<PAGE>

                                                                   EXHIBIT 10.63

                                FOURTH AMENDMENT

         THIS FOURTH AMENDMENT (the "AMENDMENT") is made and entered into as of
the 18th day of November, 2003, by and between CA-THE CONCOURSE LIMITED
PARTNERSHIP, A DELAWARE LIMITED PARTNERSHIP ("LANDLORD"), and BROCADE
COMMUNICATIONS SYSTEMS, INC., A DELAWARE CORPORATION ("TENANT").

                                    RECITALS

A.       Landlord (as successor by conversion to EOP-The Concourse, L.L.C., a
         Delaware limited liability company, the successor in interest to
         Spieker Properties, L.P., a California limited partnership) and Tenant
         are parties to that certain lease dated December 17, 1999 (the
         "ORIGINAL LEASE"), which Original Lease has been previously amended by
         instruments dated February 16, 2000 (the "FIRST AMENDMENT"), August 11,
         2000 (the "SECOND AMENDMENT") and November 30, 2000 (the "THIRD
         AMENDMENT") (collectively, the "LEASE"). Pursuant to the Lease,
         Landlord has leased to Tenant space currently containing approximately
         271,387 rentable square feet (the "PREMISES") comprised of all of the
         rentable area, approximately 210,667 rentable square feet, in the
         building commonly known as The Concourse VI located at 1745 Technology
         Drive, San Jose, California ("BUILDING VI"), and 60,710 rentable square
         feet described as Suite Nos. 250, 260, 300 and 500 on the 2nd, 3rd and
         5th floors of the building commonly known as The Concourse V located at
         1741 Technology Drive, San Jose, California ("BUILDING V") in the
         project commonly known as "THE CONCOURSE".

B.       Tenant and Landlord mutually desire that the Lease be amended on and
         subject to the following terms and conditions. Terms capitalized and
         not defined herein shall have the definition given them in the Lease or
         Exhibit A attached hereto.

         NOW, THEREFORE, in consideration of the mutual covenants and agreements
herein contained and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Landlord and Tenant agree as
follows:

I.       APPROVAL OF ALTERATIONS. Landlord and Tenant agree that, effective as
         of the date hereof, Tenant has made and Landlord approved all
         Alterations made by Tenant in Building VI prior to the date this
         Amendment, including, without limitation, certain Alterations related
         to the Building VI HVAC systems (the "HVAC Related Alterations") and
         certain other Alterations, all as listed in Section I of Exhibit A
         attached hereto (collectively the "EXISTING ALTERATIONS"). Landlord and
         Tenant acknowledge and agree that certain other Alterations listed on
         Section II of Exhibit A attached hereto are Existing Alterations that
         have been made and shall be deemed approved by Landlord, on the same
         terms and conditions contained herein (subject to no additional fee or
         removal requirements except as expressly provided in the initial
         approval and in this Amendment), if Tenant provides Landlord with
         written evidence of such approval no later than December 31, 2003. If
         no such evidence of approval (or deemed approval pursuant to Section
         12(C) of the Lease) is provided to Landlord by December 31, 2003, then
         such Alterations shall thereafter be subject to Landlord's approval and
         may be required to be removed upon expiration or termination of this
         Lease to the extent provided in the Lease.

         A.       Conditions of Approval of Alterations.

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         (1)      Administrative Fee. Tenant shall, upon the execution of this
                  Amendment, pay Landlord an administration fee of fifty
                  thousand dollars ($50,000), which shall be deemed to be equal
                  to 5% of the cost of the Existing Alterations, as required by
                  Section 12 of the Lease.

         (2)      Representations and Warranties. As of the date of this
                  Amendment, Tenant represents and warrants that the following
                  are true:

                  (a)      Exhibit A hereto contains an accurate and complete
                           description of all Alterations constructed by or on
                           behalf of Tenant in the Premises after the date of
                           the Original Lease and through the date of this
                           Amendment (provided that the parties acknowledge that
                           some of the items listed on Exhibit A are Tenant
                           Improvements).

                 (b)       All of the HVAC Related Alterations were installed in
                           accordance with all applicable laws and regulations
                           and as otherwise described in Exhibit A hereto.

                 (c)       Tenant has provided Landlord a copy of all invoices,
                           receipts, written documentation and warranties
                           applicable to the Existing Alterations and the work
                           performed in connection with the Existing Alterations
                           to the extent required under the Lease.

                 (d)       The terms of Section 8 (regarding Insurance and
                           Indemnification) of the Lease shall specifically
                           apply to the HVAC Related Alterations, including,
                           without limitation, to the roof space or portions of
                           the roof accessed or utilized by Tenant, its
                           representatives, agents, employees or contractors.

B.       Covenants. As of January 1, 2004, and for the remainder of the term of
         this Amendment, including any extension thereof (the "TERM"), Tenant
         shall be responsible for those matters set forth in this Section B:

                 (a)       Tenant shall be responsible for any increased costs
                           to Building VI or The Concourse generated by the HVAC
                           Related Alterations, including, without limitation,
                           all taxes, assessments, charges, fees and other
                           governmental impositions levied or assessed on or
                           because of the HVAC Related Alterations, and any
                           increased electrical costs and the cost of installing
                           a submeter if Building VI becomes a multi-tenant
                           building and Landlord requires such a submeter to
                           measure electrical consumption.

                 (b)       Tenant shall be solely responsible for the
                           maintenance of the building systems serving Building
                           VI for HVAC and all controls appurtenant thereto,
                           including, without limitation, the HVAC Related
                           Alterations (collectively, the "HVAC SYSTEMS"),
                           provided that the HVAC Systems shall not include any
                           component of those systems listed on Exhibit B
                           hereto. In addition, Tenant shall be solely
                           responsible for the maintenance of the secondary
                           distribution electrical systems (as more

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                           clearly defined in Section III of Exhibit A attached
                           hereto) serving the laboratories in Building VI and
                           all controls appurtenant thereto (the "SECONDARY
                           DISTRIBUTION SYSTEMS"). Tenant shall at all times
                           maintain the HVAC Systems and Secondary Distribution
                           Systems in a condition consistent with the operation
                           of a first-class office building. Tenant's
                           obligations hereunder include, without limitation,
                           the replacement, at Tenant's sole cost and expense,
                           of any portions of the HVAC Related Alterations and
                           portions of the Existing Alterations related to the
                           Secondary Distribution Systems (but not the remaining
                           portions of the HVAC Systems or base building
                           electrical systems, which shall be subject to Section
                           1(B)(g) below) if it would be commercially prudent to
                           replace, rather than repair, such portions of the
                           HVAC Related Alterations and such other Existing
                           Alterations, regardless of whether such replacement
                           would be considered a capital expenditure, provided
                           that Tenant may, in its reasonable discretion and
                           provided that such removal shall not negatively
                           affect the overall condition of Building VI, elect to
                           remove instead of replace any component of the HVAC
                           Related Alterations, other Existing Alterations or
                           Secondary Distribution Systems at the end of such
                           component's useful life. In addition, Tenant shall
                           comply with all applicable laws (including permitting
                           requirements) and shall provide Landlord with a copy
                           of any drawings and correspondence related to
                           securing permits, at the time transmitted, as well as
                           current as-built drawings of the electrical plans for
                           Building VI on June 30 and December 31 of each
                           calendar year, which as-built plans shall depict the
                           Secondary Distribution Systems.

                  (c)      In connection with Tenant's maintenance and repair
                           obligations contained in subsection (b) above, Tenant
                           shall, at its own cost and expense, enter into a
                           preventive maintenance service contract with a
                           maintenance contractor approved by Landlord, in its
                           reasonable discretion, for servicing the HVAC
                           Systems, and shall provide copies of such contract
                           and periodic maintenance reports to Landlord. At
                           Landlord's option, at any time in which an uncured
                           event of default exists under the Lease, the
                           maintenance service contract shall be prepaid on an
                           annual basis. The maintenance contract shall
                           specifically name Landlord as a third party
                           beneficiary, with the right to receive copies of all
                           notices delivered under such contract and the ability
                           to exercise Tenant's rights thereunder, at Landlord's
                           election, in connection with any cure of Tenant's
                           default by Landlord, or any assumption by Landlord of
                           Tenant's maintenance obligations with respect to the
                           HVAC Systems.

                  (d)      Upon expiration or earlier termination of the Lease
                           or Tenant's right to possession of the Premises,
                           Landlord, at its option, shall have the right to
                           require Tenant, at Tenant's sole cost and expense, to
                           restore or remove any Alterations installed after the
                           date of this Amendment (to the extent provided in the
                           Lease). In addition, Tenant acknowledges and agrees
                           that it shall, upon the expiration or earlier
                           termination of the Lease or Tenant's right to
                           possession of the

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                           Premises, remove the 2'x4' ceiling grids and
                           referenced in the letter to Sigmatech from Landlord
                           (or its predecessor in interest) regarding same and
                           dated February 17, 2000 (the "REMOVABLES"), and
                           Tenant shall restore the area(s) affected by the
                           Removables to the condition such areas were in prior
                           to installation of such Removables, ordinary wear and
                           tear excepted. Landlord acknowledges and agrees that
                           Tenant shall not be required to remove or to restore
                           the Transformer Alterations, Main Electrical
                           Switchboard Alterations or any HVAC Equipment (each
                           as defined in Exhibit A hereto) to the condition the
                           transformers and related conductors were in prior to
                           commencement of the Lease.

                  (e)      Any damage related to the installation, maintenance,
                           operation and removal of the Existing Alterations (to
                           the extent not caused by Landlord's gross negligence
                           or intentional misconduct), shall be repaired by
                           Tenant, in a manner previously approved by Landlord,
                           or, in Landlord's sole and absolute discretion and
                           with prior notice to Tenant, by Landlord or a third
                           party contractor hired by Landlord, at the sole cost
                           and expense of Tenant. This provision shall survive
                           any expiration or termination of the Lease.

                  (f)      The Existing Alterations shall remain the property of
                           Tenant (unless expressly the property of PG&E as
                           provided in Exhibit A hereto) until the expiration or
                           earlier termination of the Lease or Tenant's right to
                           possession of the Premises, at which time they shall
                           become the property of Landlord unless they are
                           required to be removed by the express terms of this
                           Amendment.

                  (g)      Landlord and Tenant acknowledge and agree that THE
                           "LANDLORD MAINTAINED SYSTEMS" set forth in Exhibit B
                           attached hereto shall be deemed included in
                           Landlord's repair, maintenance and replacement
                           obligations as provided in Section 10 of the Lease,
                           and the cost of such repair, maintenance and
                           replacement shall be included in Operating Expenses
                           to be billed to Tenant as provided in Section 7 of
                           the Lease to the extent they are properly included in
                           the definition of "Operating Expenses". In addition,
                           Landlord (and not Tenant) shall be responsible for
                           the replacement of portions of the HVAC Systems and
                           Electrical Systems, other than the HVAC Related
                           Alterations and other Existing Alterations, and the
                           cost of such replacement shall be included in
                           Operating Expenses to be billed to Tenant as provided
                           in Section 7 of the Lease to the extent they are
                           properly included in the definition of "Operating
                           Expenses". Landlord shall reasonably cooperate to
                           maintain the BMS (as defined in Exhibit B) in the
                           manner required for Tenant to operate the HVAC
                           Systems, including by making temperature adjustments
                           and programming adjustments reasonably required and
                           noticed by Tenant during regular building hours. In
                           connection with such maintenance of the BMS, Landlord
                           shall promptly notify Tenant if as soon as reasonably
                           possible after Landlord learns that the BMS is at any
                           time not working, impaired or

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                           damaged, and at the Tenant's request shall provide
                           current documentation with respect to such
                           maintenance and access to the BMS for purposes of
                           inspection of same by the Tenant or as required by
                           applicable Regulations. Landlord further agrees that
                           Tenant shall have the right to add a stand-alone BMS
                           unit within Building VI that would be sufficient to
                           support the HVAC Systems, subject to Landlord's
                           reasonable approval as provided in the Lease for
                           Alterations. Landlord also agrees that Tenant shall
                           have the right to read meters in the electrical
                           rooms, inspect the base electrical systems in
                           Building VI and make electrical repairs, provided
                           that such repairs shall be performed by MCM,
                           Cupertino Electric or another electrical contractor
                           previously approved in writing by Landlord, and
                           further provided that Tenant shall notify Landlord,
                           in writing, of the nature of repairs.

II.      OTHER PERTINENT PROVISIONS. Landlord and Tenant agree that, effective
         as of the date of this Amendment (unless different effective date(s)
         is/are specifically referenced in this Section), the Lease shall be
         amended in the following additional respects:

         A.       Landlord's Notice Addresses. The Basic Lease Information
                  section of the Lease is hereby amended to reflect that notices
                  to Landlord shall be addressed as follows:

<TABLE>
<CAPTION>
Landlord:                               With a copy to:
<S>                                     <C>
CA-The Concourse Limited Partnership    Equity Office
c/o Equity Office                       One Market Street
1740 Technology Drive                   Spear Tower, Suite 600
Suite 150                               San Francisco, CA 94105
San Jose, California  95110             Attention: Regional Counsel- San Jose Region
Attention:  Property Manager
</TABLE>

                  Rent shall be made payable to the entity, and sent to the
                  address, Landlord designates and shall be made by good and
                  sufficient check or by other means acceptable to Landlord.

         B.       Waivers.

                  1.       Tenant hereby waives any and all rights under and
                           benefits of subsection 1 of Section 1932, Sections
                           1941 and 1942 of the California Civil Code (Repairs
                           and Alterations), or any similar or successor laws
                           now or hereinafter in effect.

                  2.       REMEDIES. TENANT HEREBY WAIVES ANY AND ALL RIGHTS
                           CONFERRED BY SECTION 3275 OF THE CIVIL CODE OF
                           CALIFORNIA AND BY SECTIONS 1174 (c) AND 1179 OF THE
                           CODE OF CIVIL PROCEDURE OF CALIFORNIA AND ANY AND ALL
                           OTHER LAWS AND RULES OF LAW FROM TIME TO TIME IN
                           EFFECT DURING THE LEASE TERM, AS AMENDED HEREBY,
                           PROVIDING THAT TENANT SHALL HAVE ANY RIGHT TO REDEEM,
                           REINSTATE OR RESTORE THIS LEASE FOLLOWING ITS
                           TERMINATION BY

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                           REASON OF TENANT'S BREACH. TENANT ALSO HEREBY WAIVES,
                           TO THE FULLEST EXTENT PERMITTED BY LAW, THE RIGHT TO
                           TRIAL BY JURY IN ANY LITIGATION ARISING OUT OF OR
                           RELATING TO THIS LEASE, AS HEREBY AMENDED.

III.     MISCELLANEOUS.

         A.       This Amendment and the following exhibits and attachments
                  attached hereto, which are hereby incorporated into and made a
                  part of this Amendment, set forth the entire agreement between
                  the parties with respect to the matters set forth herein.
                  There have been no additional oral or written representations
                  or agreements. In no event shall this Amendment be construed
                  as giving Tenant any new entitlement to any Rent abatement,
                  improvement allowance, leasehold improvements, or other work
                  to the Premises, or any similar economic incentives that may
                  have been provided Tenant in connection with entering into the
                  Lease, unless specifically set forth in this Amendment.

         B.       Except as herein modified or amended, the provisions,
                  conditions and terms of the Lease shall remain unchanged and
                  in full force and effect.

         C.       In the case of any inconsistency between the provisions of the
                  Lease and this Amendment, the provisions of this Amendment
                  shall govern and control.

         D.       Submission of this Amendment by Landlord is not an offer to
                  enter into this Amendment but rather is a solicitation for
                  such an offer by Tenant. Landlord shall not be bound by this
                  Amendment until Landlord has executed and delivered the same
                  to Tenant.

         E.       The capitalized terms used in this Amendment shall have the
                  same definitions as set forth in the Lease to the extent that
                  such capitalized terms are defined therein and not redefined
                  in this Amendment.

         F.       Tenant hereby represents to Landlord that Tenant has dealt
                  with no broker in connection with this Amendment. Tenant
                  agrees to indemnify and hold Landlord, its members,
                  principals, beneficiaries, partners, officers, directors,
                  employees, mortgagee(s) and agents, and the respective
                  principals and members of any such agents (collectively, the
                  "LANDLORD RELATED PARTIES") harmless from all claims of any
                  brokers claiming to have represented Tenant in connection with
                  this Amendment. Landlord hereby represents to Tenant that
                  Landlord has dealt with no broker in connection with this
                  Amendment. Landlord agrees to indemnify and hold Tenant, its
                  members, principals, beneficiaries, partners, officers,
                  directors, employees, and agents, and the respective
                  principals and members of any such agents (collectively, the
                  "TENANT RELATED PARTIES") harmless from all claims of any
                  brokers claiming to have represented Landlord in connection
                  with this Amendment.

         G.       Each signatory of this Amendment represents hereby that he or
                  she has the authority to execute and deliver the same on
                  behalf of the party hereto for which such signatory is acting.

         H.       This Amendment may be executed in any number of identical
                  counterparts, any

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                  or all of which may contain the signatures of less than all of
                  the parties, and all of which shall be construed together as
                  but a single instrument.

         IN WITNESS WHEREOF, Landlord and Tenant have duly executed this
Amendment as of the day and year first above written.

                  LANDLORD:

                  CA-THE CONCOURSE LIMITED PARTNERSHIP, A
                  DELAWARE LIMITED PARTNERSHIP

                  By: EOM GP, L.L.C., a Delaware limited liability company, its
                      general partner

                      By: Equity Office Management, L.L.C., a
                          Delaware limited liability company, its non-member
                          manager

                          By:    /s/ John W. Petersen
                                 -----------------------------------------------
                          Name:  John W. Petersen
                          Title: Regional Senior Vice President

                  TENANT:

                  BROCADE COMMUNICATIONS SYSTEMS, INC., A
                  DELAWARE CORPORATION

                  By:    /s/ Antonio Canova
                         --------------------------------
                  Name:  Antonio Canova
                  Title: CFO

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                                    EXHIBIT A
                              EXISTING ALTERATIONS

I.       The following Alterations have been made by Tenant to Building VI and
         have been previously approved by Landlord, some of which are part of
         the Tenant Improvements:

A.       "MAIN ELECTRICAL SWITCHBOARD ALTERATIONS".

         Prior to the completion of the Existing Alterations, 2 main electrical
         lines, or busses, served the electrical demand for Tenant's Premises in
         Building VI (i.e. 1745 Technology Drive). These busses were fed from
         one main electrical switchboard located in the main electrical room on
         the ground floor of Building VI. In order to balance the electrical
         load between these busses, Tenant relocated one of the busses from the
         main electrical switchboard to the second main electrical switchboard,
         also located in the main electrical room on the ground floor of
         Building VI.

         Tenant completed the work, in accordance with the plans titled
         "Existing & New Single Line Design", consisting of Sheet E-39, prepared
         by Sasco, as engineered by Charles C. Nguyen, dated 2/1/2000, as last
         revised 6/9/2000, and referring to "Cadfile #0122-S dated 3/11/2002"
         (the "MAIN ELECTRICAL SWITCHBOARD ALTERATIONS"). Such work was
         performed by Sasco Valley Electric.

B.       "TRANSFORMER ALTERATIONS".

         Tenant replaced two existing 1000 KVA transformers servicing Building
         VI (i.e. 1745 Technology Drive) and belonging to Pacific Gas & Electric
         (PG&E), with two 2500 KVA transformers, and increased the conductors,
         or feeders, from the transformers to the main switchboard in order to
         facilitate the full capacity of the two larger replacement transformers
         (the "TRANSFORMER ALTERATIONS"). Such work was performed by, and the
         replacement transformers belong to, PG&E.

C.       "TEMPORARY CHILLERS AND GENERATORS".

         Tenant acknowledges and agrees that it has installed, used and removed
         temporary chillers and generators in the parking lot of the Building in
         order to provide electrical power and HVAC service to Building VI
         during the Service Interruption Period.

D.       "SUPPLEMENTAL HVAC".

         Tenant acknowledges and agrees that it has installed in Building VI,
         including on the roof, the following equipment (the "HVAC EQUIPMENT")
         for use in connection with Tenant's computer/data center room and lab
         rack systems in the Premises located in Building VI:

         Two (2) RTAA 200 ton Chillers
         Chilled Water Pumps

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                  Emergency
                  Generator Uninterrupted Power Supply Systems
                  Expansion Tank
                  Heat Pumps
                  Sput System Fan Coil Units ACI-1
                  Sput System Condensing Unit ACI-1
                  Two (2) Liebert Systems (LU5-1 and LU5-2)
                  Twenty Three (23) Fan Coil Units

                  The HVAC Equipment installed on the roof of Building VI does
                  not exceed, in the aggregate, 400 tons of chilling capacity.
                  The area in which Landlord has approved and Tenant has
                  installed the HVAC Equipment on the roof space is the "CHILLER
                  ROOF SPACE".

                  Tenant acknowledges and agrees that it has also installed in
                  Building V, the following equipment, which will also be part
                  of the HVAC Equipment, for use in connection with Tenant's
                  systems in the Premises located in Building V:

                  Five (5) Heat Pumps

E.       FIRST FLOOR EAST SIDE OFFICE RECONFIGURATION

II.      The following Alterations have been made by Tenant to Building VI and
         remain subject to Landlord's approval and the requirement, to the
         extent provided in the Lease, that they be removed upon expiration or
         earlier termination of the Lease, unless and until Tenant either
         submits proof of prior approval (or deemed approval pursuant to the
         Lease) as provided in the Amendment to which this Exhibit A is attached
         or obtains Landlord's approval hereafter:

         A.       Supplemental HVAC

                  Three (3) thirty ton floor-mounted air handler units

         B.       Electrical

                  Two (2) Panels
                  One (1) Transformer
                  Electrical Distribution within the Laboratories

III.     "SECONDARY DISTRIBUTION SYSTEMS" shall mean all electrical feeds from
         electrical panels. The following functions with respect to Secondary
         Distribution Systems may be performed by Tenant without Landlord's
         prior consent:

       Additions, deletions and rerouting of circuitry supporting laboratories

       Wire mold

       Starline BUS

       Dedicated outlets

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                                    EXHIBIT B
                           LANDLORD MAINTAINED SYSTEMS

The following shall be "LANDLORD MAINTAINED SYSTEMS":

Life safety systems and core generator
Base electrical systems other than Secondary Distribution Systems
Base plumbing systems and all controls appurtenant thereto
Siemens DDC Building Management System in Garage that services the HVAC Systems
and the life safety systems (the "BMS")
Roof Membrane
Windows and window frames
Elevators
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