<PAGE>   1

                                                                  Exhibit 10.25


                                 FIRST AMENDMENT
                               TO CREDIT AGREEMENT


        This FIRST AMENDMENT TO CREDIT AGREEMENT is entered into as of March 21,
2000 (this "Amendment") by and between BROCADE COMMUNICATIONS, a California
corporation, with its principal place of business at 1901 Guadalupe Parkway, San
Jose, California 95131 ("Borrower"), and COMERICA BANK-CALIFORNIA ("Bank"), a
California banking corporation, with its principal place of business at 333 West
Santa Clara Street, San Jose, California 95113.


                                    RECITALS

        A. Borrower and Bank have previously entered into that certain Credit
Agreement dated as of January 5, 2000 (the "Credit Agreement").

        B. Pursuant to the terms of the Credit Agreement, Borrower wishes to
obtain an additional standby letter of credit from Bank, and Bank desires to
issue such standby letter of credit.

        C. This Amendment sets forth the terms on which Bank will issue the
additional standby letter of credit to Borrower.


                                    AGREEMENT

        For good and valuable consideration, receipt of which is hereby
acknowledged, the parties agree as set forth below.

        1. Incorporation by Reference; Definitions. The foregoing Recitals and
the Credit Agreement are incorporated herein by this reference as though set
forth in full herein. Any terms not defined herein shall have the meanings given
in the Credit Agreement. As used herein, and unless otherwise defined herein,
the following terms shall have the following respective meanings except as the
context shall otherwise require:

           "Letter of Credit" means the standby letter of credit issued by Bank
for the account of Borrower, and for the benefit of Spieker Properties in the
amount of One Million Three Hundred Thirty Five Thousand Two Hundred Seventy One
and 00/100 Dollars ($1,335,271.00).

           "Letter of Credit Agreement" means the Standby Letter of Credit
Application and Agreement dated the date hereof by and between Borrower and
Bank.



                                      -1-
<PAGE>   2


           "Letter of Credit Documents" means this Amendment, the Letter of
Credit, and the Letter of Credit Agreement.

           "Maturity Date" means March 30, 2003.

        2. Amendment to the Credit Agreement. The parties have agreed to the
amendments to the Credit Agreement set forth in this Section 2.

           2.1 Additional Letter of Credit. The Letter of Credit is issued as an
additional letter of credit under the Credit Agreement and is subject to the
terms and conditions of the Credit Agreement. The Letter of Credit Documents are
incorporated herein by this reference.

           2.2 Issuance of Letters of Credit. Subject to the terms and
conditions of the Letter of Credit Documents and the Credit Agreement, Bank
shall issue the Letter of Credit.

           2.3 Letter of Credit Fees. Borrower shall pay to Bank fees upon the
issuance of the Letter of Credit, and upon the payment by Bank of each draft
under the Letter of Credit, in accordance with Bank's standard fees and charges
in effect at the time the Letter of Credit is issued or amended or any draft is
paid. In addition, Borrower shall prepay to Bank an annual fee with respect to
the Letter of Credit in an amount equal to three quarters of one percent (0.75%)
per annum of the original face amount of the Letter of Credit for the time
period equal to the date of issuance of the Letter of Credit to and including
the Maturity Date.

           2.4 Reimbursement by Borrower. Borrower shall immediately reimburse
Bank for all sums paid by Bank on account of the Letter of Credit in accordance
with the Letter of Credit Agreement.

           2.5 Indemnity. Borrower shall indemnify Bank as set forth in the
Letter of Credit Agreement.

           2.6 Payment of Drafts. Bank shall pay drafts as set forth in the
Letter of Credit Agreement.

        3. Conditions Precedent.

           3.1 Issuance of the Letter of Credit. As conditions precedent to
Bank's obligation to issue the Letter of Credit, Borrower shall deliver, or
cause to be delivered, to Bank:

               3.1.1 A duly executed copy of this Amendment;

               3.1.2 A duly executed copy of the Letter of Credit Agreement;


                                      -2-
<PAGE>   3

               3.1.3 All representations and warranties of Borrower to Bank set
forth in the Letter of Credit Documents shall be accurate and complete in all
respects; and

               3.1.4 There shall not exist an Event of Default or an event which
with the giving of notice of passage of time, or both, would be an Event of
Default.

        4. No Modification of Other Obligations. Except as is otherwise
specifically set forth herein, the Credit Agreement shall remain unmodified and
in full force and effect, and is hereby ratified and confirmed.

        5. Conflicts. If a conflict exists between the provisions of the Credit
Agreement and the provisions of this Amendment, the provisions of this Amendment
shall control.

        6. Further Assurances. Borrower agrees to make and execute such other
documents as may be necessary or required to effectuate the terms and conditions
of this Amendment.

        7. Future Modifications. This Amendment does not entitle, or imply any
consent or agreement to, any further or future modification of, amendment to,
waiver of, or consent with respect to any provision of the Credit Agreement. Any
amendments hereto shall be in writing and signed by the parties.

        8. Integration. This Amendment is an integrated agreement, and
supercedes all negotiations and agreement regarding the subject matter hereof,
and taken together with the Credit Agreement, constitutes the final agreement of
the parties with respect to the subject matter hereof and thereof.

        9. Fees and Costs. Borrower shall pay Bank's attorneys' fees and costs
incurred in the preparation of this Amendment.

        10. Severability. In the event any one or more of the provisions
contained in this Amendment is held to be invalid, illegal or unenforceable in
any respect, then such provision shall be ineffective only to the extent of such
prohibition or invalidity, and the validity, legality, and enforceability of the
remaining provisions contained herein shall not in any way be affected or
impaired thereby.

        11. Interpretation. This Amendment and all agreements relating to the
subject matter hereof are the product of negotiation and preparation by and
among each party and its respective attorneys, and shall be construed
accordingly. The parties waive the provisions of California Civil Code Section
1654.

        12. Counterparts. This Amendment may be signed in any number of
counterparts, each of which shall be an original, with the same effect as if all
signatures were upon the same




                                      -3-
<PAGE>   4

instrument. Delivery of an executed counterpart of the signature page to this
Amendment by telefacsimile shall be effective as delivery of a manually executed
counterpart of this Amendment, and any party delivering such an executed
counterpart of the signature page to this Amendment by telefacsimile to any
other party shall thereafter also promptly deliver a manually executed
counterpart of this Amendment to such other party, provided that the failure to
deliver such manually executed counterpart shall not affect the validity,
enforceability, or binding effect of this Amendment.

        IN WITNESS WHEREOF, the parties have caused this Amendment to be
executed as of the day and year first written above.



BROCADE COMMUNICATIONS


By:
   --------------------------------


Title:
      -----------------------------


COMERICA BANK-CALIFORNIA



Mary Beth Suhr
Vice President




                                      -4-