
<PAGE>   1
                                                                     EXHIBIT 8.2

                  [TESTA, HURWITZ & THIBEAULT, LLP LETTERHEAD]

                                  May 14, 1999


GeoTel Communications Corporation
900 Chelmsford Street
Tower II, Floor 12
Lowell, MA 01851

Ladies and Gentlemen:

     We have acted as counsel for GeoTel Communications Corporation, a Delaware
corporation ("Company"), in connection with the preparation and execution of
the Agreement and Plan of Merger and Reorganization dated as of April 12, 1999
(the "Agreement") by and among Company, Cisco Systems Inc., a California
corporation ("Parent"), and Geronimo Merger Corp., a Delaware corporation
wholly-owned by Parent ("Merger Sub"). Pursuant to the Agreement, Merger Sub
will merge with and into Company (the "Merger"), and Company will become a
wholly-owned subsidiary of Parent. Unless otherwise defined, capitalized terms
referred to herein have the meanings set forth in the Agreement.

     You have requested our opinion regarding certain United States federal
income tax consequences of the Merger. In delivering this opinion, we have
reviewed and are relying upon (without any independent investigation or
examination thereof) the truth and accuracy, at all relevant times, of the
facts, statements, descriptions, covenants, representations and warranties set
forth in the Registration Statement on Form S-4 filed by Parent with the
Securities and Exchange Commission (which contains a proxy
statement/prospectus) (the "Registration Statement"), the Agreement (including
exhibits), representations by Company, Parent and Merger Sub respectively (the
"Corporate Representations") and such other documents pertaining to the Merger
as we have deemed necessary or appropriate.

     In connection with rendering this opinion, we have also assumed and are
relying upon (without any independent investigation or examination thereof) the
following:

     1.   Original documents (including signatures) are authentic, documents
submitted to us as copies conform to the original documents, and there has been
(or will be by the Effective Time) due execution and delivery of all documents
where due execution and delivery are prerequisites to effectiveness thereof;

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GeoTel Communications Corporation
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May 14, 1999


     2.   Any statement made in any of the documents referred to herein "to the
knowledge of" or "to the best of the knowledge" of any person or party or
similarly qualified is correct without such qualification;

     3.   All facts, statements, descriptions, covenants, representations and
warranties contained in any of the documents referred to herein or otherwise
made to us (including, without limitation, the aforementioned Corporate
Representations) are true and correct in all material respects and no actions
have been (or will be) taken which are inconsistent with such positions; and

     4.   The Merger will be consummated in accordance with the terms of the
Agreement and without any waiver or breach of any material provision thereof,
and the Merger will be effective under applicable state law.

     Based upon our examination of the foregoing items and subject to the
assumptions, limitations and qualifications set forth herein, it is our opinion
that, under current law, the statements regarding United States federal income
tax considerations set forth under the heading "THE MERGER AND RELATED
TRANSACTIONS - Certain Federal Income Tax Considerations" in the Registration
Statement, insofar as they constitute statements of law or legal conclusions,
although general in nature, are correct in all material respects. The United
States federal income tax consequences of the Merger to a holder of Company
common stock will, however, depend upon that holder's particular situation and
we express no opinion as to the completeness of the discussion set forth in "THE
MERGER AND RELATED TRANSACTIONS - Certain Federal Income Tax Considerations" as
applied to any particular holder.

     This opinion represents and is based upon our best judgment regarding the
application of federal income tax laws including the Internal Revenue Code of
1986, as amended, existing judicial decisions, administrative regulations and
published rulings and procedures. Our opinion is not binding upon the Internal
Revenue Service or the courts, and there is no assurance that the Internal
Revenue Service will not successfully assert a contrary position. Furthermore,
no assurance can be given that future legislative, judicial or administrative
changes, on either a prospective or retroactive basis, would not adversely
affect the accuracy of the conclusions stated herein. Nevertheless, we undertake
no responsibility to advise you of any new developments in the application or
interpretation of the federal income tax laws.

     This opinion addresses only the matters set forth above, and does not
address any other federal, state, local or foreign tax consequences that may
result from the Merger or any other transaction (including any transaction
undertaken in connection with the Merger).

     No opinion is expressed as to any transaction other than the Merger as
described in the Agreement (whether or not undertaken in connection with the
Merger) or as to any transaction whatsoever, including the Merger, if the
transactions described in the Agreement are not consummated in accordance with
the terms of such Agreement, and without waiver or breach of any material
provision thereof or if any of the facts, statements, descriptions, covenants,
representations, warranties or assumptions upon which we relied (including,
without limitation,
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GeoTel Communications Corporation
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May 14, 1999

the aforementioned Corporate Representations) are not true and accurate at all
relevant times. In the event any one of the facts, statements, descriptions,
covenants, representations, warranties or assumptions upon which we have relied
to issue this opinion is incorrect, our opinion might be adversely affected and
may not be relied upon.

      This opinion has been delivered to you for the purpose of being included
as an exhibit to the Registration Statement; it may not be relied upon for any
other purpose or by any other person or entity and may not be made available to
any other person or entity without our prior written consent. We hereby consent
to the filing of this opinion as an exhibit to the Registration Statement, and
to the references to our firm name under the heading "THE MERGER AND RELATED
TRANSACTIONS -- Certain Federal Income Tax Considerations" in the Registration
Statement. In giving this consent, however, we do not admit that we are in the
category of persons whose consent is required under Section 7 of the Securities
Act of 1933, as amended.


                                          Very truly yours,

                                          /s/ Testa, Hurwitz & Thibeault, LLP
                                          --------------------------------------
                                          TESTA, HURWITZ & THIBEAULT, LLP


