EXHIBIT 5.1


                                                          September 8, 2000

Sycamore Networks, Inc.
10 Elizabeth Drive
Chelmsford, Massachusetts  01824

      Re:  Sirocco Systems, Inc. 1998 Stock Plan

Ladies and Gentlemen:

          We have acted as counsel to Sycamore Networks, Inc., a Delaware
corporation (the "Registrant"), in connection with the preparation of a
Registration Statement on Form S-8 (the "Registration Statement"), for the
purpose of registering with the Securities and Exchange Commission (the
"Commission"), under the Securities Act of 1933, as amended (the
"Securities Act"), an aggregate of 1,654,352 shares of the Registrant's
common stock, par value $.001 per share ("Common Stock"), issuable pursuant
to the Sirocco Systems, Inc. 1998 Stock Plan (the "Plan"). The Plan
provided for the grant of options to purchase common stock, $.01 par value
per share, of Sirocco Systems, Inc. to officers, directors, employees and
consultants of Sirocco Systems, Inc. However, pursuant to the Agreement and
Plan of Merger among the Registrant, Tropical Acquisition Corporation, a
Delaware corporation and wholly owned subsidiary of the Registrant, and
Sirocco Systems, Inc. ("Sirocco"), dated as of June 5, 2000, each of these
options has been converted into an option to purchase a fraction of a share
of Common Stock (collectively, the "Options").

          This opinion is being furnished in accordance with the
requirements of Item 601(b)(5) of Regulation S-K under the Securities Act.

          In connection with this opinion, we have examined originals or
copies, certified or otherwise identified to our satisfaction, of (i) the
Registration Statement, as filed with the Commission on the date hereof
under the Securities Act; (ii) a specimen certificate representing the
Common Stock; (iii) the Amended and Restated Certificate of Incorporation
of the Registrant, as presently in effect; (iv) the Amended and Restated
By-Laws of the Registrant, as presently in effect; (v) certain resolutions
of the Board of Directors of Sirocco and the Registrant relating to the
Plan; and (vi) the Plan. We have also examined originals or copies,
certified or otherwise identified to our satisfaction, of such records of
the Registrant and such other documents, certificates and records as we
have deemed necessary or appropriate as a basis for the opinions set forth
herein.

          In our examination, we have assumed the legal capacity of all
natural persons, the genuineness of all signatures, the authenticity of all
documents submitted to us as originals, the conformity to original
documents of all documents submitted to us as certified, conformed or
photostatic copies and the authenticity of the originals of such copies. In
making our examination of documents executed or to be executed by parties
other than the Registrant, we have assumed that such parties had or will
have the power, corporate or other, to enter into and perform all
obligations thereunder and have also assumed the due authorization by all
requisite action, corporate or other, and execution and delivery by such
parties of such documents and the validity and binding effect thereof on
such parties. As to any facts material to the opinions expressed herein
which we have not independently established or verified, we have relied
upon statements and representations of officers and other representatives
of the Registrant and others. We have assumed that each award agreement
setting forth the terms of each grant of options or other awards under the
Plan is consistent with the Plan and will be duly authorized and validly
executed and delivered by the parties thereto, and that the consideration
received by the Registrant for the Common Stock delivered pursuant to the
Plan will be in an amount at least equal to the par value of such Common
Stock. We have also assumed that, upon issuance, the stock certificates
evidencing such Common Stock will be manually signed by an authorized
officer of the transfer agent and registrar for the Common Stock and
registered by such transfer agent and registrar and will conform to the
specimen stock certificate examined by us.

          Members of our firm are admitted to the Bar of the Commonwealth
of Massachusetts and we do not express any opinion as to the laws of any
jurisdiction other than the corporate laws of the State of Delaware and we
do not express any opinion as to the effect of any other laws on the
opinion stated herein.

          Based upon and subject to the foregoing, we are of the opinion
that the shares of Common Stock initially issuable upon exercise of the
Options have been duly authorized for issuance by the Registrant and, when
such shares of Common Stock are issued upon exercise of the Options in
accordance with the terms of the Plan, such shares of Common Stock will be
validly issued, fully paid and nonassessable.

          We hereby consent to the filing of this opinion with the
Commission as an exhibit to the Registration Statement. In giving such
consent, we do not thereby admit that we are included in the category of
persons whose consent is required under Section 7 of the Securities Act or
the rules and regulations of the Commission promulgated thereunder.

          This opinion is furnished by us, as the Registrant's special
counsel, in connection with the filing of the Registration Statement with
the Commission and, except as provided in the immediately preceding
paragraph, is not to be used, circulated, quoted or otherwise referred to
for any other purpose or relied upon by any other person without our prior
written permission.

                             Very truly yours,


                             /s/ Skadden, Arps, Slate, Meagher & Flom LLP
                             Skadden, Arps, Slate, Meagher & Flom LLP