTEMS INC.



INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE



THIS CERTIFIES THAT





SEE REVERSE FOR

CERTAIN DEFINITIONS





is the owner of



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

AVICI SYSTEMS INC. transferable on the books of the Corporation in person or by attorney upon surrender of this Certificate properly
endorsed. This Certificate and the shares represented hereby are subject to the laws of the State of Delaware and to the Certificate of Incorporation and the By-laws of the Corporation as from time to time amended.

This Certificate is not valid until countersigned and registered by the Transfer Agent and 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/    Paul F. Brauneis | /s/    Steven  B.  Kaufman
|----------------------- | --------------------------



|TREASURER AND | PRESIDENT  AND
|------------- | --------------

|CHIEF FINANCIAL OFFICER | CHIEF  EXECUTIVE  OFFICER
|----------------------- | -------------------------









COUNTERSIGNED AND REGISTERED:

MELLON INVESTOR SERVICES LLC

TRANSFER AGENT

AND REGISTRAR

BY



AUTHORIZED SIGNATURE



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AVICI SYSTEMS INC.



The Corporation is authorized to issue more than one class of stock. A copy of the full text of the preferences, voting
powers, qualifications and special or relative rights of the shares of each class of stock will be provided to the holder hereof 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 laws or regulations:



|TEN COM |  |  |  | as tenants in common                                                   |  |  |  | UNIF GIFT MIN ACT |  |  |  | .................Custodian..............                                           
|------- |  | - |  | ---------------------------------------------------------------------- |  |  |  | ----------------- |  | - |  | -----------------------------------------------------------------------------------
|TEN ENT |  |  |  | as tenants by the entiretios                                           |  |  |  |                   |  |   |  | (Cust)                    (Minor)                                                  
|JT TEN  |  |  |  | as joint tenants with right ofsurvivorship and not as tenantsin common |  |  |  |                   |  |   |  | under Uniform Gifts to MinorsAct...........................................
(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

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(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)



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



Dated, ____________________________________



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| | 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:

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THE SIGNATURE(S) MUST 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-16.



This certificate also evidences and entitles the holder hereof to certain Rights as set forth in a Rights Agreement between Avici
Systems Inc. and Mellon Investor Services LLC, dated as of December 6, 2001, as amended, (the Rights Agreement), the terms of which are hereby incorporated herein by reference and a copy of which is on file at the principal executive
offices of Avici Systems Inc. Under certain circumstances, as set forth in the Rights Agreement, such Rights will be evidenced by separate certificates and will no longer be evidenced by this certificate. Avici Systems Inc. will mail to the holder
of this certificate a copy of the Rights Agreement without charge after receipt of a written request therefor. Under certain circumstances, as set forth in the Rights Agreement, Rights that are or were acquired or beneficially owned by Acquiring
Persons (as defined in the Rights Agreement) may become null and void.