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                                                                   Exhibit 10.3
                                    GUARANTY

         THIS GUARANTY (this "Guaranty") dated as of December 14, 1998 is made
by FORE SYSTEMS, INC., a Delaware corporation (the "Guarantor"), in favor of
MELLON BANK, N.A., as Construction Lender (together with the various financial
institutions which may from time to time become lenders under the Construction
Loan Agreement referred to below, and in the event of a Refinancing, the Tranche
A Lenders and the Tranche B Lender, collectively, the "Lender") and MELLON
FINANCIAL SERVICES CORPORATION #4, as Certificate Purchaser (together with its
successors and assigns in such capacity, the "Certificate Purchaser"; the Lender
and the Certificate Purchaser are referred to collectively as the "Funding
Parties" and individually as a "Funding Party").

                              W I T N E S S E T H:

         WHEREAS, pursuant to a Construction Loan Agreement, dated as of the
date hereof (together with all amendments, restatements and other modifications,
if any, from time to time hereafter made thereto, the "Loan Agreement"), between
Wilmington Trust Company, not in its individual capacity except as expressly
stated therein, but solely as Certificate Trustee and the Construction Lender,
the Construction Lender has agreed, subject to the terms and conditions thereof,
to make Loans to the Certificate Trustee;

         WHEREAS, pursuant to the terms of a Trust Agreement, dated as of the
date hereof (together with all amendments, restatements and other modifications,
if any, from time to time hereafter made thereto, the "Trust Agreement"),
between Wilmington Trust Company, not in its individual capacity except as
expressly stated therein, but solely as Certificate Trustee and the Certificate
Purchaser, the Certificate Purchaser has agreed to purchase the Certificate;

         WHEREAS, as a condition to the occurrence of the Documentation Date
under the Participation Agreement, dated as of the date hereof (together with
all amendments, restatements and other modifications, if any, from time to time
hereafter made thereto, the "Participation Agreement"), among the Guarantor, the
Certificate Trustee, the Certificate Purchaser, and the Construction Lender, the
Guarantor is required to execute and deliver this Guaranty in favor of the
Funding Parties;

         WHEREAS, the Guarantor has duly authorized the execution, delivery and
performance of this Guaranty; and

         WHEREAS, it is in the best interests of the Guarantor to execute this
Guaranty inasmuch as the Guarantor will derive substantial benefits from the
transactions contemplated by the Loan Agreement, the Trust Agreement and the
Participation Agreement;

         NOW, THEREFORE, for good and valuable consideration the receipt of
which is hereby acknowledged, and in order to induce the Construction Lender to
enter into the Loan Agreement and the Certificate Purchaser to purchase the
Certificate, and intending to be legally bound hereby, the Guarantor agrees, for
the benefit of the Funding Parties, as follows:


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                                    ARTICLE I

                                   DEFINITIONS

         SECTION 1.1. Certain Terms. The following terms (whether or not
underscored) when used in this Guaranty, including its preamble and recitals,
shall have the following meanings (such definitions to be equally applicable to
the singular and plural forms thereof):

         "Guarantor" is defined in the preamble.

         "Guaranty" is defined in the preamble.

         "Holder" shall mean any assignee or transferee of an Interest which is
assigned or otherwise transferred (in accordance with Section 12.1 of the
Participation Agreement to the extent that Section 12.1 is applicable to such
transfer).

         "Interest" shall mean all or any of the right, title, or interest and
obligations of any Funding Party or any Holder in and to the Loan Agreement or
the other Operative Documents, and in the event of a Refinancing, the
Refinancing Loan Documents, all or any of the benefits, advantages and
obligations of any Funding Party or any Holder under the Loan Agreement or the
other Operative Documents, and in the event of a Refinancing, the Refinancing
Loan Documents, and all or any right, title and interest of any Funding Party or
any Holder in and to the Property.

         "Lender" is defined in the preamble.

         "Lessee" means the Guarantor, in its capacity as Lessee under the
Participation Agreement and the Lease.

         "Loan Agreement" is defined in the first recital.

         "Obligations" means all of the following, in each case howsoever
created, arising or evidenced, whether direct or indirect, joint or several,
absolute or contingent, or now or hereafter existing, or due or to become due:
all principal of the Note, interest accrued thereon, the Certificate Amount,
Certificate Yield accrued thereon and all additional amounts and other sums at
any time due and owing, and required to be paid, to the Participants by the
Certificate Trustee under the terms of the Loan Agreement, the Note or any other
Operative Documents and, in the event of a Refinancing, to the Tranche A Lenders
and the Tranche B Lender under any Refinancing Loan Document (including, without
limitation, Section 2.6 of the Construction Loan Agreement); provided, however,
that with respect to an exercise of the Remarketing Option pursuant to Section
20.1 of the Lease the amount guaranteed hereunder shall not exceed the aggregate
amounts required to be paid by the Lessee pursuant to the Lease in connection
with such exercise; and, provided further, that prior to the commencement of the
Base Term, the aggregate amount payable by the Guarantor hereunder and Holding
under the Guaranty Agreement with respect to a Construction Agency Event of
Default arising solely from an Unrelated Event shall be subject to the
limitations on recourse liability applicable to the Lessee set forth in Section
13.1B.(2) of the Participation Agreement.

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                                                                       Guaranty

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         "Organic Document" means, relative to the Guarantor, its certificate of
incorporation, its by-laws and all shareholder agreements, voting trusts and
similar arrangements applicable to any of its authorized shares of capital
stock.

         "Participation Agreement" is defined in the third recital.

         "Trust Agreement" is defined in the second recital.

         "U.C.C." means the Uniform Commercial Code as in effect in the
Commonwealth of Pennsylvania.

         SECTION 1.2. Participation Agreement Definitions. Capitalized terms
used but not otherwise defined in this Guaranty have the respective meanings
specified in Appendix 1 to the Participation Agreement; and the rules of
interpretation set forth in Appendix 1 to the Participation Agreement shall
apply to this Guaranty.

         SECTION 1.3. U.C.C. Definitions. Unless otherwise defined herein or the
context otherwise requires, terms for which meanings are provided in the U.C.C.
are used in this Guaranty, including its preamble and recitals, with such
meanings.

                                   ARTICLE II

                               GUARANTY PROVISIONS

         SECTION 2.1.  Guaranty. The Guarantor hereby absolutely,
unconditionally and irrevocably

                  (a) guarantees the full and punctual payment when due (after
         giving effect to any applicable grace period), whether at stated
         maturity, by required prepayment, declaration, acceleration, demand or
         otherwise, of all Obligations, whether for principal, interest, fees,
         expenses or otherwise (including all such amounts which would become
         due but for the operation of the automatic stay under Section 362(a) of
         the United States Bankruptcy Code, 11 U.S.C. Section 362(a)), and the
         operation of Sections 502(b) and 506(b) of the United States Bankruptcy
         Code, 11 U.S.C. Section 502(b) and Section 506(b)), and

                  (b) indemnifies and holds harmless each Funding Party and each
         Holder for any and all costs and expenses (including reasonable
         attorney's fees and expenses) incurred by each Funding Party or such
         Holder, as the case may be, in enforcing any rights under this
         Guaranty.

This Guaranty constitutes a guaranty of payment when due and not of collection,
and the Guarantor specifically agrees that it shall not be necessary or required
that any Funding Party or any Holder exercise any right, assert any claim or 
demand or enforce any remedy whatsoever against the Lessee (or any other Person)
before or as a condition to the obligations of the Guarantor hereunder.



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                                                                       Guaranty

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         SECTION 2.2. Guaranty Absolute, etc. This Guaranty shall in all
respects be a continuing, absolute, unconditional and irrevocable guaranty of
payment, and shall remain in full force and effect until all Obligations have
been paid in full, all obligations of the Guarantor hereunder shall have been
paid in full and all commitments of the Funding Parties under the Loan Agreement
and the other Operative Documents, and in the event of a Refinancing, under the
Refinancing Loan Documents, shall have terminated. The Guarantor guarantees that
the Obligations will be paid strictly in accordance with the terms of the Loan
Agreement and each other Operative Document, and in the event of a Refinancing,
each of the Refinancing Loan Documents, under which they arise, regardless of
any law, regulation or order now or hereafter in effect in any jurisdiction
affecting any of such terms or the rights of any Funding Party or any Holder
with respect thereto. The liability of the Guarantor under this Guaranty shall
be absolute, unconditional and irrevocable irrespective of:

                  (a) any lack of validity, legality or enforceability of the
         Loan Agreement or any other Operative Document or Refinancing Loan
         Document;

                  (b)  the failure of any Funding Party or any Holder

                           (i) to assert any claim or demand or to enforce any
                  right or remedy against the Certificate Trustee or any other
                  Person (including any other guarantor) under the provisions of
                  the Loan Agreement, any other Operative Document or
                  Refinancing Loan Document or otherwise, or

                           (ii) to exercise any right or remedy against any
                  other guarantor of, or collateral securing, any Obligations;

                  (c) any change in the time, manner or place of payment of, or
         in any other term of, all or any of the Obligations, or any other
         extension, compromise or renewal of any Obligation;

                  (d) any reduction, limitation, impairment or termination of
         the Obligations for any reason, including any claim of waiver, release,
         surrender, alteration or compromise, and shall not be subject to (and
         the Guarantor hereby waives any right to or claim of) any defense or
         setoff, counterclaim, recoupment or termination whatsoever by reason of
         the invalidity, illegality, nongenuineness, irregularity, compromise,
         unenforceability of, or any other event or occurrence affecting, the
         Obligations;

                  (e) any amendment to, rescission, waiver, or other
         modification of, or any consent to departure from, any of the terms of
         the Loan Agreement or any other Operative Document or Refinancing Loan
         Document;

                  (f) any addition, exchange, release, surrender or
         nonperfection of any collateral, or any amendment to or waiver or
         release or addition of, or consent to departure from, any other
         guaranty, held by any Funding Party or any Holder securing any of 
         the Obligations; or

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                                                                       Guaranty

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                  (g) any other circumstance which might otherwise constitute a
         defense available to, or a legal or equitable discharge of, the
         Certificate Trustee, any surety or any guarantor.

         SECTION 2.3. Reinstatement, etc. The Guarantor agrees that this
Guaranty shall continue to be effective or be reinstated, as the case may be, if
at any time any payment (in whole or in part) of any of the Obligations is
rescinded or must otherwise be restored by any Funding Party or any Holder, upon
the insolvency, bankruptcy or reorganization of Lessee or otherwise, as though
such payment had not been made.

         SECTION 2.4. Waiver, etc. The Guarantor hereby waives promptness,
diligence, notice of acceptance and any other notice with respect to any of the
Obligations of the Lessee and this Guaranty and any requirement that any Funding
Party or any Holder protect, secure, perfect or insure any security interest or
Lien, or any property subject thereto, or exhaust any right or take any action
against the Certificate Trustee or any other Person (including any other
guarantor) or entity or any collateral securing the Obligations.

         SECTION 2.5. Waiver of Subrogation. The Guarantor hereby irrevocably
waives any claim or other rights which it may now or hereafter acquire against
the Certificate Trustee that arise from the existence, payment, performance or
enforcement of the Guarantor's obligations under this Guaranty or any other
Operative Document or Refinancing Loan Document, including any right of
subrogation, reimbursement, exoneration, or indemnification, any right to
participate in any claim or remedy of any Funding Party or any Holder against
the Lessee or any collateral which any Funding Party now has or hereafter
acquires, whether or not such claim, remedy or right arises in equity, or under
contract, statute or common law, including the right to take or receive from the
Certificate Trustee, directly or indirectly, in cash or other property or by
set-off or in any manner, payment or security on account of such claim or other
rights. If any amount shall be paid to the Guarantor in violation of the
preceding sentence and the Obligations shall not have been paid in cash in full
and all Commitments of the Funding Parties under the Loan Agreement and the
other Operative Documents, and in the event of a Refinancing, under the
Refinancing Loan Documents, have not been terminated, such amount shall be
deemed to have been paid to the Guarantor for the benefit of, and held in trust
for, the Funding Parties or any Holder or Holders, and shall forthwith be paid
to the Funding Parties for the benefit of the Funding Parties and any such
Holder or Holders to be credited and applied upon the Obligations, whether
matured or unmatured. The Guarantor acknowledges that it will receive benefits
from the financing and other arrangements contemplated by the Loan Agreement,
the Trust Agreement, the Participation Agreement and, in the event of a
Refinancing, under the Refinancing Loan Documents, and that the waiver set forth
in this Section is knowingly made in contemplation of such benefits.

         SECTION 2.6. Successors, Transferees and Assigns; Transfers of 
Interests, etc. This Guaranty shall:

                  (a) be binding upon the Guarantor and its successors,
         transferees and assigns; and


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                                                                       Guaranty

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                  (b) inure to the benefit of and be enforceable by each Funding
         Party, each Holder and each of their respective successors.

Without limiting the generality of clause (b), any Lender and/or any Holder may,
in accordance with Article 12 of the Participation Agreement, and the
Certificate Purchaser and any Tranche A Lenders and the Tranche B Lender may,
assign or otherwise transfer (in whole or in part) any Interest held by it to
any Holder, and such Holder shall thereupon become vested with all rights and
benefits in respect thereof granted to the respective Funding Parties under any
Operative Document (including this Guaranty), Refinancing Document or otherwise.

         SECTION 2.7.  Consent to Jurisdiction; Waiver of Immunities.  The 
Guarantor hereby acknowledges and agrees that:

                  (a) It irrevocably submits to the jurisdiction of any federal
         court sitting in Pennsylvania in any action or proceeding arising out
         of or relating to this Guaranty, and the Guarantor hereby irrevocably
         agrees that all claims in respect of such action or proceeding may be
         heard and determined in such federal court. The Guarantor hereby
         irrevocably waives, to the fullest extent it may effectively do so, the
         defense of an inconvenient forum to the maintenance of such action or
         proceeding. The Guarantor agrees that a final judgment in any such
         action or proceeding shall be conclusive and may be enforced in other
         jurisdictions by suit on the judgment or in any other manner provided
         by law.

                  (b) Nothing in this Section shall affect the right of any
         Funding Party or any Holder to serve legal process in any other manner
         permitted by law or affect the right of any Funding Party or any Holder
         to bring any action or proceeding against the Guarantor or its property
         in the courts of any other jurisdictions.

                                   ARTICLE III

                         REPRESENTATIONS AND WARRANTIES

         SECTION 3.1. Representations and Warranties. The Guarantor hereby
represents and warrants to each Funding Party as set forth in Section 8.3 and
8.4 of the Participation Agreement, which representations and warranties are
hereby incorporated by reference.

                                   ARTICLE IV

                                    COVENANTS

         SECTION 4.1. Covenants. The Guarantor covenants and agrees that, so
long as any portion of the Obligations shall remain unpaid or any Funding Party
shall have any outstanding Commitments under the Loan Agreement or the other
Operative Documents, or in the event of a Refinancing, under any Refinancing
Loan Document, the Guarantor will, unless each Funding Party shall otherwise
consent in writing, comply with and perform the obligations set forth in

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                                                                       Guaranty

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Section 10.1 of the Participation Agreement, which covenants are hereby
incorporated by reference.

                                   ARTICLE V.

                            MISCELLANEOUS PROVISIONS

         SECTION 5.1.  Operative Document. This Guaranty is an Operative 
Document executed pursuant to the Loan Agreement and the Participation 
Agreement.

         SECTION 5.2. Binding on Successors, Transferees and Assigns; Assignment
of Guaranty. In addition to, and not in limitation of, Section 2.6, this
Guaranty shall be binding upon the Guarantor and its successors, transferees and
assigns and shall inure to the benefit of and be enforceable by each Funding
Party and each Holder and their respective successors and assigns (to the full
extent provided pursuant to Section 2.6); provided, however, that the Guarantor
may not assign any of its obligations hereunder without the prior written
consent of each Funding Party and each Holder.

         SECTION 5.3. Amendments, etc. No amendment to or waiver of any
provision of this Guaranty, nor consent to any departure by the Guarantor
herefrom, shall in any event be effective unless the same shall be in writing
and signed by each Funding Party and each Holder, and then such waiver or
consent shall be effective only in the specific instance and for the specific
purpose for which given.

         SECTION 5.4. Addresses for Notices. All notices, demands, requests,
consents, approvals and other communications hereunder shall be in writing and
directed to the address described in, and deemed received in accordance with the
provisions of, Section 14.3 of the Participation Agreement.

         SECTION 5.5. No Waiver; Remedies. In addition to, and not in limitation
of, Section 2.2 and Section 2.4, no failure on the part of any Funding Party or
any Holder to exercise, and no delay in exercising, any right hereunder shall
operate as a waiver thereof; nor shall any single or partial exercise of any
right hereunder preclude any other or further exercise thereof or the exercise
of any other right. The remedies herein provided are cumulative and not
exclusive of any remedies provided by law.

         SECTION 5.6. Section Captions. Section captions used in this Guaranty
are for convenience of reference only, and shall not affect the construction of
this Guaranty.

         SECTION 5.7. Severability. Wherever possible each provision of this
Guaranty shall be interpreted in such manner as to be effective and valid under
applicable law, but if any provision of this Guaranty shall be prohibited by or
invalid under such law, such provision shall be ineffective to the extent of
such prohibition or invalidity, without invalidating the remainder of such
provision or the remaining provisions of this Guaranty.

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                                                                       Guaranty

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         SECTION 5.8. Termination of Guaranty. The Guarantor's obligations under
this Guaranty shall terminate on the date upon which all Obligations are
satisfied in full, subject to reinstatement as provided in Section 2.3 hereof.

         SECTION 5.9. GOVERNING LAW. THIS GUARANTY SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH LAWS OF THE COMMONWEALTH OF PENNSYLVANIA, WITHOUT
REGARD TO CONFLICTS OF LAW PRINCIPLES. FOR PURPOSES OF ANY ACTION OR PROCEEDING
INVOLVING THIS GUARANTY, THE GUARANTOR HEREBY EXPRESSLY SUBMITS TO THE
JURISDICTION OF ALL FEDERAL AND STATE COURTS LOCATED IN THE COMMONWEALTH OF
PENNSYLVANIA AND CONSENTS THAT IT MAY BE SERVED WITH ANY PROCESS OR PAPER BY
REGISTERED MAIL OR BY PERSONAL SERVICE WITHIN OR WITHOUT THE COMMONWEALTH OF
PENNSYLVANIA.

         SECTION 5.10. WAIVER OF JURY TRIAL. THE GUARANTOR HEREBY KNOWINGLY,
VOLUNTARILY AND INTENTIONALLY WAIVES ANY RIGHTS IT MAY HAVE TO A TRIAL BY JURY
IN RESPECT OF ANY LITIGATION BASED HEREON, OR ARISING OUT OF, UNDER, OR IN
CONNECTION WITH, THIS GUARANTY. THE GUARANTOR ACKNOWLEDGES AND AGREES THAT IT
HAS RECEIVED FULL AND SUFFICIENT CONSIDERATION FOR THIS PROVISION AND THAT THIS
PROVISION IS A MATERIAL INDUCEMENT FOR THE LENDER ENTERING INTO THE LOAN
AGREEMENT AND THE CERTIFICATE PURCHASER PURCHASING THE CERTIFICATE.

/s/ BEH
-------
Initial

         SECTION 5.11. POWER TO CONFESS JUDGMENT. THE GUARANTOR HEREBY
AUTHORIZES AND EMPOWERS THE PROTHONOTARY OR ANY ATTORNEY OF ANY COURT OF RECORD
WITHIN THE COMMONWEALTH OF PENNSYLVANIA OR ELSEWHERE AFTER AN EVENT OF DEFAULT
UNDER THIS GUARANTY, THE LEASE, THE CONSTRUCTION AGENCY AGREEMENT, OR ANY OF THE
OPERATIVE DOCUMENTS AND, IN THE EVENT OF A REFINANCING, ANY REFINANCING LOAN
DOCUMENT, TO APPEAR FOR THE GUARANTOR, AND, WITH OR WITHOUT DECLARATION FILED,
CONFESS JUDGMENT AGAINST THE GUARANTOR IN FAVOR OF THE FUNDING PARTIES, AS 
OF ANY TERM, FOR THE UNPAID OBLIGATIONS HEREUNDER, AND INCLUDING, WITHOUT
LIMITATION, ALL ACCRUED AND UNPAID INTEREST, CHARGES, EXPENSES OR OTHER AMOUNTS
PAYABLE UNDER THIS GUARANTY, THE LEASE, THE CONSTRUCTION AGENCY AGREEMENT, OR
UNDER ANY OF THE OPERATIVE DOCUMENTS, AND, IN THE EVENT OF A REFINANCING, ANY
REFINANCING LOAN DOCUMENT, WHETHER BY ACCELERATION OR OTHERWISE WITH COSTS OF
SUIT AND A REASONABLE ATTORNEY'S COMMISSION AS CERTIFIED BY THE FUNDING PARTIES
WITH RELEASE OF ALL ERRORS, WAIVING ALL LAWS EXEMPTING REAL OR PERSONAL PROPERTY
FROM EXECUTION TO THE EXTENT THAT SUCH LAWS 


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                                                                       Guaranty

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MAY LAWFULLY BE WAIVED BY THE GUARANTOR. NO SINGLE EXERCISE OF THE FOREGOING
POWER TO CONFESS JUDGMENT SHALL BE DEEMED TO EXHAUST THE POWER, WHETHER OR NOT
ANY SUCH EXERCISE SHALL BE HELD BY ANY COURT TO BE VALID, VOIDABLE, OR VOID, BUT
THE POWER SHALL CONTINUE UNDIMINISHED AND IT MAY BE EXERCISED FROM TIME TO TIME
AS OFTEN AS THE FUNDING PARTIES SHALL ELECT, UNTIL SUCH TIME AS THE FUNDING
PARTIES SHALL HAVE RECEIVED PAYMENT IN FULL OF THE OBLIGATIONS, INTEREST AND
COSTS AND ALL AMOUNTS PAYABLE UNDER THIS GUARANTY.

         BY SIGNING THIS INSTRUMENT, THE GUARANTOR HEREBY ACKNOWLEDGES THAT IT
HAS READ, HAS HAD THE OPPORTUNITY TO HAVE IT REVIEWED BY LEGAL COUNSEL,
UNDERSTANDS, AND AGREES TO THE PROVISIONS CONTAINED HEREIN, INCLUDING THE
CONFESSION OF JUDGMENT PROVISION AND UNDERSTANDS THAT A CONFESSION OF JUDGMENT
CONSTITUTES A WAIVER OF RIGHTS IT OTHERWISE WOULD HAVE TO PRIOR NOTICE AND A
HEARING BEFORE A JUDGMENT IS ENTERED AGAINST IT AND WHICH MAY RESULT IN A COURT
JUDGMENT AGAINST THE GUARANTOR WITHOUT PRIOR NOTICE OR HEARING AND THAT THE
AMOUNTS PAYABLE UNDER THIS GUARANTY MAY BE COLLECTED FROM THE GUARANTOR
REGARDLESS OF ANY CLAIM THE GUARANTOR MAY HAVE AGAINST ANY FUNDING PARTY.

/s/ BEH
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Initial
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                                                                       Guaranty

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         SECTION 5.12. POWER TO EXECUTE ON A JUDGMENT WITHOUT HEARING. THE
GUARANTOR HEREBY AUTHORIZES AND EMPOWERS THE PROTHONOTARY OR ANY ATTORNEY OF ANY
COURT OF RECORD OR THE SHERIFF WITHIN THE COMMONWEALTH OF PENNSYLVANIA OR
ELSEWHERE, TO TAKE ALL ACTION ALLOWED BY OR PROVIDED FOR IN THE PENNSYLVANIA
RULES OF CIVIL PROCEDURE OR OTHER APPLICABLE RULES OF CIVIL PROCEDURE TO EXECUTE
ON ANY JUDGMENT ENTERED AGAINST THE GUARANTOR PURSUANT TO THE CONFESSION OF
JUDGMENT SET FORTH ABOVE WITHOUT PRIOR NOTICE OR HEARING OF ANY NATURE
WHATSOEVER, WAIVING ALL LAWS EXEMPTING REAL OR PERSONAL PROPERTY FROM EXECUTION,
TO THE EXTENT THAT SUCH LAWS MAY LAWFULLY BE WAIVED BY THE GUARANTOR. NO SINGLE
EXERCISE OF THE FOREGOING POWER TO EXECUTE ON JUDGMENT WITHOUT A HEARING SHALL
BE DEEMED TO EXHAUST THE POWER, WHETHER OR NOT ANY SUCH EXERCISE SHALL BE HELD
BY ANY COURT TO BE VALID, VOIDABLE OF VOID, BUT THE POWER SHALL CONTINUE
UNDIMINISHED AND IT MAY BE EXERCISED FROM TIME TO TIME AS OFTEN AS THE FUNDING
PARTIES SHALL ELECT UNTIL SUCH TIME AS THE FUNDING PARTIES SHALL HAVE RECEIVED
PAYMENT IN FULL OF THE OBLIGATIONS, INTEREST AND COSTS AND ALL AMOUNTS PAYABLE
UNDER THIS GUARANTY.

/s/ BEH
-------

         BY SIGNING THIS INSTRUMENT THE GUARANTOR HEREBY ACKNOWLEDGES THAT IT
HAS READ, HAS HAD THE OPPORTUNITY TO HAVE IT REVIEWED BY LEGAL COUNSEL,
UNDERSTANDS AND AGREES TO THE PROVISIONS CONTAINED HEREIN, INCLUDING THE POWER
TO EXECUTE ON JUDGMENT WITHOUT A HEARING, AND UNDERSTANDS THAT THE POWER TO
EXECUTE ON A JUDGMENT WITHOUT A HEARING CONSTITUTES A WAIVER OF RIGHTS IT
OTHERWISE WOULD HAVE TO PRIOR NOTICE AND A HEARING BEFORE EXECUTION ON A
JUDGMENT, AND THAT THE AMOUNTS PAYABLE UNDER THIS GUARANTY MAY BE COLLECTED FROM
THE GUARANTOR REGARDLESS OF ANY CLAIM THAT THE GUARANTOR MAY HAVE AGAINST ANY
FUNDING PARTY.

         SECTION 5.13. SUBMISSION OF JURISDICTION. THE FUNDING PARTIES AND ALL
HOLDERS HEREBY SUBMIT TO THE NONEXCLUSIVE JURISDICTION OF THE UNITED STATES
DISTRICT COURT FOR THE WESTERN DISTRICT OF PENNSYLVANIA AND ANY COURT OF THE
COMMONWEALTH OF PENNSYLVANIA FOR PURPOSES OF ALL LEGAL PROCEEDINGS ARISING OUT
OF OR IN CONNECTION WITH THIS GUARANTY AND THE TRANSACTIONS CONTEMPLATED HEREBY.
THE FUNDING PARTIES AND ALL HOLDERS IRREVOCABLY WAIVE, TO THE FULLEST EXTENT
PERMITTED BY LAW, ANY OBJECTION WHICH SUCH PARTY MAY NOW OR HEREAFTER HAVE TO
THE LAYING OF THE VENUE OF ANY SUCH PROCEEDINGS BROUGHT IN SUCH A COURT AND ANY
CLAIM THAT ANY SUCH PROCEEDING BROUGHT IN SUCH A COURT HAS BEEN BROUGHT IN AN
INCONVENIENT FORUM.


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                                                                       Guaranty


<PAGE>   11

         IN WITNESS WHEREOF, the Guarantor has caused this Guaranty to be duly
executed and delivered by its officer thereunto duly authorized as of the date
first above written.


Attest:                                 FORE SYSTEMS, INC.



/s/  CHRISTOPHER H. GEBHARDT            By:  /s/ BRUCE E. HANEY             
----------------------------            ---------------------------------------
                                        Name: Bruce E. Haney                  
                                              ---------------------------------
                                        Title: SR. VP & Chief Financial Officer
                                               --------------------------------


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                                                                       Guaranty