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                                                                     EXHIBIT 5.1
 
                                  May 14,1999
 
The United States Securities
and Exchange Commission
450 Fifth Street, N.W.
Judiciary Plaza
Washington, D.C. 20549
 
     Re: Cisco Systems, Inc. Registration Statement on Form S-4
         for Issuance of 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 proposed public offering of the
Company's Common Stock (the "Shares"), as described in the Company's
Registration Statement on Form S-4 ("Registration Statement") filed on the date
hereof with the Securities and Exchange Commission under the Securities Act of
1933, as amended (the "Act").
 
     This opinion is being furnished in accordance with the requirements of Item
21 of Form S-4 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 issuance and sale of the Shares, and
a certificate of a Company officer regarding (among other things) the Company's
receipt of consideration upon the issuance and sale of the Shares. Based on such
review and assuming the Registration Statement becomes and remains effective,
and all applicable state and federal laws are complied with, we are of the
opinion that the Shares when issued will be validly issued, fully paid and
nonassessable shares of the Common Stock of the Company.
 
     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
