Exhibit 5.1

[WILSON SONSINI GOODRICH & ROSTI LOGO]
Wilson Sonsini Goodrich & Rosati
PROFESSIONAL CORPORATION

November 22, 2005

Brocade Communications Systems, Inc.
1745 Technology Drive
San Jose, California 95110

RE: REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

          We have examined the Registration Statement on Form S-8 to be filed by you with the Securities and Exchange Commission on or about November 22, 2005 (the “Registration Statement”) in connection with the registration under the Securities Act of 1933, as amended (the “Act”) of 424,822 additional shares available for issuance under the Therion Software Corporation 2004 Stock Plan. Such shares of Common Stock are referred to herein as the “Shares,” and such plan is referred to herein as the “Plan.” As your counsel in connection with this transaction, we have examined the proceedings taken and are familiar with the proceedings proposed to be taken by you in connection with the issuance and sale of the Shares pursuant to the Plan.

          It is our opinion that, when issued and sold in the manner described in the Plan and pursuant to the agreements which accompany each grant or purchase under the Plan, the Shares will be legally and validly issued, fully-paid and non-assessable.

          We consent to the use of this opinion as an exhibit to the Registration Statement, and consent to the use of our name wherever appearing in the Registration Statement and any amendments thereto.

Sincerely,

WILSON SONSINI GOODRICH & ROSATI
Professional Corporation

/s/ Wilson Sonsini Goodrich & Rosati