UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of report (Date of earliest event reported): July 22, 2013

CISCO SYSTEMS, INC.

(Exact name of registrant as specified in its charter)

California

(State or other jurisdiction of incorporation)

(408) 526-4000

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

On July 23, 2013, Cisco Systems, Inc. (“Cisco”) announced its entry into a definitive agreement for Cisco to acquire Sourcefire, Inc. (“Sourcefire”). Sourcefire is a leader in intelligent cybersecurity solutions.

Under the terms of the agreement, Cisco will pay $76 per share in cash in exchange for each share of Sourcefire and assume outstanding equity awards for an aggregate purchase price of approximately $2.7 billion, including retention-based incentives. The acquisition has been approved by the board of directors of each company.

The acquisition is expected to close during the second half of calendar year 2013, subject to customary closing conditions and regulatory reviews. Cisco expects the acquisition to be slightly dilutive to non-GAAP earnings in fiscal year 2014 due to normal purchase accounting adjustments and integration costs.

Certain Sourcefire stockholders that hold in the aggregate approximately 1.9% of the total outstanding shares of Sourcefire common stock, have agreed to vote their shares in favor of the transaction.

Prior to the close, Cisco and Sourcefire will continue to operate as separate companies. Upon completion of the transaction, Sourcefire employees will join the Cisco Security Group led by Christopher Young.

On July 23, 2013, Cisco and Sourcefire issued a press release relating to the transaction. A copy of the press release is attached hereto as Exhibit 99.1.

This filing may be deemed to contain forward-looking statements, which are subject to the safe harbor provisions of the Private Securities Litigation Reform Act of 1995, including the expected completion of the acquisition and the time frame in which this will occur, the expected benefits to Cisco and its customers from completing the acquisition, the expected financial performance of Cisco (including earnings projections) following completion of the acquisition, and plans regarding Sourcefire personnel. Statements regarding future events are based on the parties’ current expectations and are necessarily subject to associated risks related to, among other things, obtaining Sourcefire’s stockholder and regulatory approval of the acquisition or that other conditions to the closing of the transaction may not be satisfied, the potential impact on the business of Sourcefire due to the uncertainty about the acquisition, the occurrence of any event, change or other circumstances that could give rise to the termination of the definitive agreement, the outcome of any legal proceedings related to the transaction, general economic conditions, the retention of employees of Sourcefire and the ability of Cisco to successfully integrate Sourcefire’s market opportunities, technology, personnel and operations and to achieve expected benefits. Therefore, actual results may differ materially and adversely from those expressed in any forward-looking statements. For information regarding other related risks, see the “Risk Factors” section of Cisco’s most recent reports on Form 10-K and Form 10-Q filed with the SEC on September 12, 2012 and May 21, 2013, respectively. Cisco undertakes no obligation to revise or update any forward-looking statements for any reason.

Additional Information and Where to Find It

In connection with the proposed acquisition and required stockholder approval, Sourcefire will file with the Securities and Exchange Commission a preliminary proxy statement and a definitive proxy statement. The proxy statement will be mailed to the stockholders of Sourcefire. Sourcefire’s stockholders are urged to read the proxy statement (including all amendments and supplements) and other relevant materials when they become available because they will contain important information. Investors may obtain free copies of these documents (when they are available) and other documents filed with the SEC at its website at http://www.sec.gov. In addition, investors may obtain free copies of the documents filed with the SEC by Sourcefire by going to Sourcefire’s Investor Relations page on its corporate website at http://investor.sourcefire.com/ or by directing a request to Sourcefire at: Sourcefire, 9770 Patuxent Woods Drive, Columbia, MD 21046.

Sourcefire and its officers and directors and other members of management and employees may be deemed to be participants in the solicitation of proxies from Sourcefire’s stockholders with respect to the acquisition. Information about Sourcefire’s executive officers and directors is set forth in the proxy statement for the Sourcefire 2013 Annual Meeting of Stockholders, which was filed with the SEC on April 24, 2013. Investors may obtain more detailed information regarding the direct and indirect interests of Sourcefire and its respective executive officers and directors in the acquisition by reading the preliminary and definitive proxy statements regarding the transaction, which will be filed with the SEC.

In addition, Cisco and its officers and directors may be deemed to have participated in the solicitation of proxies from Sourcefire’s stockholders in favor of the approval of the transaction. Information concerning Cisco’s directors and executive officers is set forth in Cisco’s proxy statement for its 2012 Annual Meeting of Shareholders, which was filed with the SEC on September 26, 2012, annual report on Form 10-K filed with the SEC on September 12, 2012, Form 8-K filed with the SEC on February 1, 2013, and Form 8-K filed with the SEC on October 4, 2012. These documents are available free of charge at the SEC’s website at www.sec.gov or by going to Cisco’s Investor Relations website at http://www.cisco.com/go/investors.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

EXHIBIT INDEX