Exhibit 5.1

March 16, 2011

Cisco Systems, Inc.

170 West Tasman Drive

San Jose, California 95134-1706

Ladies and Gentlemen:

We have examined the Registration Statement on Form S-3 (File No. 333-157177) (the “Registration Statement”) filed by Cisco Systems, Inc., a California corporation (the “Company”), on February 9, 2009 with the United States Securities and Exchange Commission (the “Commission”) under Rule 462(e) of the Securities Act of 1933, as amended (the “Securities Act”), relating to the proposed issuance and sale, from time to time by the Company of one or more series of its debt securities (the “Debt Securities”) issuable pursuant to an indenture (the “Indenture”) between the Company and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”), with an indeterminate aggregate principal amount of the Debt Securities as may at various times be issued at indeterminate prices, in reliance on Rule 456(b) and Rule 457(r) under the Securities Act. With the prospectus and prospectus supplements, which comprise part of the Registration Statement, the Company may offer and sell the Debt Securities from time to time or on a continuous basis.

The Company currently proposes to sell up to an aggregate of $4,000,000,000 Debt Securities, consisting of an aggregate of $1,250,000,000 principal amount of the Company’s Floating Rate Notes due 2014, $2,000,000,000 principal amount of the Company’s 1.625% Senior Notes due 2014, and $750,000,000 principal amount of the Company’s 3.150% Senior Notes due 2017 (collectively, the “Takedown Securities”), under the Registration Statement (the “Offering”), all of which will be sold pursuant to the Underwriting Agreement, dated as of March 9, 2011 (the “Underwriting Agreement”), by and among the Company and the entities referred to therein as the Underwriters.

In rendering this opinion, we have examined the following:

Cisco Systems, Inc.

March 16, 2011

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In our examination of documents for purposes of this opinion, we have assumed, and express no opinion as to, the genuineness of all signatures on original documents, the authenticity and completeness of all documents submitted to us as originals, the conformity to originals and completeness of all documents submitted to us as copies, the legal capacity of all persons or entities executing the same, the lack of any undisclosed termination, modification, waiver or amendment to any documents reviewed by us, and the due authorization, execution and delivery of all documents where authorization, due execution and delivery are prerequisites to the effectiveness thereof (other than the due authorization of the documents described above by the Company). We have also assumed that, if and to the extent that the Takedown Securities are issued in certificated form, the certificates representing the Takedown Securities will be, when issued, properly signed by authorized officers of the Company or their agents, properly authenticated in accordance with the terms of the Indenture and delivered to the intended recipients with the intent that the Company be bound thereby.

Cisco Systems, Inc.

March 16, 2011

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In connection with our opinion expressed below, we have assumed that, at or prior to the time of the delivery of any of the Takedown Securities, there will not have occurred any change in law affecting the validity of the Takedown Securities. We also assume that the Indenture has been validly executed and delivered by the Trustee and the Underwriting Agreement has been validly executed and delivered by the Underwriters.

As to matters of fact relevant to this opinion, we have relied solely upon our examination of the documents referred to above and have assumed the current accuracy and completeness of the information included in the documents referred to above and the representations and warranties made by representatives of the Company to us, including, but not limited to, those set forth in the Management Certificate. We have made no independent investigation or other attempt to verify the accuracy of any of such information or to determine the existence or non-existence of any other factual matters.

We are admitted to practice law in the state of California, and this opinion is rendered only with respect to, and no opinion is expressed herein concerning the application or effect of the laws of any jurisdiction other than, (i) the existing laws of the United States of America, (ii) the existing laws of the state of California, and (iii) solely with respect to whether or not the Debt Securities are the valid and binding obligations of the Company, the existing laws of the state of New York.

This opinion is qualified by, and is subject to, and we render no opinion with respect to, the following limitations and exceptions to the enforceability of the Takedown Securities:

Based upon the foregoing, we are of the opinion that the Takedown Securities, when issued, sold and delivered (i) in the manner and for the consideration approved by an authorized officer of the Company in accordance with the Resolutions and stated in the Prospectus Supplement, and (ii) pursuant to the terms of the Underwriting Agreement, then the Takedown Securities will be validly issued and will constitute valid and binding obligations of the Company.

Cisco Systems, Inc.

March 16, 2011

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We consent to the use of this opinion as an exhibit to the Registration Statement and further consent to all references to us, if any, in the Registration Statement, the Base Prospectus, the Prospectus Supplement and any amendments or supplements thereto. In giving this consent we do not thereby admit that we come within the category of persons whose consent is required by the Securities Act or by the rules and regulations promulgated thereunder.

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This opinion is intended solely for use in connection with the issuance and sale of the Takedown Securities under the Registration Statement and is not to be relied upon for any other purpose. This opinion speaks as of the date first written above, and we assume no obligation to advise you, or any other person or entity, of any fact, circumstance, event or change in the law or the facts that may hereafter be brought to our attention whether or not such occurrence would affect or modify the opinions expressed herein.