EXHIBIT 99(A)--CERTIFICATION OF CHIEF EXECUTIVE OFFICER

                           CERTIFICATION PURSUANT TO
                            18 U.S.C. SECTION 1350,
                            AS ADOPTED PURSUANT TO
                 SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

   In connection with the Annual Report of Sycamore Networks, Inc. (the
"Company") on Form 10-K for the period ending July 31, 2002 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), I, Daniel
E. Smith, Chief Executive Officer of the Company, certify, pursuant to 18
U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley
Act of 2002, that to the best of my knowledge:

      (1) The Report fully complies with the requirements of section 13(a) or
   15(d) of the Securities Exchange Act of 1934; and

      (2) The information contained in the Report fairly presents, in all
   material respects, the financial condition and results of operations of the
   Company, as of, and for the periods presented in the Report.


                                                      /s/  DANIEL E. SMITH
                                                  -----------------------------
                                                         Daniel E. Smith
                                                  President and Chief Executive
                                                             Officer

October 24, 2002