
<PAGE>   1
                                                                    EXHIBIT 99.6

                              CISCO SYSTEMS, INC.

                        STOCK OPTION ASSUMPTION AGREEMENT
                            AMTEVA TECHNOLOGIES, INC.


                             1996 STOCK OPTION PLAN
                            1998 STOCK INCENTIVE PLAN


OPTIONEE:  [[Employee]]

               STOCK OPTION ASSUMPTION AGREEMENT effective as of the 2nd day of
June, 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 Amteva
Technologies, Inc. (formerly Aum Tech of Virginia, Inc.), a Virginia corporation
("Amteva"), which were granted to Optionee under the Amteva 1996 Stock Option
Plan or 1998 Stock Incentive Plan (the "Plans") and are each evidenced by a
Stock Option Agreement (the "Option Agreement").

               WHEREAS, Amteva has been acquired by Cisco through the merger of
Amteva with and into Cisco (the "Merger") pursuant to the Agreement and Plan of
Merger and Reorganization, by and between Cisco and Amteva (the "Merger
Agreement").

               WHEREAS, the provisions of the Merger Agreement require Cisco to
assume all obligations of Amteva under all outstanding options under the Plans
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 Merger Agreement, the
exchange ratio (the "Exchange Ratio") in effect for the Merger is 0.104902
shares of Cisco common stock ("Cisco Stock") for each outstanding share of
Amteva common stock ("Amteva Stock").

               WHEREAS, this Agreement became effective immediately upon the
consummation of the Merger (the "Effective Time") in order to reflect certain
adjustments to Optionee's outstanding options 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 Amteva Stock subject to the options
held by Optionee immediately prior to the Effective Time (the "Amteva 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 Amteva under each of the Amteva Options. In connection with such assumption,
the number of shares of Cisco Stock purchasable under each Amteva

<PAGE>   2

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 Amteva 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 Amteva Option shall also be
as indicated for that option in attached Exhibit(s) A.

               2. The intent of the foregoing adjustments to each assumed Amteva
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 Amteva Stock subject to the Amteva 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 Amteva Option immediately prior to the Merger.

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

                             (a) Unless the context otherwise requires, all
               references in each Option Agreement and, if applicable, in the
               Plans (as incorporated into such Option Agreement) (i) to the
               "Company" shall mean Cisco, (ii) to "Shares" 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 Amteva Option and all other provisions which govern
               either the exercise or the termination of the assumed Amteva
               Option shall remain the same as set forth in the Option Agreement
               and the Plan, applicable to that option, and the provisions of
               the Option Agreement and the Plan 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 vest and become exercisable on an accelerated
               basis upon the consummation of the Merger. Each Amteva Option
               shall be assumed by Cisco as of the Effective Time. Each such
               assumed Amteva 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 shares
               subject to each such installment shall be adjusted to reflect the
               Exchange Ratio.

                                       2

<PAGE>   3

                        (d) For purposes of applying any and all provisions of
                the Option Agreement and/or the Plan relating to Optionee's
                status as an employee or a consultant of Amteva, 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 Amteva Options upon
                Optionee's cessation of service as an employee or a consultant
                of Amteva shall hereafter be applied on the basis of Optionee's
                cessation of employee or consultant status with Cisco and its
                subsidiaries, and each assumed Amteva 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. Specifically, an assumed Amteva Option which was
                originally granted under the 1996 Stock Option Plan must be
                exercised by the Optionee within fifteen (15) days of the date
                of the Optionee's termination by Cisco or the assumed Amteva
                Option will terminate. An assumed Amteva Option which was
                originally granted under the 1998 Stock Incentive Plan must be
                exercised within three (3) months of the date of the Optionee's
                termination by Cisco or the assumed Amteva Option will
                terminate.

                        (e) The adjusted exercise price payable for the Cisco
                Stock subject to each assumed Amteva 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 Amteva Stock prior to the Merger shall be
                taken into account.

                        (f) In order to exercise each assumed Amteva 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

<PAGE>   4

               IN WITNESS WHEREOF, Cisco Systems, Inc. has caused this Stock
Option Assumption Agreement to be executed on its behalf by its duly-authorized
officer effective as of the 2nd day of June, 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 Amteva Options hereby assumed by Cisco are as set
forth in the Option Agreement, the Plan, as applicable, and such Stock Option
Assumption Agreement.


                                            ------------------------------------
                                            [[EMPLOYEE]], OPTIONEE



DATED:          , 1999
      ---------


                                       4


