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                                                                     EXHIBIT 4.4


                                AMENDMENT NO. 2
                                       TO
                          SECOND AMENDED AND RESTATED
                           INVESTOR RIGHTS AGREEMENT
                           -------------------------

          This Amendment dated as of August 5, 1999 is entered into by and among
Sycamore Networks, Inc., a Delaware corporation (the "Company"), and the persons
and entities listed on Schedule I hereto.

          WHEREAS, the Company has entered into a Second Amended and Restated
Investor Rights Agreement dated as of February 26, 1999 and amended on July 23,
1999 (the "Agreement");

          WHEREAS, the Company and the requisite parties necessary to effect an
amendment to the Agreement pursuant to Section 7(b) thereof desire that the
Agreement be amended in the manner set forth below; and

          WHEREAS, the parties hereto desire to amend the Agreement pursuant to
this Amendment No. 2.

          NOW THEREFORE, in consideration of the mutual covenants contained
herein and for other valuable consideration, the receipt and sufficiency of
which is hereby acknowledged, the parties hereto agree as follows:

          1.  Section 2 of Article V of the Agreement is hereby amended to add
the word "parents" as persons to whom a Founder (as such term is defined in the
Agreement) may transfer shares of the capital stock of the Company.

          2.  The Agreement, as supplemented and modified by this Amendment,
together with the other writings referred to in the Agreement or delivered
pursuant thereto which form a part thereof, contain the entire agreement among
the parties with respect to the subject matter thereof and amend, restate and
supersede all prior and contemporaneous arrangements or understandings with
respect thereto.

          3.  Upon the effectiveness of this Amendment, on and after the date
hereof, each reference in the Agreement to "this Agreement," "hereunder,"
"hereof," "herein" or words of like import, and each reference in the other
documents entered into in connection with the Agreement, shall mean and be a
reference to the Agreement, as amended hereby.  Except as specifically amended
above, the Agreement shall remain in full force and effect and is hereby
ratified and confirmed.
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          4.  This Amendment shall be governed by the laws of the State of
Delaware, notwithstanding the conflict-of-law doctrines of Delaware or any other
jurisdiction to the contrary.

          5.  This Amendment may be executed in any number of counterparts, and
each such counterpart shall be deemed to be an original instrument, but all such
counterparts together shall constitute but one agreement.

          6.  This Amendment shall be binding on all parties to the Agreement as
and when executed by the Company, Founding Stockholders holding at least a
majority by voting power of the shares of capital stock held by the Founding
Stockholders and Investors holding at least a majority of the shares of Common
Stock issued or issuable upon conversion of the shares of the Series A Preferred
Stock and/or the Series B Preferred Stock and/or the Series C Preferred Stock
and/or the Series D Preferred Stock.


               [Remainder of this Page Intentionally Left Blank]

                                       2
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          IN WITNESS WHEREOF the parties hereto have executed this Amendment on
the date first above written.

                              COMPANY:

                              SYCAMORE NETWORKS, INC.


                              By: /s/ Daniel Smith
                                  ---------------------
                                  Daniel Smith
                                  President


                              Matrix Partners V, L.P.
                              Bay Colony Corporate Center
                              1000 Winter Street, Suite 4500
                              Waltham, Massachusetts 02154

                              By:   Matrix V Management Co., L.C.C.,
                                    its General Partner

                              By: /s/ Paul J. Ferri
                                  ---------------------


                              Matrix V Entrepreneurs Fund, L.P.
                              Bay Colony Corporate Center
                              1000 Winter Street, Suite 4500
                              Waltham, Massachusetts 02154

                              By:   Matrix V Management Co., L.C.C.,
                                    its General Partner


                              By /s/ Paul J. Ferri
                                 ----------------------


                              /s/ Gururaj Deshpande
                              ------------------------
                              Gururaj Deshpande


                              /s/ Daniel Smith
                              ------------------------
                              Daniel Smith

                                       3
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                              FOUNDING STOCKHOLDERS:


                              /s/ Gururaj Deshpande
                              ------------------------
                              Gururaj Deshpande

                              Address:   9 Sparta Way
                                         Andover, MA 01810


                              /s/ Richard Barry
                              ------------------------
                              Richard Barry

                              Address:   1284 Beacon Street, #815
                                         Brookline, MA 02138

                                       4
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                                   Schedule 1
                                   ----------

                                   Investors
                                   ---------


Matrix Partners V, L.P.
Matrix V Entrepreneurs Fund, L.P.
Gururaj Deshpande
Daniel Smith
Richard Barry