<PAGE>

                           CERTIFICATE OF AMENDMENT
                                    OF THE
                             AMENDED AND RESTATED
                         CERTIFICATE OF INCORPORATION
                                      OF
                              CLARENT CORPORATION

  CLARENT CORPORATION, a corporation organized and existing under and by virtue
of the General Corporation Law of the state of Delaware, does hereby certify:

  FIRST: The name of the corporation is CLARENT CORPORATION.

  SECOND: The date on which the Certificate of Incorporation of the corporation
was filed with the Secretary of State of the State of Delaware was April 6,
1999.  A Certificate of Amendment to the Certificate of Incorporation was filed
with the Secretary of State of the State of Delaware on June 28, 1999.  An
Amended and Restated Certificate of Incorporation of the corporation was filed
with the Secretary of State of the State of Delaware on July 7, 1999.

  THIRD: The Board of Directors of the corporation, acting in accordance with
the provisions of Section 242 of the General Corporation Law of the State of
Delaware, adopted resolutions to amend the Amended and Restated Certificate of
Incorporation of the corporation by deleting the first paragraph of Article IV
and substituting therefor a new first paragraph of Article IV in the following
form:

           "This corporation is authorized to issue two classes of stock to be
        designated, respectively, `Common Stock' and `Preferred Stock.' The
        total number of shares which the corporation is authorized to issue is
        two hundred five million (205,000,000) shares. Two hundred million
        (200,000,000) shares shall be Common Stock, each having a par value of
        one-tenth of one cent ($.001). Five million (5,000,000) shares shall be
        Preferred Stock, each having a par value of one-tenth of one cent
        ($.001)."

  FOURTH: Thereafter, pursuant to a resolution of the Board of Directors, this
Certificate of Amendment was submitted to the stockholders of the corporation
for their approval and was duly adopted in accordance with the provision of
Section 242 of the General Corporation Law of the State of Delaware.

  IN WITNESS WHEREOF, Clarent Corporation has caused this Certificate of
Amendment to be signed by its President and Chief Executive Officer and attested
to by its Secretary this 28 day of February, 2000.

                                         CLARENT CORPORATION


                                         /s/ Jerry Shaw-Yau Chang
                                         -------------------------------------
                                         Jerry Shaw-Yau Chang
                                         President and Chief Executive Officer
ATTEST:


/s/ Richard J. Heaps
Richard J. Heaps
Secretary
<PAGE>

                              AMENDED AND RESTATED
                          CERTIFICATE OF INCORPORATION
                                       OF
                              CLARENT CORPORATION


  CLARENT CORPORATION, a corporation organized and existing under the laws of
the state of Delaware (the "Corporation") hereby certifies that:

  1.  The name of the Corporation is Clarent Corporation.

  2.  The date of filing of the Corporation's original Certificate of
Incorporation was April 6, 1999.

  3.  The Amended and Restated Certificate of Incorporation of the Corporation
as provided in Exhibit A hereto was duly adopted in accordance with the
provisions of Section 242 and Section 245 of the General Corporation Law of the
State of Delaware by the Board of Directors of the Corporation.

  4.  Pursuant to Section 245 of the Delaware General Corporation Law, approval
of the stockholders of the Corporation has been obtained.

  5.  The Amended and Restated Certificate of Incorporation so adopted reads in
full as set forth in Exhibit A attached hereto and is hereby incorporated by
reference.

  IN WITNESS WHEREOF, the undersigned has signed this certificate this 7 day of
July, 1999, and hereby affirms and acknowledges under penalty of perjury that
the filing of this Amended and Restated Certificate of Incorporation is the act
and deed of Clarent Corporation.

                                 CLARENT CORPORATION



                                 By /s/ Jerry Chang
                                    -----------------------
                                    Jerry Chang
                                    Chief Executive Officer
<PAGE>

                              AMENDED AND RESTATED                     Exhibit A
                          CERTIFICATE OF INCORPORATION
                                       OF
                              CLARENT CORPORATION


                                       I.

  The name of this corporation is Clarent Corporation.

                                      II.

  The address of the registered office of the corporation in the State of
Delaware is 9 Loockerman Street, City of Dover, County of Kent and the name of
the registered agent of the corporation in the State of Delaware at such address
is National Registered Agents, Inc.

                                      III.

  The purpose of this corporation is to engage in any lawful act or activity for
which a corporation may be organized under the General Corporation Law of the
State of Delaware.

                                      IV.

  This corporation is authorized to issue two classes of stock to be designated,
respectively, "Common Stock" and "Preferred Stock."  The total number of shares
which the corporation is authorized to issue is Fifty Five Million (55,000,000)
shares. Fifty Million (50,000,000) shares shall be Common Stock, each having a
par value of one tenth of one cent ($.001).  Five Million (5,000,000) shares
shall be Preferred Stock, each having a par value of one tenth of one cent
($.001).

  The Preferred Stock may be issued from time to time in one or more series.
The Board of Directors is hereby authorized, by filing a certificate (a
"Preferred Stock Designation") pursuant to the Delaware General Corporation Law,
to fix or alter from time to time the designation, powers, preferences and
rights of the shares of each such series and the qualifications, limitations or
restrictions of any wholly unissued series of Preferred Stock, and to establish
from time to time the number of shares constituting any such series or any of
them; and to increase or decrease the number of shares of any series subsequent
to the issuance of shares of that series, but not below the number of shares of
such series then outstanding.  In case the number of shares of any series shall
be decreased in accordance with the foregoing sentence, the shares constituting
such decrease shall resume the status that they had prior to the adoption of the
resolution originally fixing the number of shares of such series.

                                       V.

  A.  For the management of the business and for the conduct of the affairs of
the Corporation, and in further definition, limitation and regulation of the
powers of the Corporation, of its directors and of its stockholders or any class
thereof, as the case may be, it is further provided that:
<PAGE>

        (1) The management of the business and the conduct of the affairs of the
Corporation shall be vested in its Board of Directors. The number of directors
which shall constitute the whole Board of Directors shall be fixed exclusively
by one or more resolutions adopted by the Board of Directors.

        (2) Subject to the rights of the holders of any series of Preferred
Stock to elect additional directors under specified circumstances, and to any
restrictions or limitations of applicable law, following the closing of the
initial public offering pursuant to an effective registration statement under
the Securities Act of 1933, as amended, covering the offer and sale of Common
Stock to the public (the "Initial Public Offering"), the directors shall be
divided into three classes designated as Class I, Class II and Class III,
respectively. Directors shall be assigned to each class in accordance with a
resolution or resolutions adopted by the Board of Directors. At the first annual
meeting of stockholders following the closing of the Initial Public Offering,
the term of office of the Class I directors shall expire and Class I directors
shall be elected for a full term of three years. At the second annual meeting of
stockholders following the Closing of the Initial Public Offering, the term of
office of the Class II directors shall expire and Class II directors shall be
elected for a full term of three years. At the third annual meeting of
stockholders following the Closing of the Initial Public Offering, the term of
office of the Class III directors shall expire and Class III directors shall be
elected for a full term of three years. At each succeeding annual meeting of
stockholders, directors shall be elected for a full term of three years to
succeed the directors of the class whose terms expire at such annual meeting.

  Notwithstanding the foregoing provisions of this Article, each director shall
serve until his successor is duly elected and qualified or until his death,
resignation or removal.  No decrease in the number of directors constituting the
Board of Directors shall shorten the term of any incumbent director.

        (3) Subject to the rights of the holders of any series of Preferred
Stock, the Board of Directors or any individual director may be removed from
office at any time (i) with cause by the affirmative vote of the holders of a
majority of the voting power of all the then-outstanding shares of voting stock
of the Corporation, entitled to vote at an election of directors (the "Voting
Stock") or (ii) without cause by the affirmative vote of the holders of at least
sixty-six and two-thirds percent (66 2/3%) of the voting power of all the then-
outstanding shares of the Voting Stock.

        (4) Subject to the rights of the holders of any series of Preferred
Stock, any vacancies on the Board of Directors resulting from death,
resignation, disqualification, removal or other causes and any newly created
directorships resulting from any increase in the number of directors, shall,
unless the Board of Directors determines by resolution that any such vacancies
or newly created directorships shall be filled by the stockholders, except as
otherwise provided by law, be filled only by the affirmative vote of a majority
of the directors then in office, even though less than a quorum of the Board of
Directors, and not by the stockholders. Any director elected in accordance with
the preceding sentence shall hold office for the remainder of the full term of
the director for which the vacancy was created or occurred and until such
director's successor shall have been elected and qualified.
<PAGE>

        (5) In the event that Section 2115(a) of the California Corporations
Code is applicable to this corporation, then the following shall apply:

            (a) Every stockholder entitled to vote in any election of directors
of this corporation may cumulate such stockholder's votes and give one candidate
a number of votes equal to the number of directors to be elected multiplied by
the number of votes to which the stockholder's shares are otherwise entitled, or
distribute the stockholder's votes on the same principle among as many
candidates as such stockholder thinks fit;

            (b) No stockholder, however, may cumulate such stockholder's votes
for one or more candidates unless (i) the names of such candidates have been
properly placed in nomination, in accordance with the Bylaws of the corporation,
prior to the voting, (ii) the stockholder has given advance notice to the
corporation of the intention to cumulative votes pursuant to the Bylaws, and
(iii) the stockholder has given proper notice to the other stockholders at the
meeting, prior to voting, of such stockholder's intention to cumulate such
stockholder's votes; and

            (c) If any stockholder has given proper notice, all stockholders may
cumulate their votes for any candidates who have been properly placed in
nomination. The candidates receiving the highest number of votes of the shares
entitled to be voted for them up to the number of directors to be elected by
such shares shall be declared elected.

    B. (1) Subject to paragraph (h) of Section 43 of the Bylaws, the Bylaws may
be altered or amended or new Bylaws adopted by the affirmative vote of at least
sixty-six and two-thirds percent (66-2/3%) of the voting power of all of the
then-outstanding shares of the Voting Stock. The Board of Directors shall also
have the power to adopt, amend, or repeal Bylaws.

        (2) The directors of the Corporation need not be elected by written
ballot unless the Bylaws so provide.

        (3) No action shall be taken by the stockholders of the Corporation
except at an annual or special meeting of stockholders called in accordance with
the Bylaws and following the closing of the Initial Public Offering no action
shall be taken by the stockholders by written consent.

        (4) Advance notice of stockholder nominations for the election of
directors and of business to be brought by stockholders before any meeting of
the stockholders of the Corporation shall be given in the manner provided in the
Bylaws of the Corporation.

                                      VI.

   A.   A director of the Corporation shall not be personally liable to the
Corporation or its stockholders for monetary damages for any breach of fiduciary
duty as a director, except for liability (i) for any breach of the director's
duty of loyalty to the Corporation or its stockholders, (ii) for acts or
omissions not in good faith or which involve intentional misconduct or a knowing
violation of law, (iii) under Section 174 of the Delaware General Corporation
Law, or (iv) for any transaction from which the director derived an improper
personal benefit.  If the Delaware
<PAGE>

General Corporation Law is amended after approval by the stockholders of this
Article to authorize corporate action further eliminating or limiting the
personal liability of directors, then the liability of a director shall be
eliminated or limited to the fullest extent permitted by the Delaware General
Corporation Law, as so amended.

   B.  Any repeal or modification of this Article VI shall be prospective and
shall not affect the rights under this Article VI in effect at the time of the
alleged occurrence of any act or omission to act giving rise to liability or
indemnification.

                                      VII.

   A.  The Corporation reserves the right to amend, alter, change or repeal any
provision contained in this Certificate of Incorporation, in the manner now or
hereafter prescribed by statute, except as provided in paragraph B. of this
Article VII, and all rights conferred upon the stockholders herein are granted
subject to this reservation.

   B.  Notwithstanding any other provisions of this Certificate of Incorporation
or any provision of law which might otherwise permit a lesser vote or no vote,
but in addition to any affirmative vote of the holders of any particular class
or series of the Voting Stock required by law, this Certificate of Incorporation
or any Preferred Stock Designation, the affirmative vote of the holders of at
least sixty-six and two-thirds percent (66-2/3%) of the voting power of all of
the then-outstanding shares of the Voting Stock, voting together as a single
class, shall be required to alter, amend or repeal Articles V, VI and VII.