Document:

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                                                                     EXHIBIT 4.1

                             STOCK OPTION AGREEMENT

        THIS STOCK OPTION AGREEMENT ("Option Agreement") is entered into as of
May 1, 2000, by and between ACT NETWORKS, INC., a Delaware corporation (the
"Company"), and CLARENT CORPORATION, a Delaware corporation (the "Grantee").

                                    RECITALS

        A. The Grantee, Copper Acquisition Sub, Inc., a Delaware corporation and
a wholly owned subsidiary of the Grantee ("Merger Sub"), and the Company are
entering into an Agreement and Plan of Merger and Reorganization of even date
herewith (as amended from time to time, the "Reorganization Agreement") which
provides (subject to the conditions set forth therein) for the merger of Merger
Sub into the Company (the "Merger").

        B. The Company is entering into this Option Agreement in order to induce
the Grantee to enter into the Reorganization Agreement.

                                    AGREEMENT

        The parties to this Option Agreement, intending to be legally bound,
agree as follows:

        1. CERTAIN DEFINITIONS. Capitalized terms used but not defined in this
Option Agreement shall have the meanings given to such terms in the
Reorganization Agreement.

        2. GRANT OF OPTION. The Company hereby grants to the Grantee an
irrevocable option (the "Option") to purchase, out of the authorized but
unissued Company Common Stock, a number of shares of Company Common Stock equal
to up to 19.9% of the number of shares of Company Common Stock outstanding as of
the date hereof (the shares of Company Common Stock purchasable pursuant to the
Option, as adjusted as set forth herein, being referred to as the "Option
Shares"), at a price per Option Share equal to the applicable Exercise Price.
For purposes of this Option Agreement, the applicable "Exercise Price" shall be
equal to $16.00.

        3. TERM. The Option shall terminate on the earliest of the following
dates (the "Termination Date"): (a) the date on which the Merger becomes
effective; (b) the date on which the Reorganization Agreement is validly
terminated pursuant to Section 8.1 thereof, if an Exercise Event (as defined in
Section 4(b)) shall not have occurred on or prior to such date; or (c) the first
anniversary of the date on which the Grantee receives written notice from the
Company of the occurrence of an Exercise Event; provided, however, that if an
Exercise Event occurs, and if, by reason of any applicable Legal Requirement,
order, judgment, decree or other legal impediment, the Option cannot be
exercised on the first anniversary of the date on which the Grantee receives the
written notice referred to in clause "(c)" of this sentence, then the
Termination Date shall be extended until the date 30 days after the date on
which such impediment is removed. The rights of the Grantee and the obligations
of the Company set forth in Sections 7 and 8 shall not terminate on the
Termination Date, but rather shall survive beyond the Termination Date as
provided in those Sections; and the representation, warranty and covenant set
forth in Section 5(b) shall survive the termination date for an indefinite
period of time.

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        4. EXERCISE OF OPTION.

            (a) The Grantee may exercise the Option, in whole or in part, at any
time and from time to time on or before the Termination Date following the
occurrence of an Exercise Event. If the Grantee exercises the Option with
respect to any Option Shares on or before the Termination Date, then,
notwithstanding anything to the contrary contained in this Option Agreement, the
Grantee shall be entitled to purchase such Option Shares in accordance with the
terms of this Option Agreement after the Termination Date.

            (b) For purposes of this Option Agreement, an "Exercise Event" shall
be deemed to have occurred if:

               (i) either the Grantee or the Company shall have the right to
        terminate the Reorganization Agreement pursuant to Section 8.1(d)
        thereof and an Acquisition Proposal shall have been previously
        disclosed, announced, commenced, submitted or made; or

               (ii) the Grantee shall have the right to terminate the
        Reorganization Agreement pursuant to Section 8.1(e) thereof.

            (c) To exercise the Option with respect to any Option Shares, the
Grantee shall deliver to the Company a written notice (an "Exercise Notice")
specifying: (i) the number of Option Shares the Grantee will purchase; (ii) the
place at which such Option Shares are to be purchased; and (iii) the date on
which such Option Shares are to be purchased, which shall not be sooner than two
business days nor later than twenty business days after the date of delivery of
such Exercise Notice to the Company. (The date of delivery of such Exercise
Notice to the Company is referred to as the applicable "Notice Date," and the
Option shall be deemed to have been validly exercised on such Notice Date with
respect to the Option Shares referred to in such Exercise Notice.) The closing
of the purchase of such Option Shares (the applicable "Closing") shall take
place at the place specified in the Exercise Notice and on the date specified in
such Exercise Notice (the applicable "Closing Date"); provided, however, that:
(A) if such purchase cannot be consummated on such Closing Date by reason of any
applicable Legal Requirement, order, judgment, decree or other legal impediment,
then the Grantee may extend the Closing Date to a date not more than 30 days
after the date on which such impediment is removed; and (B) if prior
notification to or approval of any Governmental Body is required, or if any
waiting period must expire or be terminated, in connection with such purchase,
then (1) the Company shall promptly cause to be filed the required notice or
application for approval and shall expeditiously process such notice or
application, (2) the Company shall cooperate with the Grantee in the filing of
any such notice or application required to be filed by the Grantee and in the
obtaining of any such approval required to be obtained by the Grantee, and (3)
the Grantee may extend the Closing Date to a date not more than 30 days after
the latest date on which any required notification has been made, any required
approval has been obtained or any required waiting period has expired or been
terminated.

        5. PAYMENT AND DELIVERY OF CERTIFICATE.

            (a) On each Closing Date, the Grantee shall pay to the Company in
immediately available funds an amount equal to the applicable Exercise Price
multiplied by the

                                       2.
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number of Option Shares to be purchased by the Grantee from the Company on such
Closing Date.

            (b) At each Closing, concurrently with the delivery of the
immediately available funds referred to in Section 5(a), the Company shall
deliver to the Grantee a certificate representing the Option Shares being
purchased at such Closing. The Company represents, warrants and covenants that
such Option Shares will be duly authorized, validly issued, fully paid,
nonassessable and free and clear of all Encumbrances, except as may be
specifically provided in this Option Agreement.

            (c) The certificate representing the Option Shares delivered at each
Closing shall be endorsed with a restrictive legend that shall read
substantially as follows:

               THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
               REGISTERED UNDER THE SECURITIES ACT OF 1933 AND MAY NOT BE
               OFFERED, SOLD OR OTHERWISE TRANSFERRED UNLESS REGISTERED UNDER
               SAID ACT OR UNLESS AN EXEMPTION FROM THE REGISTRATION
               REQUIREMENTS OF SAID ACT IS AVAILABLE.

If at any time the Grantee delivers to the Company evidence (in the form of a
copy of a letter from the staff of the SEC or an opinion of counsel reasonably
satisfactory to the Company, or in some other form) that the legend referred to
above is not required for purposes of the Securities Act, then the Company shall
promptly replace such certificate with a certificate that does not include such
legend.

        6. ADJUSTMENT UPON CHANGES IN CAPITALIZATION, ETC.

            (a) If the outstanding shares of Company Common Stock are changed
into a different number or class of shares by reason of any stock split,
division or subdivision of shares, stock dividend, reverse stock split,
reclassification, recapitalization or other similar transaction, then the type
and number of shares or other securities subject to the Option, the applicable
Exercise Price, the Designated Price (as defined in Section 7(c)) and the other
numbers and dollar amounts referred to in this Option Agreement shall be
adjusted appropriately, and the Company shall ensure that proper provision is
made in the agreements and other documents governing such transaction so that
the Grantee shall receive upon exercise of the Option the same class and number
of outstanding shares or other securities or property that the Grantee would
have received if the Option had been exercised immediately prior to such
transaction or the record date for determining the stockholders entitled to
participate in such transaction, as applicable.

            (b) If any additional shares of Company Common Stock are issued
after the date of this Option Agreement (other than pursuant to a transaction
described in Section 6(a)), then the number of shares of Company Common Stock
then remaining subject to the Option shall be increased to the number by which
(i) 19.9% of the number of shares of the Company Common Stock outstanding after
the issuance of such additional shares exceeds (ii) the number of shares
(adjusted in accordance with Section 6(a)) previously issued to the Grantee upon
exercise of the Option.

                                       3.
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            (c) If the Company shall enter into an agreement (i) to consolidate,
exchange shares or merge with any Person, other than the Grantee or one of the
Grantee's subsidiaries, and, in the case of a merger, shall not be the
continuing or surviving corporation, (ii) to permit any Person, other than the
Grantee or one of the Grantee's subsidiaries, to merge into the Company and the
Company shall be the continuing or surviving corporation, but, in connection
with such merger, the then outstanding shares of Company Common Stock shall be
changed into or exchanged for stock or other securities of the Company or any
other Person or cash or any other property, or the shares of Company Common
Stock outstanding immediately before such merger shall after such merger
represent less than 50% of the common shares and common share equivalents of the
Company outstanding immediately after the merger, or (iii) to sell, lease or
otherwise transfer all or substantially all of its assets to any Person, other
than the Grantee or one of the Grantee's subsidiaries, then, and in each such
case, the Company shall ensure that proper provision is made in the agreements
and other documents governing such transaction so that the Option shall, upon
the consummation of such transaction, become exercisable for the stock,
securities, cash or other property that would have been received by the Grantee
if the Grantee had exercised the Option immediately prior to such transaction or
the record date for determining the stockholders entitled to participate in such
transaction, as appropriate.

            (d) The provisions of Sections 7 and 8 shall apply (with appropriate
adjustments) to any securities for which the Option becomes exercisable pursuant
to this Section 6.

        7. REPURCHASE AT THE REQUEST OF THE GRANTEE.

            (a) If, at any time during the period commencing upon the occurrence
of the first Exercise Event and ending on the first anniversary of the date on
which the Grantee receives written notice from the Company of the occurrence of
such Exercise Event, the Grantee delivers to the Company a notice stating that
the Grantee is exercising its rights under this Section 7, then the Company
shall repurchase from the Grantee (i) that portion of the Option that then
remains unexercised, (ii) all of the shares of Company Common Stock beneficially
owned by the Grantee that were previously purchased by the Grantee upon exercise
of the Option, and (iii) the Grantee's right to receive all Option Shares with
respect to which the Option was previously exercised but which have not yet been
issued and delivered to the Grantee. (The date on which the Grantee delivers
such notice to the Company is referred to as the "Request Date.") Such
repurchase shall be at an aggregate price (the "Repurchase Consideration") equal
to the sum of:

               (i) the aggregate number of Option Shares with respect to which
        the Option has been exercised on or prior to the Request Date and which
        are beneficially owned by the Grantee, multiplied by the Designated
        Price;

               (ii) the aggregate number of Option Shares with respect to which
        the Option has been exercised on or prior to the Request Date but which
        have not, as of the Request Date been issued and delivered to the
        Grantee, multiplied by the Designated Price; and

               (iii) the number of Option Shares with respect to which the
        Option has not been exercised on or prior to the Request Date multiplied
        by the amount (if any) by which the Designated Price exceeds the
        applicable Exercise Price.

                                       4.
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            (b) If the Grantee exercises its rights under this Section 7, then
the Company shall, within two business days after the Request Date, pay the
Repurchase Consideration to the Grantee in immediately available funds, and the
Grantee shall thereupon surrender to the Company the certificate or certificates
evidencing any shares of Company Common Stock repurchased by the Company
pursuant to this Section 7.

            (c) For purposes of this Option Agreement, the "Designated Price"
means the highest of (i) the highest purchase price per share paid after the
date of this Option Agreement and on or prior to the Request Date pursuant to
any tender or exchange offer made for shares of Company Common Stock, (ii) the
highest price per share paid or to be paid by any Person for shares of Company
Common Stock pursuant to any agreement contemplating a merger or other business
combination transaction involving the Company that was entered into after the
date of this Option Agreement and on or prior to the Request Date, (iii) the
highest bid price per share of Company Common Stock as quoted on The Nasdaq
National Market (or if Company Common Stock is not quoted on The Nasdaq National
Market, the highest bid price per share of Company Common Stock as quoted on any
other market comprising a part of The Nasdaq Stock Market or, if the shares of
Company Common Stock are not quoted thereon, on the principal trading market (as
defined in Regulation M under the Exchange Act) on which such shares are traded
as reported by a recognized source) during the 90-day period ending on the
Request Date, or (iv) the highest Exercise Price paid or payable for any Option
Shares. If any portion of the consideration paid or to be paid pursuant to
clause "(i)" or "(ii)" of the preceding sentence is in a form other than cash,
then the value of the non-cash portion of such consideration shall be determined
in good faith by an independent nationally recognized investment banking firm
selected by the Company, and such firm's determination of such value shall be
final and conclusive for all purposes under this Option Agreement.

        8. REGISTRATION RIGHTS.

            (a) The Company shall, if requested by the Grantee at any time and
from time to time within two years after the first exercise of the Option (the
"Registration Period"), as expeditiously as practicable, prepare, file and cause
to be declared effective up to two registration statements under the Securities
Act (including, in the sole discretion of the Grantee, a "shelf" registration
statement under Rule 415 under the Securities Act or any successor provision),
if registration under the Securities Act is (in the Grantee's good faith
judgment) necessary or desirable in order to permit the offering, sale and
delivery, in accordance with the intended method of sale or other disposition
stated by the Grantee, of any or all shares of Company Common Stock or other
securities that have been acquired or are issuable upon exercise of the Option.
No other securities may be included in any such registration statement without
the Grantee's prior written consent. The Company shall use all reasonable
efforts to cause each such registration statement to become effective, to obtain
all consents or waivers of other parties that are required therefor and to keep
such registration statement effective for such period as may be as reasonably
necessary to effect such sale or other disposition. In addition, the Company
shall use all reasonable efforts to register such shares or other securities
under any applicable state securities laws. The obligations of the Company
hereunder to file a registration statement and to maintain its effectiveness may
be suspended for one or more periods of time not exceeding 30 days in the
aggregate if the board of directors of the Company shall have determined in good
faith that the filing of such registration statement or the maintenance of its
effectiveness would require disclosure of material nonpublic information and
that the disclosure thereof would materially and adversely affect the Company.
For purposes of determining

                                       5.
<PAGE>   6

whether two requests have been made under this Section 8, only requests relating
to a registration statement that has become effective under the Securities Act
and pursuant to which the Grantee has disposed of all shares covered thereby in
the manner contemplated therein shall be counted.

            (b) The expenses associated with the preparation and filing of any
registration statement pursuant to this Section 8 and any sale covered thereby
(including any fees related to blue sky qualifications and filing fees in
respect of the National Association of Securities Dealers, Inc.) shall be borne
and paid by, and shall be for the sole account of, the Company (except for
underwriting discounts or commissions in respect to shares to be sold by the
Grantee and the fees and disbursements of the Grantee's counsel).

            (c) The Grantee shall provide all information reasonably requested
by the Company for inclusion in any registration statement to be filed
hereunder. If, during the Registration Period, the Company shall propose to
register under the Securities Act the offering, sale and delivery of Company
Common Stock for cash pursuant to a firm commitment underwriting, it shall
(without limiting the Company's other obligations under this Section 8) allow
the Grantee the right to participate in such registration (provided that the
Grantee participates in the underwriting); provided, however, that, if the
managing underwriter of such offering advises the Company in writing that in its
opinion the number of shares of Company Common Stock requested to be included in
such registration exceeds the number that can be sold in such offering, the
Company shall, after fully including therein all securities to be sold by the
Company, include the shares requested to be included therein by Grantee pro rata
(based on the number of shares intended to be included therein) with the shares
intended to be included therein by Persons other than the Company. In connection
with any offering, sale and delivery of Company Common Stock pursuant to a
registration effected pursuant to this Section 8, the Company and the Grantee
shall provide each other and each underwriter of the offering with customary
representations, warranties and covenants, including covenants of
indemnification and contribution.

        9. PROFIT LIMITATION. Notwithstanding anything to the contrary contained
in this Option Agreement, the Grantee may not exercise its rights pursuant to
this Option Agreement in a manner that would result in a cash payment to the
Grantee of an aggregate amount under this Option Agreement and under Section
8.3(b) and Section 8.3(c) of the Reorganization Agreement of more than the sum
of (a) the aggregate exercise price paid and/or payable by the Grantee for any
Option Shares as to which the Option has theretofore been exercised, plus (b) an
amount equal to the Termination Fee, plus (c) $1,833,333.

        10. LISTING. If, at the time of the occurrence of an Exercise Event, the
Company Common Stock (or any other class of securities subject to the Option) is
listed on The Nasdaq National Market or on any other market or exchange, then
the Company, upon the occurrence of such Exercise Event, shall promptly file an
application to list on The Nasdaq National Market and on such other market or
exchange the shares of the Company Common Stock (or other securities) subject to
the Option, and shall use its best efforts to cause such application to be
approved as promptly as practicable.

        11. REPLACEMENT OF AGREEMENT. Upon receipt by the Company of evidence
reasonably satisfactory to it of the loss, theft, destruction or mutilation of
this Option Agreement, and (in the case of loss, theft or destruction) of
reasonably satisfactory indemnification, and upon

                                       6.
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surrender and cancellation of this Option Agreement, if mutilated, the Company
will execute and deliver a new Option Agreement of like tenor and date.

        12. MISCELLANEOUS.

            (a) WAIVER. No failure on the part of any party to exercise any
power, right, privilege or remedy under this Option Agreement, and no delay on
the part of any party in exercising any power, right, privilege or remedy under
this Option Agreement, shall operate as a waiver of such power, right, privilege
or remedy; and no single or partial exercise of any such power, right, privilege
or remedy shall preclude any other or further exercise thereof or of any other
power, right, privilege or remedy. No party shall be deemed to have waived any
claim arising out of this Option Agreement, or any power, right, privilege or
remedy under this Option Agreement, unless the waiver of such claim, power,
right, privilege or remedy is expressly set forth in a written instrument duly
executed and delivered on behalf of such party; and any such waiver shall not be
applicable or have any effect except in the specific instance in which it is
given.

            (b) NOTICES. Any notice or other communication required or permitted
to be delivered to any party under this Option Agreement shall be in writing and
shall be deemed properly delivered, given and received (a) when delivered by
hand, or (b) two business days after sent by internationally recognized courier
or express delivery service (such as Federal Express) or by facsimile (with
confirmation of receipt) to the address or facsimile telephone number set forth
beneath the name of such party below (or to such other address or facsimile
telephone number as such party shall have specified in a written notice given to
the other parties hereto):

                             if to the Company, to it at:

                                    26707 W. Agoura Road
                                    Calabasas, CA 91302
                                    Facsimile: (818) 871-6405
                                    Attention: Chief Executive Officer

                             if to the Grantee, to it at:

                                    700 Chesapeake Drive
                                    Redwood City, CA 94063
                                    Facsimile:  (650) 306-7512
                                    Attention: Chief Executive Officer

            (c) ENTIRE AGREEMENT; COUNTERPARTS. This Option Agreement and the
agreements referred to herein constitute the entire agreement and supersede all
prior agreements and understandings, both written and oral, between the parties
with respect to the subject matter hereof and thereof. This Option Agreement may
be executed in several counterparts, each of which shall be deemed an original
and all of which shall constitute one and the same instrument.

            (d) BINDING EFFECT; BENEFIT; ASSIGNMENT. This Option Agreement shall
be binding upon, and shall be enforceable by and inure solely to the benefit of,
the parties hereto and their respective successors and assigns; provided,
however, that neither this Option

                                       7.
<PAGE>   8

Agreement nor any of the Company's rights, interest or obligations hereunder may
be assigned by the Company without the prior written consent of the Grantee, and
any attempted assignment of this Option Agreement or any of such rights,
interest or obligations by the Company without such consent shall be void and of
no effect. Nothing in this Option Agreement, express or implied, is intended to
or shall confer upon any Person (other than the parties hereto and the Grantee's
successors and assigns) any right, benefit or remedy of any nature under or by
reason of this Option Agreement.

            (e) AMENDMENT AND MODIFICATION. This Option Agreement may be amended
with the approval of the respective boards of directors of the Company and the
Grantee at any time. This Option Agreement may not be amended except by an
instrument in writing signed on behalf of each of the parties hereto.

            (f) FURTHER ACTIONS. The Company agrees to cooperate fully with the
Grantee and to execute and deliver such further documents, certificates,
agreements and instruments and to take such other actions as may be reasonably
requested by the Grantee to evidence or reflect the transactions contemplated by
this Option Agreement and to carry out the intent and purposes of this Option
Agreement.

            (g) HEADINGS. The bold-faced headings contained in this Option
Agreement are for convenience of reference only, shall not be deemed to be a
part of this Option Agreement and shall not be referred to in connection with
the construction or interpretation of this Option Agreement.

            (h) APPLICABLE LAW; JURISDICTION. This Option Agreement shall be
governed by, and construed in accordance with, the laws of the State of
Delaware, regardless of the laws that might otherwise govern under applicable
principles of conflicts of laws thereof. In any action between any of the
parties arising out of or relating to this Option Agreement or any of the
transactions contemplated by this Option Agreement: (a) each of the parties
irrevocably and unconditionally consents and submits to the jurisdiction and
venue of the state and federal courts located in the State of Delaware; (b) each
of the parties irrevocably waives the right to trial by jury; and (c) each of
the parties irrevocably consents to service of process by first class certified
mail, return receipt requested, postage prepaid, to the address at which such
party is to receive notice in accordance with Section 12(b).

            (i) SEVERABILITY. In the event that any provision of this Option
Agreement, or the application of any such provision to any Person (as such term
is defined in the Reorganization Agreement) or set of circumstances, shall be
determined to be invalid, unlawful, void or unenforceable to any extent, the
remainder of this Option Agreement, and the application of such provision to
Persons or circumstances other than those as to which it is determined to be
invalid, unlawful, void or unenforceable, shall not be impaired or otherwise
affected and shall continue to be valid and enforceable to the fullest extent
permitted by law.

            (j) SPECIFIC PERFORMANCE. The Company agrees that (i) in the event
of any breach or threatened breach by the Company of any covenant, obligation or
other provision set forth in this Option Agreement, the Grantee shall be
entitled (in addition to any other remedy that may be available to it), to (A) a
decree or order of specific performance or mandamus to enforce the observance
and performance of such covenant, obligation or other provision, and (B) an
injunction restraining such breach or threatened breach, and (ii) neither the
Grantee nor

                                       8.
<PAGE>   9

any other Person shall be required to provide any bond or other security in
connection with any such decree, order or injunction or in connection with any
related action or proceeding.

            (k) ATTORNEYS' FEES. In any action at law or suit in equity to
enforce this Option Agreement or the rights of any of the parties hereunder, the
prevailing party in such action or suit shall be entitled to receive a
reasonable sum for its attorneys' fees and all other reasonable costs and
expenses incurred in such action or suit.

            (l) NON-EXCLUSIVITY. The rights and remedies of the Grantee under
this Option Agreement are not exclusive of or limited by any other rights or
remedies which it may have, whether at law, in equity, by contract or otherwise,
all of which shall be cumulative (and not alternative). Without limiting the
generality of the foregoing, the rights and remedies of the Grantee under this
Option Agreement, and the obligations and liabilities of the Company under this
Option Agreement, are in addition to their respective rights, remedies,
obligations and liabilities under all applicable Legal Requirements and under
the Reorganization Agreement. The covenants and obligations of the Company set
forth in this Option Agreement shall be construed as independent of any other
agreement or arrangement between the Company, on the one hand, and the Grantee,
on the other. The existence of any claim or cause of action by the Company
against the Grantee shall not constitute a defense to the enforcement of any of
such covenants or obligations against the Company.

            (m) CONSTRUCTION.

               (i) For purposes of this Agreement, whenever the context
requires: the singular number shall include the plural, and vice versa; the
masculine gender shall include the feminine and neuter genders; the feminine
gender shall include the masculine and neuter genders; and the neuter gender
shall include the masculine and feminine genders.

               (ii) The parties hereto agree that any rule of construction to
the effect that ambiguities are to be resolved against the drafting party shall
not be applied in the construction or interpretation of this Agreement.

               (iii) As used in this Agreement, the words "include" and
"including," and variations thereof, shall not be deemed to be terms of
limitation, but rather shall be deemed to be followed by the words "without
limitation."

               (iv) Unless otherwise specified, references in this Agreement to
"Sections" are intended to refer to Sections of this Agreement.

                                       9.
<PAGE>   10

        IN WITNESS WHEREOF, the Company and the Grantee have caused this Option
Agreement to be executed as of the date first written above.

                                          ACT NETWORKS, INC.

                                          By: /s/ ANDRE DE FUSCO
                                             -----------------------------------
                                          Name:  Andre de Fusco

                                          Title: President and Chief Executive
                                                 Officer

                                          CLARENT CORPORATION

                                          By: /s/ RICHARD J. HEAPS
                                             -----------------------------------
                                          Name:  Richard J. Heaps

                                          Title: Chief Operating Officer and
                                                 Chief Financial Officer

                                      10.<PAGE>   1

                                                                    EXHIBIT 10.1

                                VOTING AGREEMENT

         THIS VOTING AGREEMENT (the "Voting Agreement") is entered into as of
May 1, 2000, by and between CLARENT CORPORATION., a Delaware corporation
("Parent"), and __________ ("Stockholder").

                                    RECITALS

         A. Parent, Copper Acquisition Sub, Inc., a Delaware corporation and a
wholly owned subsidiary of Parent ("Merger Sub"), and Aluminum, Inc., a Delaware
corporation (the "Company"), are entering into an Agreement and Plan of Merger
and Reorganization of even date herewith (as amended from time to time, the
"Reorganization Agreement"), which provides (subject to the conditions set forth
therein) for the merger of Merger Sub into the Company (the "Merger").

         B. As a condition to the willingness of Parent and Merger Sub to enter
into the Reorganization Agreement, Parent and Merger Sub have required that
Stockholder enter into this Voting Agreement; and Stockholder is entering into
this Voting Agreement in order to induce Parent and Merger Sub to enter into the
Reorganization Agreement.

                                    AGREEMENT

         The parties to this Voting Agreement, intending to be legally bound,
agree as follows:

SECTION 1.        CERTAIN DEFINITIONS

                  (a) All capitalized terms used but not otherwise defined in
this Voting Agreement have the meanings given to them in the Reorganization
Agreement.

                  (b) "SUBJECT SECURITIES" shall mean: (i) all securities of the
Company (including shares of Company Common Stock and options and other rights
to acquire shares of Company Common Stock) Owned by Stockholder as of the date
of this Voting Agreement; and (ii) all additional securities of the Company
(including any additional shares of Company Common Stock and any additional
options or other rights to acquire shares of Company Common Stock) of which
Stockholder acquires Ownership during the period from the date of this Voting
Agreement through the Expiration Date.

                  (c) Stockholder shall be deemed to "OWN" or to have acquired
"OWNERSHIP" of a security if Stockholder: (i) is the record owner of such
security; or (ii) is the "beneficial owner" (within the meaning of Rule 13d-3
under the Exchange Act) of such security.

                  (d) "EXPIRATION DATE" shall mean the earlier of the date upon
which the Reorganization Agreement is validly terminated or the date upon which
the Effective Time occurs.

<PAGE>   2

                  (e) The "RECORD DATE" for a particular matter shall be the
date fixed for Persons entitled: (i) to receive notice of, and to vote at, a
meeting of the stockholders of the Company called for the purpose of voting on
such matter; or (ii) to take action by written consent of the stockholders of
the Company with respect to such matter.

SECTION 2.        TRANSFER OF SUBJECT SECURITIES

         2.1 NO DISPOSITION OR ENCUMBRANCE OF SUBJECT SECURITIES. Stockholder
covenants and agrees that, from the date of this Voting Agreement until the
Expiration Date, Stockholder will not, directly or indirectly: (i) offer, sell,
offer to sell, contract to sell, pledge, grant any option to purchase or
otherwise dispose of or transfer (or permit or announce any offer, sale, offer
of sale, contract of sale or grant of any option for the purchase of, or permit
or announce any other disposition or transfer of) any of the Subject Securities,
or any interest in any of the Subject Securities, to any Person other than
Parent; (ii) create or permit to exist any Encumbrance on or otherwise affecting
any of the Subject Securities; or (iii) reduce Stockholder's Ownership of,
interest in or risk relating to any of the Subject Securities.

         2.2 TRANSFER OF VOTING RIGHTS. Stockholder covenants and agrees that,
from the date of this Voting Agreement until the Expiration Date, Stockholder
will not deposit any of the Subject Securities into a voting trust or grant a
proxy or enter into a voting agreement or similar Contract with respect to any
of the Subject Securities.

SECTION 3.        VOTING OF SHARES

         3.1 VOTING AGREEMENT. Stockholder covenants and agrees that, from the
date of this Voting Agreement until the Expiration Date, at any meeting of the
stockholders of the Company, however called, and in any written action by
consent of stockholders of the Company, Stockholder shall (unless otherwise
directed in writing by Parent) cause to be voted all outstanding shares of
capital stock of the Company that (as of the Record Date for any of the matters
referred to in this Section 3.1) are Owned by Stockholder:

                           (i) in favor of the Merger and the adoption of the
         Reorganization Agreement and in favor of each of the other actions
         contemplated by the Reorganization Agreement and any action that could
         reasonably be expected to facilitate the consummation of the Merger;
         and

                           (ii) against the following actions (other than the
         Merger and the other transactions contemplated by the Reorganization
         Agreement): (A) any extraordinary corporate transaction, such as a
         merger, consolidation or other business combination involving any of
         the Acquired Companies; (B) any sale, lease or transfer of a material
         amount of assets of any of the Acquired Companies (other than in the
         ordinary course of business); (C) any reorganization, recapitalization,
         dissolution or liquidation of any of the Acquired Companies; (D) any
         removal of or change in a majority of the board of directors of the
         Company; (E) any amendment to the Company's certificate of
         incorporation; (F) any material change in the capitalization of the
         Company or the

                                       2
<PAGE>   3

         Company's corporate structure; and (G) any other action that is
         inconsistent with the Merger or that is intended, or could reasonably
         be expected, to impede, interfere with, delay, postpone, discourage or
         adversely affect the Merger or any of the other transactions
         contemplated by the Reorganization Agreement or this Voting Agreement.

Stockholder shall not, from the date of this Voting Agreement until the
Expiration Date, enter into any agreement or understanding with any Person to
vote or give instructions inconsistent with clause "(i)" or "(ii)" of the
preceding sentence.

         3.2      PROXY.

                  (a) Concurrently with the execution of this Voting Agreement:
(i) Stockholder shall deliver to Parent a proxy in the form attached hereto as
Exhibit A, which shall be irrevocable to the fullest extent permitted by law,
with respect to the shares referred to therein (the "Proxy"); and (ii)
Stockholder shall cause to be delivered to Parent an additional proxy (in the
form attached hereto as Exhibit A) executed on behalf of the record owner of any
outstanding shares of Company Common Stock that are owned beneficially (but are
not owned of record) by Stockholder.

                  (b) After the execution of this Voting Agreement until the
Expiration Date, Stockholder shall execute or cause to be executed such further
proxies as may be requested by Parent with respect to any additional shares of
outstanding capital stock of the Company of which Stockholder acquires
Ownership, and Stockholder shall promptly notify Parent upon acquiring Ownership
of any additional securities of the Company.

SECTION 4.        WAIVER OF APPRAISAL RIGHTS.

         Stockholder hereby irrevocably and unconditionally waives any rights of
appraisal, dissenters' rights or similar rights that Stockholder may have in
connection with the Merger, and Stockholder shall cause to be irrevocably and
unconditionally waived any such rights that any affiliate of Stockholder may
have in connection with the Merger.

SECTION 5.        NO SOLICITATION

         Stockholder covenants and agrees that, during the period commencing on
the date of this Voting Agreement and ending on the Expiration Date, Stockholder
shall not, directly or indirectly, and shall not authorize or permit any
Representative of Stockholder, directly or indirectly, to (i) solicit, initiate,
encourage or induce the making, submission or announcement of any Acquisition
Proposal or take any action that would, individually or in the aggregate,
reasonably be expected to lead to an Acquisition Proposal, (ii) furnish any
information regarding any of the Acquired Companies to any Person in connection
with or in response to an Acquisition Proposal or an inquiry or indication of
interest that could lead to an Acquisition Proposal, (iii) engage in discussions
with any Person with respect to any Acquisition Proposal, (iv) approve, endorse
or recommend any Acquisition Proposal, or (v) enter into any letter of intent or
similar document or any Contract contemplating or otherwise relating to any

                                       3
<PAGE>   4

Acquisition Transaction. Stockholder shall immediately cease any existing
discussions with any Person that relate to any Acquisition Proposal. Stockholder
shall promptly (and in no event later than 24 hours after receiving, or
obtaining knowledge of, any Acquisition Proposal, any inquiry or indication of
interest that could reasonably be expected to lead to an Acquisition Proposal or
any request for nonpublic information) advise Parent orally and in writing of
any Acquisition Proposal, any inquiry or indication of interest that could
reasonably be expected to lead to an Acquisition Proposal or any request for
nonpublic information relating to any of the Acquired Companies (including the
identity of the Person making or submitting such Acquisition Proposal, inquiry,
indication of interest or request, and the terms thereof) that is made or
submitted by any Person.

SECTION 6.        REPRESENTATIONS AND WARRANTIES OF STOCKHOLDER

         Stockholder hereby represents and warrants to Parent as follows:

         6.1 AUTHORIZATION, ETC. Stockholder has all requisite power and
capacity to execute and deliver this Voting Agreement and the Proxy and to
perform Stockholder's obligations hereunder and thereunder. This Voting
Agreement and the Proxy have been duly executed and delivered by Stockholder
and, assuming the due authorization, execution and delivery by Parent,
constitute the legal, valid and binding obligations of Stockholder, enforceable
against Stockholder in accordance with their respective terms, subject to (i)
laws of general application relating to bankruptcy, insolvency and the relief of
debtors, and (ii) rules of law governing specific performance, injunctive relief
and other equitable remedies.

         6.2      NO CONFLICTS, REQUIRED FILINGS AND CONSENTS.

                  (a) The execution and delivery of this Voting Agreement and
the Proxy by Stockholder do not, and the performance of this Voting Agreement
and the Proxy by Stockholder will not: (i) conflict with or violate any Legal
Requirement, order, decree or judgment applicable to Stockholder or by which
Stockholder or any of Stockholder's properties is bound or affected; or (ii)
result in any breach of or constitute a default (with notice or lapse of time,
or both) under, or give to others any rights of termination, amendment,
acceleration or cancellation of, or result in the creation of an Encumbrance on
or otherwise affecting any of the Subject Securities pursuant to, any Contract
to which Stockholder is a party or by which Stockholder or any of Stockholder's
properties is bound or affected.

                  (b) The execution and delivery of this Voting Agreement and
the Proxy by Stockholder do not, and the performance of this Voting Agreement
and the Proxy by Stockholder will not, require any Consent of any Person.

         6.3 TITLE TO SUBJECT SECURITIES. As of the date hereof, Stockholder
Owns in the aggregate (including shares owned of record and shares owned
beneficially) the number of outstanding shares of capital stock of the Company
specified below Stockholder's name on the signature page hereof, and the number
of options and other rights to acquire shares of Company Common Stock specified
below Stockholder's name on the signature page hereof, and Stockholder does not
directly or indirectly Own any other securities of the Company.

                                       4
<PAGE>   5

         6.4 ACCURACY OF REPRESENTATIONS. The representations and warranties
contained in this Voting Agreement are accurate in all respects as of the date
of this Voting Agreement, will be accurate in all respects at all times through
the Expiration Date and will be accurate in all respects as of the date of the
consummation of the Merger as if made on that date.

SECTION 7.        COVENANTS OF STOCKHOLDER

         Stockholder agrees to cooperate fully with Parent and to execute and
deliver (at Parent's expense) such further documents, certificates, agreements
and instruments and to take such other actions (at Parent's expense) as may be
reasonably requested by Parent to evidence or reflect the transactions
contemplated by this Voting Agreement and to carry out the intent and purposes
of this Voting Agreement.

SECTION 8.        LIMITATIONS

         This Voting Agreement is intended to bind Stockholder only with respect
to the specific matters set forth herein, and shall not prohibit Stockholder
from acting in accordance with his or her fiduciary duties as an officer or
director of the Company. Stockholder will retain at all times the right to vote
Stockholder's Subject Securities, in Stockholder's sole discretion, on all
matters other than those set forth in Section 3.1 which are at any time or from
time to time presented to the Company's stockholders generally.

SECTION 9.        MISCELLANEOUS

         9.1 SURVIVAL OF REPRESENTATIONS, WARRANTIES AND AGREEMENTS. None of the
representations, warranties and agreements made by Stockholder in this Voting
Agreement shall survive the Expiration Date; provided, however that the
termination of this Voting Agreement shall not relieve any party from any
liability for any breach of this Voting Agreement.

         9.2 INDEMNIFICATION. Without in any way limiting any of the rights or
remedies otherwise available to Parent, Stockholder shall hold harmless and
indemnify Parent from and against, and shall compensate and reimburse Parent
for, any loss, damage, injury, decline in value, lost opportunity, liability,
exposure, claim, demand, settlement, judgment, award, fine, penalty, tax, fee,
charge, cost or expense of any nature (whether or not relating to a third party
claim) which is directly or indirectly suffered or incurred at any time by
Parent or any of Parent's affiliates or to which Parent or any of Parent's
affiliates otherwise becomes subject and that arises from any inaccuracy in or
breach of any representation, warranty, covenant or obligation of Stockholder
contained in this Voting Agreement.

         9.3 EXPENSES. All costs and expenses incurred in connection with the
transactions contemplated by this Voting Agreement shall be paid by the party
incurring such costs and expenses.

         9.4 NOTICES. Any notice or other communication required or permitted to
be delivered to any party under this Voting Agreement shall be in writing and
shall be deemed properly delivered, given and received (a) when delivered by
hand, or (b) one business day after sent by

                                       5
<PAGE>   6

internationally recognized courier or express delivery service (such as Federal
Express) or by facsimile (with confirmation of receipt), to the address or
facsimile telephone number set forth beneath the name of such party below (or to
such other address or facsimile telephone number as such party shall have
specified in a written notice given to the other parties hereto):

                  if to Stockholder:

                           at the address set forth below Stockholder's
                           signature on the signature page hereto;

                  if to Parent:

                           Clarent Corporation
                           700 Chesapeake Drive
                           Redwood City, CA 94063
                           Facsimile:  (650) 306-7512
                           Attention:  Chief Executive Officer

         9.5 SEVERABILITY. In the event that any term, provision, covenant or
restriction contained in this Voting Agreement, or the application of any such
term, provision, covenant or restriction to any Person or set of circumstances,
shall be determined to be invalid, unlawful, void or unenforceable to any
extent, the remainder of this Voting Agreement, and the application of such
term, provision, covenant or restriction to Persons or circumstances other than
those as to which it is determined to be invalid, unlawful, void or
unenforceable, shall not be impaired or otherwise affected and shall continue to
be valid and enforceable to the fullest extent permitted by law.

         9.6 ENTIRE AGREEMENT; COUNTERPARTS. This Voting Agreement, the Proxy,
the Reorganization Agreement and any Affiliate Agreement between Stockholder and
Parent constitute the entire agreement and supersede all prior agreements and
understandings, both written and oral, between the parties with respect to the
subject matter hereof and thereof. No addition to or modification of any
provision of this Voting Agreement shall be binding upon either party hereto
unless made in writing and signed by both parties hereto. This Voting Agreement
may be executed in several counterparts, each of which shall be deemed an
original and all of which shall constitute one and the same instrument.

         9.7 ASSIGNMENT; BINDING EFFECT. Except as provided herein, neither this
Voting Agreement nor any of the obligations hereunder of Stockholder shall be
assigned or delegated by Stockholder (whether by operation of law or otherwise)
without the prior written consent of Parent. Subject to the preceding sentence,
this Voting Agreement shall be binding upon Stockholder and Stockholder's heirs,
successors and assigns, and shall inure to the benefit of Parent and its
successors and assigns. Without limiting any of the restrictions set forth in
Section 2 or elsewhere in this Voting Agreement, this Voting Agreement shall be
binding upon any Person to whom any Subject Securities are transferred.
Notwithstanding anything contained in this Voting Agreement to the contrary,
nothing in this Voting Agreement, express or implied, is

                                       6
<PAGE>   7

intended to confer on any Person, other than Parent and its successors and
assigns, any rights or remedies of any nature.

         9.8 SPECIFIC PERFORMANCE. Stockholder agrees that in the event of any
breach or threatened breach by Stockholder of any material covenant, obligation
or other provision contained in this Voting Agreement, Parent shall be entitled
(in addition to any other remedy that may be available to Parent) to: (a) a
decree or order of specific performance or mandamus to enforce the observance
and performance of such covenant, obligation or other provision; and (b) an
injunction restraining such breach or threatened breach. Stockholder further
agrees that neither Parent nor any other Person shall be required to obtain,
furnish or post any bond or similar instrument in connection with or as a
condition to obtaining any remedy referred to in this Section 9.8, and
Stockholder irrevocably waives any right he or she may have to require the
obtaining, furnishing or posting of any such bond or similar instrument.

         9.9 OTHER AGREEMENTS. Nothing in this Voting Agreement shall limit any
of the rights or remedies of Parent or obligations of Stockholder under any
Affiliate Agreement or under any other agreement between Parent and Stockholder
or any certificate or instrument executed by Stockholder in favor of Parent; and
nothing in the Reorganization Agreement or in any other agreement, certificate
or instrument shall limit any of the rights or remedies of Parent or any of the
obligations of Stockholder under this Voting Agreement.

         9.10 APPLICABLE LAW; JURISDICTION. THIS VOTING AGREEMENT SHALL BE
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF
DELAWARE, REGARDLESS OF THE LAWS THAT MIGHT OTHERWISE GOVERN UNDER APPLICABLE
PRINCIPLES OF CONFLICTS OF LAWS THEREOF. In any action between any of the
parties arising out of or relating to this Voting Agreement or any of the
transactions contemplated by this Voting Agreement: (a) each of the parties
irrevocably and unconditionally consents and submits to the jurisdiction and
venue of the state and federal courts located in the State of Delaware; (b) if
any such action is commenced in any other state or federal court, then, subject
to applicable law, no party shall object to the removal of such action to any
federal court located in the State of Delaware; (c) each of the parties
irrevocably waives the right to trial by jury; and (d) the parties irrevocably
consent to service of process in the manner contemplated by Section 9.4.

         9.11 CONSTRUCTION.

                  (a) For purposes of this Voting Agreement, whenever the
context requires: the singular number shall include the plural, and vice versa;
the masculine gender shall include the feminine and neuter genders; the feminine
gender shall include the masculine and neuter genders; and the neuter gender
shall include masculine and feminine genders.

                  (b) The parties hereto agree that any rule of construction to
the effect that ambiguities are to be resolved against the drafting party shall
not be applied in the construction or interpretation of this Voting Agreement.

                                       7
<PAGE>   8

                  (c) As used in this Voting Agreement, the words "include" and
"including," and variations thereof, shall not be deemed to be terms of
limitation, but rather shall be deemed to be followed by the words "without
limitation."

                  (d) Except as otherwise indicated, all references in this
Voting Agreement to "Sections" and "Exhibits" are intended to refer to Sections
of this Voting Agreement and Exhibits to this Voting Agreement.

                  (e) The headings contained in this Voting Agreement are for
convenience of reference only, shall not be deemed to be a part of this Voting
Agreement and shall not be referred to in connection with the construction or
interpretation of this Voting Agreement.

                                       8
<PAGE>   9

         IN WITNESS WHEREOF, Parent and Stockholder have caused this Voting
Agreement to be executed as of the date first written above.

                                    CLARENT CORPORATION

                                    By:
                                       -----------------------------------------
                                    Name:  Richard J. Heaps
                                    Title: Chief Operating Officer and Chief
                                           Financial Officer

                                    --------------------------------------------
                                    Name:

                                    Address:
                                            ------------------------------------

                                    --------------------------------------------

                                    --------------------------------------------

                                    Facsimile:
                                              ----------------------------------

                                    Number of outstanding shares of Company
                                    Common Stock owned of record as of the date
                                    of this Voting Agreement:
                                                             -------------------

                                    Number of additional outstanding shares of
                                    Company Common Stock owned beneficially (but
                                    not of record) as of the date of this Voting
                                    Agreement:
                                              ----------------------------------

                                       9
<PAGE>   10

                                    Number of options and other rights to
                                    acquire shares of Company Common Stock owned
                                    of record as of the date of this Voting
                                    Agreement:____________________

                                    Number of additional options and other
                                    rights to acquire shares of Company Common
                                    Stock owned beneficially (but not of record)
                                    as of the date of this Voting
                                    Agreement:____________________

                                       10

<PAGE>   11

                                    EXHIBIT A

                                IRREVOCABLE PROXY

         The undersigned stockholder of ACT Networks, Inc., a Delaware
corporation (the "Company"), hereby irrevocably (to the fullest extent permitted
by law) appoints and constitutes Jerry Shaw-Yau Chang, Richard J. Heaps and
Clarent Corporation, a Delaware corporation ("Parent"), and each of them, the
attorneys and proxies of the undersigned with full power of substitution and
resubstitution, to the full extent of the undersigned's voting rights on the
matters referred to in the third paragraph of this proxy with respect to (i) the
issued and outstanding shares of capital stock of the Company owned of record by
the undersigned as of the date of this proxy, which shares are specified beneath
the undersigned's signature on the signature page of this proxy and (ii) any and
all other shares of capital stock of the Company which the undersigned may
acquire after the date hereof. (The shares of the capital stock of the Company
referred to in clauses "(i)" and "(ii)" of the immediately preceding sentence
are collectively referred to as the "Shares.") Upon the execution hereof, all
prior proxies given by the undersigned with respect to any of the Shares are
hereby revoked, and no subsequent proxies will be given with respect to any of
the Shares.

         This proxy is irrevocable, is coupled with an interest and is granted
in connection with the Voting Agreement, dated as of the date hereof, between
Parent and the undersigned (the "Voting Agreement"), and is granted in
consideration of Parent entering into the Agreement and Plan of Merger and
Reorganization, dated as of the date hereof, among Parent, Copper Acquisition
Sub, Inc., a Delaware corporation and wholly owned subsidiary of Parent, and the
Company (the "Reorganization Agreement"). Capitalized terms used but not
otherwise defined in this proxy have the meanings given to such terms in the
Reorganization Agreement.

         The attorneys and proxies named above will be empowered, and may
exercise this proxy, to vote the Shares at any time until the earlier of the
date upon which the Reorganization Agreement is validly terminated or the date
upon which the Effective Time occurs (the "Expiration Date") at any meeting of
the stockholders of the Company, however called, or in any action by written
consent of stockholders of the Company:

                           (i) in favor of the Merger and the adoption of the
         Reorganization Agreement and in favor of each of the other actions
         contemplated by the Reorganization Agreement and any action that could
         reasonably be expected to facilitate the consummation of the Merger;
         and

                           (ii) in their discretion, with respect to the
         following actions (other than the Merger and the transactions
         contemplated by the Reorganization Agreement): (A) any extraordinary
         corporate transaction, such as a merger, consolidation or other
         business combination involving any of the Acquired Companies; (B) any
         sale, lease or transfer of a material amount of assets of any of the
         Acquired Companies (other than in the ordinary course of business); (C)
         any reorganization, recapitalization, dissolution or liquidation of any
         of the Acquired Companies; (D) any removal of or change in a majority
         of the board of directors of the Company; (E) any amendment to the
         Company's certificate of

                                       1

<PAGE>   12

         incorporation; (F) any material change in the capitalization of the
         Company or the Company's corporate structure; or (G) any other action
         which is intended, or could reasonably be expected to, impede,
         interfere with, delay, postpone, discourage or adversely affect the
         Merger or any of the other transactions contemplated by the
         Reorganization Agreement or the Voting Agreement.

         The undersigned stockholder may vote the Shares on all matters other
than the matters referred to in the preceding paragraph.

         This proxy shall be binding upon the heirs, successors and assigns of
the undersigned (including any transferee of any of the Shares).

         In the event that any term, provision, covenant or restriction
contained in this proxy, or the application of any such term, provision,
covenant or restriction to any Person or set of circumstances, shall be
determined to be invalid, unlawful, void or unenforceable to any extent, the
remainder of this proxy, and the application of such term, provision, covenant
or restriction to Persons or circumstances other than those as to which it is
determined to be invalid, unlawful, void or unenforceable, shall not be impaired
or otherwise affected and shall continue to be valid and enforceable to the
fullest extent permitted by law..

         This proxy shall terminate upon the Expiration Date.

Dated:  May 1, 2000

                                    --------------------------------------------
                                    Name:

                                    Number of shares of Company Common Stock
                                    owned of record as of the date of this
                                    proxy:
                                           -------------------------------------

                                       2

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