<PAGE>

                                                                 EXHIBIT - 10.69

                GUARANTY OF BROCADE COMMUNICATIONS SYSTEMS, INC.

         The undersigned, Brocade Communications Systems, Inc., a Delaware
corporation ("Guarantor"), hereby absolutely and unconditionally guarantees (x)
to EOP Skyport I, L.L.C., a Delaware limited liability company and its
successors and assigns (collectively, the "EOP Owner"), payment by Brocade
Communications Systems Skyport LLC, a Delaware limited liability corporation
(together with its successors and assigns which are its Affiliates (as defined
in the Declaration), the "Brocade Owner") of the Brocade Owner's obligations
(the "Declaration Obligations") under that certain Skyport Plaza (Adjusted
Parcel II) Declaration of Common Easements, Covenants, Conditions and
Restrictions dated as of November 18, 2003 (as amended from time to time, the
"Declaration"), and (y) to EOP Skyport II, L.L.C., a Delaware limited liability
company and its successors and assigns ( "EOP II"), payment by the Brocade Owner
of the Brocade Owner's obligations (the "License Obligations", and together with
the Declaration Obligations, the "Obligations") under that certain License
Agreement for Space dated as of November 18, 2003 (as amended from time to time,
the "License"), all subject to the limitations below .

         In the event of any default on the part of the Brocade Owner with
respect to the Obligations, the EOP Owner or EOP II, as applicable, may give a
written notice of such default to Guarantor and a demand (a "Demand for
Performance by Guarantor") that Guarantor perform, prior to the expiration of
any applicable cure period in the Declaration or the License, as applicable, and
to the extent such obligation is not performed by the Brocade Owner, that
portion of the Obligations with respect to which the Brocade Owner is in
default. Such Demand for Performance by Guarantor shall state in reasonable
detail the nature of the default, together with the applicable cure period for
the cure of such default and the payment amount that the EOP Owner or EOP II (as
applicable) demands from the Brocade Owner or from Guarantor pursuant to this
Guaranty in order to cure such default. The other provisions of this Guaranty to
the contrary notwithstanding, Guarantor shall not be required to perform the
Obligations with respect to any default until the EOP Owner or EOP II (as
applicable) has given to Guarantor a Demand for Performance by Guarantor, and
the Guarantor shall have the benefit of all cure periods provided to the Brocade
Owner in the Declaration or the License (as applicable). Any performance by
Guarantor of the Obligations prior to expiration of any applicable cure period
under the Declaration or the License (as applicable) shall be deemed a cure of
the applicable default by Brocade Owner under the Declaration or the License (as
applicable), to the extent of such performance by Guarantor.

         To the extent that the performance by the Brocade Owner of any of the
Obligations is expressly waived in a writing executed and delivered by the EOP
Owner or EOP II (as applicable) to the Brocade Owner pursuant to the Declaration
or the License, Guarantor shall have the benefit of such waiver. Without
limiting the foregoing, no Obligation shall be deemed to have been waived, or
the performance of such Obligation excused, by any failure or delay in the
enforcement of such Obligation by the EOP Owner or EOP II (as applicable).

         Except as otherwise provided hereunder and to the extent provided
herein, the obligations of Guarantor hereunder are unconditional and absolute
and not subject to termination for any reason other than the satisfaction of the
Obligations or expiration of this Guaranty. In addition,

<PAGE>

the Obligations of Guarantor hereunder are not subject to counterclaim, set-off,
abatement, deferment or defense based upon any claim that Guarantor may have
against the EOP Owner or EOP II:

                  (a)      unrelated to the transaction giving rise to the
Obligations herein; or

                  (b)      regarding (i) the release or discharge of the Brocade
Owner in any creditor's receivership, bankruptcy or other proceedings or (ii)
the impairment, limitation, modification or termination of the liabilities of
the Brocade Owner or the estate of the Brocade Owner in bankruptcy, or any
remedy for the enforcement of the Brocade Owner's liability under the
Declaration or the License, resulting from the operation of any present or
future provision of Title 11 of the United States Code or other statute or from
the decision in any court.

         Except as expressly provided herein and to the extent provided herein,
Guarantor irrevocably waives acceptance hereof, presentment, demand, protest, to
the fullest extent permitted by law, any notice not provided for herein, as well
as any requirement that at any time any action be taken by any person against
the Brocade Owner or Guarantor. Until payment of the Obligations by the Brocade
Owner under the Declaration and the License, any right of subrogation on the
part of Guarantor shall be in all respects subordinate to all rights and claims
of the EOP Owner and EOP II for all other payments or damages which shall be or
become due and payable by the Brocade Owner under the provisions of the
Declaration or the License. The EOP Owner may compromise or adjust any part of
the Declaration Obligations, EOP II may compromise or adjust any part of the
License Obligations, and the EOP Owner or EOP II make any other accommodation
with the Brocade Owner or exercise any other right or remedy available to it
against the Brocade Owner without affecting or impairing in any way the
liability of the Guarantor under this Guaranty except to the extent the
Obligations have been performed; provided that neither the EOP Owner or EOP II
shall have no right to increase the liability of the Guarantor hereunder without
the express written consent of Guarantor.

         In the event of any litigation between the EOP Owner and/or EOP II, on
the one hand and the Guarantor, on the other hand, seeking a declaration of
rights hereunder, damages for breach or any other remedy pertaining to this
Guaranty, the prevailing party shall recover its reasonable attorneys' fees and
court costs. For purposes hereof, the EOP Owner and/or EOP II shall be deemed
the prevailing party in the event that the result of any proceeding requires
Guarantor to pay any amount to the EOP Owner or EOP II or perform any obligation
for the benefit of the EOP Owner and/or EOP II.

         Notwithstanding anything to the contrary herein, the total obligations
of Guarantor hereunder shall be limited to the sum of One Million Fifteen
Thousand Dollars ($1,015,000) (the "Limit"); provided, however, that any
Obligations that arise out of the Brocade Owner's obligations under Section
17.13 of the Declaration, the penultimate sentence of Section 24 of the License
or the obligations of the Guarantor under the immediately preceding paragraph
for reasonable attorneys' fees and court costs (the "Non-Capped Obligations")
shall not be subject to the Limit. . All payments by Guarantor hereunder, other
than those related to the Non-Capped Obligations shall be credited against the
Limit. Upon termination of the License and satisfaction

<PAGE>

of all obligations under the License by Brocade Owner, the Limit, as it may have
already been reduced as of such time in accordance with the previous sentence,
shall be further reduced by the difference between (a) $15,000 and (b) the
amount of previous reduction in the Limit in respect of Guarantor's payment of
License Obligations.

         This Guaranty shall remain in full force and effect as to any
Obligation for so long as the Brocade Owner named herein or an Affiliate thereof
owns the Brocade Building. After any sale or conveyance of the entire Brocade
Building by the Brocade Owner named herein to a party other than an Affiliate of
the Brocade Owner, this Guaranty shall terminate and be of no further force or
effect (except as to claims against Guarantor hereunder then existing, which
claims shall survive such termination) and the EOP Owner and EOP II shall
execute an agreement acknowledging the termination of this Guaranty, and the EOP
Owner and EOP II shall not be entitled to seek or obtain a replacement guaranty
for this Guaranty from any successor in interest to the Brocade Owner named
herein.

         IN WITNESS WHEREOF, Guarantor has executed this Guaranty as of the 18th
day of November, 2003.

                                    GUARANTOR:

                                    BROCADE COMMUNICATIONS SYSTEMS, INC.,
                                    a Delaware corporation

                                    By:   /s/ Antonio Canova
                                          --------------------------------------
                                    Name: Antonio Canova
                                    Its:  Chief Financial Officer