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                                                                     EXHIBIT 5.1

                   OPINION OF BROBECK, PHLEGER & HARRISON LLP


                                 March 20, 2001


Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California  95134

        Re:    Cisco Systems, Inc. Registration  Statement on Form S-3 for
               Resale of 4,526,756 Shares of Common Stock

Ladies and Gentlemen:

        We have acted as counsel to Cisco Systems, Inc., a California
corporation (the "Company"), in connection with the registration of resales of
(i) 4,516,707 outstanding shares of Common Stock of the Company (the "Shares")
and (ii) 10,049 shares of Common Stock of the Company that are subject to
issuance upon the exercise of certain outstanding stock purchase options (the
"Option Shares"), as described in the Company's Registration Statement on Form
S-3 ("Registration Statement") to which this opinion is attached as Exhibit 5.1.

        This opinion is being furnished in accordance with the requirements of
Item 16 of Form S-3 and Item 601(b)(5)(i) of Regulation S-K.

        We have reviewed the Company's charter documents, the corporate
proceedings taken by the Company in connection with the original issuance and
sale of the Shares and the Company's assumption of the Exio Communications, Inc.
1999 Stock Plan (the "Plan") and the option agreements relating to the Option
Shares (the "Options"), and such other documents, records and matters of law as
we have deemed necessary for purposes of this opinion. Based on such review and
on the assumptions set forth below, we are of the opinion that (i) the Shares
are duly authorized, validly issued, fully paid and nonassessable and (ii) the
Option Shares are duly authorized and, when issued and delivered upon exercise
of the Options in accordance with the provisions of the Options and the Plan
against payment of the exercise price therefor as provided in the Options and
the Plan, will be validly issued, fully paid and nonassessable.

        In rendering the foregoing opinion, we have (i) assumed (a) that the
Plan and each other instrument (collectively, the "Instruments") pursuant to
which any of the Option Shares are to be issued will at the time of such
issuance constitute valid, binding, and enforceable obligations of the Company,
(b) that any issuance of Option Shares pursuant to the Instruments will be
effected in accordance with the provisions of the Instruments, (c) that the
resolutions of the board of directors of the Company authorizing the Company to
issue the Option Shares will remain in full force and effect until all of the
Option Shares have been issued, and (d) the authenticity of all documents
submitted to us as originals and the conformity to original documents of all
documents submitted to us as copies and (ii) relied, as to certain factual
matters, without any independent investigation, inquiry or verification, upon
statements or certificates of public officials and of representatives of the
Company. In addition, our examination of matters of law has been limited to the
General Corporation Law of the State of California and the federal laws of the
United States of America, in each case as in effect on the date hereof.

        We consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the prospectus which is part of the Registration Statement.
In giving this consent, we do not thereby admit that we are within the category
of persons whose consent is required under Section 7 of the Act, the rules and
regulations of the Securities and Exchange Commission promulgated thereunder or
Item 509 of Regulation S-K.

        Our opinion is expressly limited to the matters set forth above and we
render no opinion, whether by implication or otherwise, as to any other matters
relating to the Company or the Shares.


                                            Very truly yours,

                                            /s/ BROBECK, PHLEGER & HARRISON LLP

                                            BROBECK, PHLEGER & HARRISON LLP




