Exhibit 5.1

Simpson Thacher & Bartlett LLP

2475 HANOVER STREET

PALO ALTO, CA 94304

TELEPHONE: +1-650-251-5000

FACSIMILE: +1-650-251-5002

Direct Dial Number E-mail Address

February 15, 2024

Cisco Systems, Inc.

170 West Tasman Drive

San Jose, California 95134

Ladies and Gentlemen:

We have acted as counsel to Cisco Systems, Inc., a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-3 (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the issuance of debt securities of the Company (collectively, the “Securities”). The Securities may be issued and sold or delivered from time to time as set forth in the Registration Statement, any amendment thereto, the prospectus contained therein (the “Prospectus”) and supplements to the Prospectus pursuant to Rule 415 under the Securities Act in an indeterminate amount.

The Securities will be issued under an indenture (the “Base Indenture” and, including any supplemental indentures relating thereto, the “Indenture”), between the Company and Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”).

We have examined the Registration Statement and the form of Base Indenture, which is an exhibit to the Registration Statement. In addition, we have examined, and have relied as to matters of fact upon, originals, or duplicates or certified or conformed copies, of such records, agreements, documents and other instruments and such certificates or comparable documents of public officials and of officers and representatives of the Company and have made such other investigations as we have deemed relevant and necessary in connection with the opinions hereinafter set forth.

BEIJING  BRUSSELS  HONG KONG  HOUSTON  LONDON  LOS ANGELES  PALO A LTO  SÃO PAULO  TOKYO  WASHIN GTON, D. C.

	Simpson Thacher & Bartlett LLP
-2-	February 15, 2024

In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies, and the authenticity of the originals of such latter documents. We also have assumed that at the time of execution, authentication, issuance and delivery, as applicable, of the Securities, the Indenture (including the applicable supplemental indentures) will be the valid and legally binding obligation of the Trustee.

In rendering the opinion set forth below, we have assumed further that, at the time of execution, authentication, issuance and delivery, as applicable, of the Indenture and the Securities, (1) the Company will be validly existing and in good standing under the law of the jurisdiction in which it is organized and the Indenture will have been duly authorized, executed and delivered by the Company in accordance with its organizational documents and the law of the jurisdiction in which it is organized, (2) the execution, delivery and performance by the Company of the Indenture will not constitute a breach or violation of its organizational documents or violate the law of the jurisdiction in which it is organized or any other jurisdiction (except that no such assumption is made with respect to the law of the State of New York or the Delaware General Corporation Law, assuming there shall not have been any change in such laws affecting the validity or enforceability of the Indenture) and (3) the execution, delivery and performance, as applicable, by the Company of the Indenture will not constitute a breach or default under any agreement or instrument which is binding upon the Company.

	Simpson Thacher & Bartlett LLP
-3-	February 15, 2024

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that assuming (a) the taking of all necessary corporate action by the Board of Directors of the Company, a duly constituted and acting committee of such Board or duly authorized officers of the Company (such Board of Directors, committee or authorized officers being hereinafter referred to as the “Company Authorizing Party”) to approve the issuance and terms of the Securities and the terms of the offering thereof so as not to violate any applicable law or agreement or instrument then binding on the Company and (b) the due execution, authentication, issuance and delivery of the Securities, upon payment therefor in accordance with the applicable definitive underwriting, purchase or similar agreement approved by the Company Authorizing Party and otherwise in accordance with the provisions of such agreement and the Indenture, the Securities will constitute valid and legally binding obligations of the Company enforceable against the Company in accordance with their terms.

Our opinion set forth above is subject to (i) the effects of bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and other similar laws relating to or affecting creditors’ rights generally, (ii) general equitable principles (whether considered in a proceeding in equity or at law) and (iii) an implied covenant of good faith and fair dealing. In addition, we express no opinion as to the validity, legally binding effect or enforceability of (A) the waiver of rights and defenses contained in Section 512 of the Base Indenture or (B) Section 110 of the Base Indenture relating to the severability of provision of the Base Indenture.

We do not express any opinion herein concerning any law other than the law of the State of New York.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the use of our name under the caption “Legal Matters” in the prospectus included in the Registration Statement.

Very truly yours,

/s/ Simpson Thacher & Bartlett LLP

SIMPSON THACHER & BARTLETT LLP