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                                                                    EXHIBIT 5.1



                                 March 29, 2001

Juniper Networks, Inc.
1194 North Mathilda Avenue
Sunnyvale, California 94089

        Re: Registration Statement on Form S-8

Ladies and Gentlemen:

        With reference to the registration statement that Juniper Networks,
Inc., a Delaware corporation (the "Company") proposes to file with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
registering an aggregate of 6,000,000 shares of its common stock, $.00001 par
value (the "Shares") to be issued and sold pursuant to the Juniper Networks,
Inc. 1999 Employee Stock Purchase Plan (the "Plan"), I am of the opinion that:

        1.      The Plan has been duly adopted by the Company.

        2.      All proper corporate proceedings have been taken so that the
                Shares have been duly authorized and, upon issuance and payment
                therefore in accordance with the Plan and the resolutions of the
                board of directors of the Company relating to the adoption of
                the Plan and the offering and sale of the Shares thereunder,
                will be legally issued, fully paid and nonassessable.

        I hereby consent to the filing of this opinion with the Securities and
Exchange Commission in connection with the registration statement referred to
above. This opinion is limited to the laws of the State of Delaware and the
federal law of the United States of America.


                                             Very truly yours,


                                             /s/ Lisa C. Berry
                                             ----------------------------------
                                             Lisa C. Berry
                                             Vice President,  General Counsel
                                             and Secretary