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                                                                     EXHIBIT 5.1

                   OPINION OF BROBECK, PHLEGER & HARRISON LLP


                                 March 13, 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 4,858,995 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) 4,842,032 outstanding shares of Common Stock of the Company (the "Shares")
and (ii) 16,963 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 to which this opinion is attached as Exhibit 5.1 ("Registration Statement").

        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 Radiata, Inc. 1999 Stock
Option 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.

        This opinion letter is rendered as of the date first written above and
we disclaim any obligation to advise you of facts, circumstances, events or
developments which hereafter may be brought to our attention and which may
alter, affect or modify the opinion expressed herein. 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




