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                                                                    EXHIBIT 99.6


                               CISCO SYSTEMS, INC.

                        STOCK OPTION ASSUMPTION AGREEMENT
                                 PIPELINKS, INC.
                             1997 STOCK OPTION PLAN


OPTIONEE:  ((Employee))

               STOCK OPTION ASSUMPTION AGREEMENT effective as of the 4th day of
March, 1999 by Cisco Systems, Inc., a California corporation ("Cisco").

               WHEREAS, the undersigned individual ("Optionee") holds one or
more outstanding options to purchase shares of the common stock of Pipelinks,
Inc., a California corporation ("Pipelinks"), which were granted to Optionee
under the Pipelinks, Inc. 1997 Stock Option Plan (the "Plan") and are each
evidenced by the following agreement between Pipelinks and Optionee: a Stock
Option Agreement (the "Option Agreement").

               WHEREAS, Pipelinks has been acquired by Cisco through the merger
of Pipelinks with and into Cisco (the "Merger") pursuant to the Amended and
Restated Agreement and Plan of Reorganization dated March 4, 1999 by and between
Cisco and Pipelinks (the "Reorganization Agreement").

               WHEREAS, the provisions of the Reorganization Agreement require
Cisco to assume all obligations of Pipelinks under all outstanding options under
the Plan at the consummation of the Merger and to issue to the holder of each
outstanding option an agreement evidencing the assumption of such option.

               WHEREAS, pursuant to the provisions of the Reorganization
Agreement, the exchange ratio (the "Exchange Ratio") in effect for the Merger is
0.100248 of a share of Cisco common stock ("Cisco Stock") for each outstanding
share of Pipelinks common stock ("Pipelinks Stock").

               WHEREAS, this Agreement is effective as of the consummation of
the Merger (the "Effective Time") in order to reflect certain adjustments to
Optionee's outstanding options under the Plan which have become necessary by
reason of the assumption of those options by Cisco in connection with the
Merger.

               NOW, THEREFORE, it is hereby agreed as follows:

               1. The number of shares of Pipelinks Stock subject to the options
held by Optionee immediately prior to the Effective Time (the "Pipelinks
Options") and the exercise price payable per share are set forth in Exhibit(s) A
hereto. Cisco hereby assumes, as of the Effective Time, all the duties and
obligations of Pipelinks under each of the Pipelinks Options. In connection with
such assumption, the number of shares of Cisco Stock purchasable under each 


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Pipelinks Option hereby assumed and the exercise price payable thereunder have
been adjusted to reflect the Exchange Ratio. Accordingly, the number of shares
of Cisco Stock subject to each Pipelinks Option hereby assumed shall be as
specified for that option in attached Exhibit(s) A, and the adjusted exercise
price payable per share of Cisco Stock under the assumed Pipelinks Option shall
also be as indicated for that option in attached Exhibit(s) A.

               2. The intent of the foregoing adjustments to each assumed
Pipelinks Option is to assure that the spread between the aggregate fair market
value of the shares of Cisco Stock purchasable under each such option and the
aggregate exercise price as adjusted pursuant to this Agreement will,
immediately after the consummation of the Merger, be not less than the spread
which existed, immediately prior to the Merger, between the then aggregate fair
market value of the Pipelinks Stock subject to the Pipelinks Option and the
aggregate exercise price in effect at such time under the Option Agreement. Such
adjustments are also intended to preserve, immediately after the Merger, on a
per share basis, the same ratio of exercise price per option share to fair
market value per share which existed under the Pipelinks Option immediately
prior to the Merger. Such adjustments are also intended to preserve, to the
extent applicable, the Incentive Stock Option status of the assumed Pipelinks
Options.

               3. The following provisions shall govern each Pipelinks Option
hereby assumed by Cisco:

                        (a) Unless the context otherwise requires, all
                references in each Option Agreement and in the Plan (as
                incorporated into such Option Agreement) (i) to the "Company"
                shall mean Cisco, (ii) to "Stock" shall mean shares of Cisco
                Stock, (iii) to the "Board" shall mean the Board of Directors of
                Cisco and (iv) to the "Committee" shall mean the Compensation
                Committee of the Cisco Board of Directors.

                        (b) The grant date and the expiration date of each
                assumed Pipelinks Option and all other provisions which govern
                either the exercise or the termination of the assumed Pipelinks
                Option shall remain the same as set forth in the Option
                Agreement applicable to that option, and the provisions of the
                Option Agreement shall accordingly govern and control Optionee's
                rights under this Agreement to purchase Cisco Stock.

                        (c) Pursuant to the terms of the Option Agreement, none
                of your options assumed by Cisco in connection with the
                transaction will terminate and cease to outstanding upon the
                consummation of the Merger. Each Pipelinks Option shall be
                assumed by Cisco as of the Effective Time. Each such assumed
                Pipelinks Option shall thereafter continue to vest for any
                remaining unvested shares of Cisco Stock subject to that option
                in accordance with the same installment vesting schedule in
                effect under the applicable Option Agreement immediately prior
                to the Effective Time; provided, however, that the number of




                                       2.

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                shares subject to each such installment shall be adjusted to
                reflect the Exchange Ratio.

                        (d) For purposes of applying any and all provisions of
                the Option Agreement and the Plan relating to Optionee's status
                as an employee or a consultant of Pipelinks, Optionee shall be
                deemed to continue in such status as an employee or a consultant
                for so long as Optionee renders services as an employee or a
                consultant to Cisco or any present or future Cisco subsidiary.
                Accordingly, the provisions of the Option Agreement governing
                the termination of the assumed Pipelinks Options upon Optionee's
                cessation of service as an employee or a consultant of Pipelinks
                shall hereafter be applied on the basis of Optionee's cessation
                of employee or consultant status with Cisco and its
                subsidiaries, and each assumed Pipelinks Option shall
                accordingly terminate, within the designated time period in
                effect under the Option Agreement for that option, following
                such cessation of service as an employee or a consultant of
                Cisco and its subsidiaries.

                        (e) The adjusted exercise price payable for the Cisco
                Stock subject to each assumed Pipelinks Option shall be payable
                in any of the forms authorized under the Option Agreement
                applicable to that option. For purposes of determining the
                holding period of any shares of Cisco Stock delivered in payment
                of such adjusted exercise price, the period for which such
                shares were held as Pipelinks Stock prior to the Merger shall be
                taken into account.

                        (f) In order to exercise each assumed Pipelinks Option,
                Optionee must deliver to Cisco a written notice of exercise in
                which the number of shares of Cisco Stock to be purchased
                thereunder must be indicated. The exercise notice must be
                accompanied by payment of the adjusted exercise price payable
                for the purchased shares of Cisco Stock and should be delivered
                to Cisco at the following address:

                         Cisco Systems, Inc.
                         255 West Tasman Drive, Building J
                         San Jose, CA 95134
                         Attention: Option Plan Administrator

               4. Except to the extent specifically modified by this Option
Assumption Agreement, all of the terms and conditions of each Option Agreement
as in effect immediately prior to the Merger shall continue in full force and
effect and shall not in any way be amended, revised or otherwise affected by
this Stock Option Assumption Agreement.


                                       3.
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               IN WITNESS WHEREOF, Cisco Systems, Inc. has caused this Stock
Option Assumption Agreement to be executed on its behalf by its duly-authorized
officer as of the 9th day of March, 1999.




                                            CISCO SYSTEMS, INC.

                                            By: /s/ LARRY R. CARTER
                                                --------------------------------
                                                    Larry R. Carter
                                                    Corporate Secretary




                                 ACKNOWLEDGMENT


               The undersigned acknowledges receipt of the foregoing Stock
Option Assumption Agreement and understands that all rights and liabilities with
respect to each of his or her Pipelinks Options hereby assumed by Cisco are as
set forth in the Option Agreement, the Plan and such Stock Option Assumption
Agreement.


                                            ------------------------------------
                                            ((EMPLOYEE)), OPTIONEE



DATED:             , 1999
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                                       4.
