<PAGE>

                                                                   Exhibit 4.3

                          SECOND AMENDED AND RESTATED
                           INVESTOR RIGHTS AGREEMENT
                           -------------------------

     This Agreement, dated as of February 26, 1999, is entered into by and among
Sycamore Networks, Inc., a Delaware corporation (the "Company"), the persons and
entities listed on Schedule I hereto under the heading "Investors"
(individually, an "Investor", and collectively, the "Investors") and the persons
listed on Schedule II hereto under the heading "Founders" (individually, a
"Founder" and collectively, the "Founders").

                                 BACKGROUND
                                 ----------

     WHEREAS, the Company and the Investors have entered into a Series C
Preferred Stock Purchase Agreement of even date herewith (the "Purchase
Agreement");

     WHEREAS, the Company, the Investors and others are parties to an Amended
and Restated Investor Rights Agreement, dated as of December 3, 1998, (the
"Prior Agreement"), which Prior Agreement the parties thereto desire to amend
and restate in its entirety pursuant to the terms hereof; and

     WHEREAS, the parties hereto wish to provide for (i) the composition of the
Board of Directors of the Company, (ii) certain arrangements with respect to the
registration of shares of capital stock of the Company under the Securities Act
of 1933, and (iii) a right of first refusal with respect to the sale of any
securities of the Company;

     NOW, THEREFORE, in consideration of the mutual promises and covenants
contained in this Agreement, and the consummation of the sale and purchase of
the Series C Convertible Preferred Stock pursuant to the Series C Purchase
Agreement, and for other valuable consideration, receipt of which is hereby
acknowledged, the parties hereto agree as follows:


ARTICLE I.  DEFINITIONS

     As used in this Agreement, the following terms shall have the following
respective meanings:

          "Commission" means the United States Securities and Exchange
           ----------
Commission, or any other federal agency at the time administering the Securities
Act.
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          "Common Stock" means the common stock, $0.001 par value per share, of
           ------------
the Company.

          "Exchange Act" means the Securities Exchange Act of 1934, as amended,
           ------------
or any similar federal statute, and the rules and regulations of the Commission
issued under such Act, as they each may, from time to time, be in effect.

          "Initial Public Offering" means the sale of shares of Common Stock in
           -----------------------
a firm commitment underwritten public offering pursuant to a Registration
Statement at a price to the public of at least $12.00 per share (adjusted for
stock splits, stock dividends and similar events) resulting in proceeds (net of
the underwriting discounts or commissions and offering expenses) to the Company
of at least $10,000,000.

          "Registration Statement" means a registration statement filed by the
           ----------------------
Company with the Commission for a public offering and sale of Common Stock by
the Company (other than a registration statement on Form S-8 or Form S-4, or
their successors, or any other form for a similar limited purpose, or any
registration statement covering only securities proposed to be issued in
exchange for securities or assets of another corporation).

          "Registration Expenses" means the expenses described in Section 4 of
           ---------------------
Article III below.

          "Registrable Shares" means (i) the shares of Common Stock issued or
           ------------------
issuable upon conversion of the Shares, (ii) shares of Common Stock held by
Gururaj Deshpande, Richard Barry, Daniel Smith and Chikong Shue, (iii) any
shares of Common Stock, and any shares of Common Stock issued or issuable upon
the conversion or exercise of any other securities, acquired by the Investors
pursuant to Article IV of this Agreement or pursuant to the Second Amended and
Restated Right of First Refusal and Co-Sale Agreement of even date herewith, as
amended, among the Company, the Investors and others, and (iv) any other shares
of Common Stock issued in respect of such shares (because of stock splits, stock
dividends, reclassifications, recapitalizations, or similar events); provided,
                                                                     --------
however, that shares of Common Stock which are Registrable Shares shall cease to
-------
be Registrable Shares (a) upon any sale of such shares pursuant to a
Registration Statement or Rule 144 under the Securities Act, (b) upon any sale
of such shares in any manner to a person or entity which, by virtue of Section 2
of Article V of this Agreement, is not entitled to the rights provided by this
Agreement, or (c) for purposes of Section 2 of Article III hereof, following the
third anniversary of the Initial Public Offering.  Wherever reference is made in
this Agreement to a request or consent of holders of a certain percentage of
Registrable Shares, the determination of such percentage shall include shares of
Common Stock issuable upon conversion of the Shares even if such conversion has
not yet been effected.

                                      -2-
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          "Securities Act" means the Securities Act of 1933, as amended, or any
           --------------
similar federal statute, and the rules and regulations of the Commission issued
under such Act, as they each may, from time to time, be in effect.

          "Shares" means the shares of Series A Convertible Preferred Stock,
           ------
Series B Convertible Preferred Stock and Series C Convertible Preferred Stock
held by Stockholders.

          "Stockholders" means the Investors, the Founders, and any persons or
           ------------
entities to whom the rights granted to Investors or Founders under this
Agreement are transferred by an Investor or Founder, or their successors or
permitted assigns, pursuant to Section 2 of Article V below.

ARTICLE II.    ELECTION OF DIRECTORS

     1.  Voting of Shares.  In any and all elections of directors of the Company
         ----------------
(whether at a meeting or by written consent in lieu of a meeting), each
Stockholder shall vote or cause to be voted all Voting Shares (as defined in
Section 2 of Article II below) owned by him, her or it, or over which he, she or
it has voting control, and otherwise use his, her or its respective best
efforts, so as to fix the number of directors at five and to elect as directors
(i) Gururaj Deshpande, (ii) two representatives designated by Matrix Partners V,
L.P. (initially Paul J. Ferri and Timothy Barrows), (iii) the Chief Executive
Officer of the Company, and (iv) one person mutually agreed upon by all of the
other members of the Board of Directors.  The obligation of the Stockholders
under this Section 1 to elect as a director designees of Matrix Partners V, L.P.
shall continue only for so long as Matrix Partners V, L.P. (together with any
affiliates, within the meaning of Rule 144 under the Securities Act) owns, after
giving effect to the conversion of all convertible preferred stock into Common
Stock, at least 1,375,000 shares of Common Stock (subject to appropriate
adjustment for stock splits, stock dividends, combinations and other similar
recapitalizations affecting such shares).

     2.  Voting Shares.  "Voting Shares" shall mean and include any and all
         -------------
shares of the Common Stock, Shares, and/or shares of capital stock of the
Company, by whatever name called, which carry voting rights (including voting
rights which arise by reason of default).

     3.  Restrictive Legend.  All certificates representing Voting Shares owned
         ------------------
or hereafter acquired by the Stockholders or any transferee bound by this
Agreement shall have affixed thereto a legend substantially in the following
form:

     "The shares of stock represented by this certificate are subject to certain
     voting agreements as set forth in a Second Amended and Restated Investor
     Rights Agreement by and among the registered owner of this

                                      -3-
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     certificate, the Company and certain other stockholders of the Company, a
     copy of which is available for inspection at the offices of the Secretary
     of the Company."

     4.   Transfers of Voting Rights.  Any transferee to whom Voting Shares are
          --------------------------
transferred by a Stockholder, whether voluntarily or by operation of law, shall
be bound by the voting obligations imposed upon the transferor under this
Agreement, to the same extent as if such transferee were a Stockholder
hereunder.


ARTICLE III.  REGISTRATION RIGHTS

     1.   Required Registrations.
          ----------------------

          (a) At any time after the earlier of February 26, 2003 or 180 days
after the closing of the Company's first underwritten public offering of shares
of Common Stock pursuant to a Registration Statement, Stockholders (other than
the Founders) holding in the aggregate at least 35% of the Registrable Shares
held by the Stockholders (other than the Founders) may request, in writing, that
the Company effect the registration on Form S-1 or Form S-2 (or any successor
form) of Registrable Shares owned by such Stockholders having an aggregate
offering price of at least $5,000,000 (based on the market price or fair value
at the time of such request).  If the Stockholders initiating the registration
intend to distribute the Registrable Shares by means of an underwriting, they
shall so advise the Company in their request.  Thereupon, the Company shall, as
expeditiously as possible, use its best efforts to effect the registration on
Form S-1 or Form S-2 (or any successor form) of all Registrable Shares which the
Company has been requested to so register.

          (b) At any time after the Company becomes eligible to file a
Registration Statement on Form S-3 (or any successor form relating to secondary
offerings), a Stockholder or Stockholders may request the Company, in writing,
to effect the registration on Form S-3 (or such successor form), of Registrable
Shares having an aggregate offering price of at least $1,000,000 (based on the
public market price at the time of such request).  Thereupon, the Company shall,
as expeditiously as possible, use its best efforts to effect the registration on
Form S-3 (or such successor form) of all Registrable Shares which the Company
has been requested to so register.

          (c) The Company shall not be required to effect more than two
registrations pursuant to paragraph (a) above or more than three registrations
pursuant to paragraph (b) above; provided, however, that such obligation shall
                                 --------  -------
be deemed satisfied only when a registration statement covering the applicable
Registrable Shares shall have (i) become effective or (ii) been withdrawn at the
request of the Stockholders requesting such registration (other than as a result
of information concerning the business or financial condition of the Company
which is

                                      -4-
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made known to the Stockholders after the date on which such registration was
requested).

          (d)  If at the time of any request to register Registrable Shares
pursuant to this Section 1, the Company is engaged or has plans to engage within
90 days of the time of the request in a registered public offering of securities
for its own account or is engaged in any other activity which, in the good faith
determination of the Company's Board of Directors, would be adversely affected
by the requested registration to the material detriment of the Company, then the
Company may at its option direct that such request be delayed for a period not
in excess of three months from the effective date of such offering or the date
of commencement of such other material activity, as the case may be, such right
to delay a request to be exercised by the Company not more than once in any 12-
month period.

     2.   Incidental Registration.
          -----------------------

          (a)  Whenever the Company proposes to file a Registration Statement at
any time and from time to time, it will, prior to such filing, give written
notice to all Stockholders of its intention to do so and, upon the written
request of a Stockholder or Stockholders, given within 10 business days after
the Company provides such notice (which request shall state the intended method
of disposition of such Registrable Shares), the Company shall use its reasonable
best efforts to cause all Registrable Shares which the Company has been
requested by such Stockholder or Stockholders to register, to be registered
under the Securities Act to the extent necessary to permit their sale or other
disposition in accordance with the intended methods of distribution specified in
the request of such Stockholder or Stockholders; provided, however, that the
                                                 --------  -------
Company shall have the right to postpone or withdraw any registration effected
pursuant to this Section 2 without obligation to any Stockholder.

          (b)  In connection with any registration under this Section 2
involving an underwriting, the Company shall not be required to include any
Registrable Shares in such registration unless the holders thereof accept the
terms of the underwriting as agreed upon between the Company and the
underwriters selected by it.  If in the opinion of the managing underwriter it
is desirable because of marketing factors to limit the number of Registrable
Shares to be included in the offering, then the Company shall be required to
include in the registration only that number of Registrable Shares, if any,
which the managing underwriter believes should be included therein; provided,
                                                                    --------
however, that no persons or entities other than the Company, the Stockholders
-------
and other persons or entities holding registration rights shall be permitted to
include securities in the offering.  If the number of Registrable Shares to be
included in the offering in accordance with the foregoing is less than the total
number of shares which the holders of Registrable Shares have requested to be
included, then the holders of Registrable Shares who have requested registration
and

                                      -5-
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other holders of securities entitled to include them in such registration shall
participate in the registration pro rata based upon their total ownership of
shares of Common Stock (giving effect to the conversion into Common Stock of all
securities convertible thereinto). If any holder would thus be entitled to
include more securities than such holder requested to be registered, the excess
shall be allocated among other requesting holders pro rata in the manner
described in the preceding sentence.

     3.   Registration Procedures.  If and whenever the Company is required by
          -----------------------
the provisions of this Agreement to use its best efforts to effect the
registration of any of the Registrable Shares under the Securities Act, the
Company shall:

          (a)  file with the Commission a Registration Statement with respect to
such Registrable Shares and use its best efforts to cause that Registration
Statement to become effective;

          (b)  as expeditiously as possible prepare and file with the Commission
any amendments and supplements to the Registration Statement and the prospectus
included in the Registration Statement as may be necessary to keep the
Registration Statement effective, in the case of a firm commitment underwritten
public offering, until each underwriter has completed the distribution of all
securities purchased by it and, in the case of any other offering, until the
earlier of the sale of all Registrable Shares covered thereby or 180 days after
the effective date thereof;

          (c)  as expeditiously as possible furnish to each selling Stockholder
such reasonable numbers of copies of the prospectus, including a preliminary
prospectus, in conformity with the requirements of the Securities Act, and such
other documents as the selling Stockholder may reasonably request in order to
facilitate the public sale or other disposition of the Registrable Shares owned
by the selling Stockholder; and

          (d)  as expeditiously as possible use its best efforts to register or
qualify the Registrable Shares covered by the Registration Statement under the
securities or Blue Sky laws of such states as the selling Stockholder shall
reasonably request, and do any and all other acts and things that may be
necessary or desirable to enable the selling Stockholder to consummate the
public sale or other disposition in such states of the Registrable Shares owned
by the selling Stockholder; provided, however, that the Company shall not be
                            --------  -------
required in connection with this paragraph (d) to qualify as a foreign
corporation or execute a general consent to service of process in any
jurisdiction.

     If the Company has delivered preliminary or final prospectuses to the
selling Stockholders and after having done so the prospectus is amended to
comply with the requirements of the Securities Act, the Company shall promptly
notify the selling Stockholders and, if requested, the selling Stockholder shall
immediately cease

                                      -6-
<PAGE>

making offers of Registrable Shares and return all prospectuses to the Company.
The Company shall promptly provide each selling Stockholder with revised
prospectuses and, following receipt of the revised prospectuses, the selling
Stockholder shall be free to resume making offers of the Registrable Shares.

     If, after a registration statement becomes effective, the Company becomes
engaged in any activity which, in the good faith determination of the Company's
Board of Directors, involves information that would have to be disclosed in the
Registration Statement but which the Company desires to keep confidential for
valid business reasons, then the Company may at its option, by notice to such
Stockholders, require that the Stockholders who have included Shares in such
Registration Statement cease sales of such Shares under such Registration
Statement for a period not in excess of three months from the date of such
notice, such right to be exercised by the Company not more than once in any 12-
month period.  If, in connection therewith, the Company considers it appropriate
for such Registration Statement to be amended, the Company shall so amend such
Registration Statement as promptly as practicable and such Stockholders shall
suspend any further sales of their Shares until the Company advises them that
such Registration Statement has been amended.  The time periods referred to
herein during which such Registration Statement must be kept effective shall be
extended for an additional number of days equal to the number of days during
which the right to sell shares was suspended pursuant to this paragraph.

     4.   Allocation of Expenses.  The Company will pay all Registration
          ----------------------
Expenses of all registrations under this Agreement.  For purposes of this
Section 4, the term "Registration Expenses" shall mean all expenses incurred by
the Company in complying with this Article III, including, without limitation,
all registration and filing fees, exchange listing fees, printing expenses, fees
and expenses of counsel for the Company to represent the selling Stockholder(s),
state Blue Sky fees and expenses, and the expense of any special audits incident
to or required by any such registration, but excluding underwriting discounts,
selling commissions and the fees and expenses of selling Stockholders' own
counsel.

     5.   Indemnification and Contribution.
          --------------------------------

          (a)  In the event of any registration of any of the Registrable Shares
under the Securities Act pursuant to this Agreement, the Company will indemnify
and hold harmless the seller of such Registrable Shares, each of such sellers',
directors and officers, each underwriter of such Registrable Shares, and each
other person, if any, who controls such seller or underwriter within the meaning
of the Securities Act or the Exchange Act against any losses, claims, damages or
liabilities, joint or several, to which such seller, underwriter or controlling
person may become subject under the Securities Act, the Exchange Act, state
securities or Blue Sky laws or otherwise, insofar as such losses, claims,
damages or liabilities (or actions in respect thereof)

                                      -7-
<PAGE>

arise out of or are based upon any untrue statement or alleged untrue statement
of any material fact contained in any Registration Statement under which such
Registrable Shares were registered under the Securities Act, any preliminary
prospectus or final prospectus contained in the Registration Statement, or any
amendment or supplement to such Registration Statement, or arise out of or are
based upon the omission or alleged omission to state a material fact required to
be stated therein or necessary to make the statements therein not misleading;
and the Company will reimburse such seller, underwriter and each such
controlling person for any legal or any other expenses reasonably incurred by
such seller, underwriter or controlling person in connection with investigating
or defending any such loss, claim, damage, liability or action; provided,
                                                                --------
however, that the Company will not be liable in any such case to a seller,
-------
underwriter or controlling person to the extent that any such loss, claim,
damage or liability arises out of or is based upon any untrue statement or
omission made in such Registration Statement, preliminary prospectus or final
prospectus, or any such amendment or supplement, in reliance upon and in
conformity with information furnished to the Company, in writing, by or on
behalf of such seller, underwriter or controlling person specifically for use in
the preparation thereof.

          (b)  In the event of any registration of any of the Registrable Shares
under the Securities Act pursuant to this Agreement, each seller of Registrable
Shares, severally and not jointly, will indemnify and hold harmless the Company,
each of its directors and officers and each underwriter (if any) and each
person, if any, who controls the Company or any such underwriter within the
meaning of the Securities Act or the Exchange Act, against any losses, claims,
damages or liabilities, joint or several, to which the Company, such directors
and officers, underwriter or controlling person may become subject under the
Securities Act, Exchange Act, state securities or Blue Sky laws or otherwise,
insofar as such losses, claims, damages or liabilities (or actions in respect
thereof) arise out of or are based upon any untrue statement or alleged untrue
statement of a material fact contained in any Registration Statement under which
such Registrable Shares were registered under the Securities Act, any
preliminary prospectus or final prospectus contained in the Registration
Statement, or any amendment or supplement to the Registration Statement, or
arise out of or are based upon any omission or alleged omission to state a
material fact required to be stated therein or necessary to make the statements
therein not misleading, if the statement or omission was made in reliance upon
and in conformity with information relating to such seller furnished in writing
to the Company by or on behalf of such seller specifically for use in connection
with the preparation of such Registration Statement, prospectus, amendment or
supplement; provided, however, that the obligations of each such Stockholder
            --------  -------
hereunder shall be limited to an amount equal to the net proceeds to such
Stockholder of Registrable Shares sold in connection with such registration.

                                      -8-
<PAGE>

          (c)  Each party entitled to indemnification under this Article III,
Section 5 (the "Indemnified Party") shall give notice to the party required to
provide indemnification (the "Indemnifying Party") promptly after such
Indemnified Party has actual knowledge of any claim as to which indemnity may be
sought, and shall permit the Indemnifying Party to assume the defense of any
such claim or any litigation resulting therefrom; provided, that counsel for the
                                                  --------
Indemnifying Party, who shall conduct the defense of such claim or litigation,
shall be approved by the Indemnified Party (whose approval shall not be
unreasonably withheld); and, provided further, that the failure of any
                             ----------------
Indemnified Party to give notice as provided herein shall not relieve the
Indemnifying Party of its obligations under this Article III, Section 5, unless
and except to the extent that the Indemnifying Party is prejudiced by the
failure of the Indemnified Party to provide timely notice.  The Indemnified
Party may participate in such defense at such party's expense; provided,
                                                               --------
however, that the Indemnifying Party shall pay such expense if representation of
-------
such Indemnified Party by the counsel retained by the Indemnifying Party would
be inappropriate due to actual or potential differing interests between the
Indemnified Party and any other party represented by such counsel in such
proceeding.  No Indemnifying Party, in the defense of any such claim or
litigation shall, except with the consent of each Indemnified Party, consent to
entry of any judgment or enter into any settlement which does not include as an
unconditional term thereof the giving by the claimant or plaintiff to such
Indemnified Party of a release from all liability in respect of such claim or
litigation, and no Indemnified Party shall consent to entry of any judgment or
settle such claim or litigation without the prior written consent of the
Indemnifying Party.

          (d)  In order to provide for just and equitable contribution to joint
liability under the Securities Act in any case in which either (i) any holder of
Registrable Shares exercising rights under this Agreement, or any controlling
person of any such holder, makes a claim for indemnification pursuant to this
Article III, Section 5 but it is judicially determined (by the entry of a final
judgment or decree by a court of competent jurisdiction and the expiration of
time to appeal or the denial of the last right of appeal) that such
indemnification may not be enforced in such case notwithstanding the fact that
this Article III, Section 5 provides for indemnification in such case, or (ii)
contribution under the Securities Act may be required on the part of any such
selling Stockholder or any such controlling person in circumstances for which
indemnification is provided under this Article III, Section 5; then, in each
such case, the Company and such Stockholder will contribute to the aggregate
losses, claims, damages or liabilities to which they may be subject (after
contribution from others) in such proportions so that such holder is responsible
for the portion represented by the percentage that the public offering price of
its Registrable Shares offered by the Registration Statement bears to the public
offering price of all securities offered by such Registration Statement, and the
Company is responsible for the remaining portion; provided, however, that, in
                                                  --------  -------
any such case, (A) no such holder will be required to contribute any amount in
excess of the net proceeds to it of all

                                      -9-
<PAGE>

Registrable Shares sold by it pursuant to such Registration Statement, and (B)
no person or entity guilty of fraudulent misrepresentation, within the meaning
of Section 11(f) of the Securities Act, shall be entitled to contribution from
any person or entity who is not guilty of such fraudulent misrepresentation.

     6.   Indemnification with Respect to Underwritten Offering.  In the event
          -----------------------------------------------------
that Registrable Shares are sold pursuant to a Registration Statement in an
underwritten offering, the Company agrees to enter into an underwriting
agreement containing customary representations and warranties with respect to
the business and operations of an issuer of the securities being registered and
customary covenants and agreements to be performed by such issuer, including
without limitation customary provisions with respect to indemnification by the
Company of the underwriters of such offering.

     7.   Information by Holder.  Each Stockholder including Registrable Shares
          ---------------------
in any registration shall furnish to the Company such information regarding such
Stockholder and the distribution proposed by such Stockholder as the Company may
reasonably request in writing and as shall be required in connection with any
registration, qualification or compliance referred to in this Agreement.

     8.   "Stand-Off" Agreement.  Each Stockholder, if requested by the Company
           --------------------
and the managing underwriter of an offering by the Company of Common Stock or
other securities of the Company pursuant to a Registration Statement, shall
agree not to sell publicly or otherwise transfer or dispose of any Registrable
Shares of the Company held by such Stockholder for a specified period of time
(not to exceed 180 days) following the effective date of such Registration
Statement; provided, that:
           --------

          (a) such agreement shall only apply to the first Registration
Statement covering Common Stock to be sold by or on behalf of the Company to the
public in an underwritten offering; and

          (b) all officers and directors of the Company and all selling
stockholders in such offering enter into similar agreements.

     9.   Limitations on Subsequent Registration Rights.  The Company shall not,
          ---------------------------------------------
without the prior written consent of Investors holding a majority of the
Registrable Shares held by all Investors, enter into any agreement (other than
this Agreement) with any holder or prospective holder of any securities of the
Company which would allow such holder or prospective holder (a) to include
securities of the Company in any Registration Statement upon terms which are
more favorable to such holder or prospective holder than the terms on which
holders of Registrable Shares may include shares in such registration, or (b) to
make a demand registration which could result in such registration statement
being declared effective prior to February 26, 2003.

                                      -10-
<PAGE>

     10.  Rule 144 Requirements.  After the earliest of (a) the closing of the
          ---------------------
sale of securities of the Company pursuant to a Registration Statement, (b) the
registration by the Company of a class of securities under Section 12 of the
Exchange Act, or (c) the issuance by the Company of an offering circular
pursuant to Regulation A under the Securities Act, the Company agrees to:

          (i)   comply with the requirements of Rule 144(c) under the Securities
Act with respect to current public information about the Company;

          (ii)  use its best efforts to file with the Commission in a timely
manner all reports and other documents required of the Company under the
Securities Act and the Exchange Act (at any time after it has become subject to
such reporting requirements); and

          (iii) furnish to any holder of Registrable Shares upon request (A) a
written statement by the Company as to its compliance with the requirements of
said Rule 144(c), and the reporting requirements of the Securities Act and the
Exchange Act (at any time after it has become subject to such reporting
requirements), (B) a copy of the most recent annual or quarterly report of the
Company, and (C) such other reports and documents of the Company as such holder
may reasonably request to avail itself of any similar rule or regulation of the
Commission allowing it to sell any such securities without registration.

ARTICLE IV.  RIGHT OF FIRST REFUSAL

     1.   Right of First Refusal
          ----------------------

          (a)   The Company shall not issue, sell or exchange, agree to issue,
sell or exchange, or reserve or set aside for issuance, sale or exchange, (i)
any shares of its Common Stock, (ii) any other equity securities of the Company,
including, without limitation, shares of preferred stock, (iii) any option,
warrant or other right to subscribe for, purchase or otherwise acquire any
equity securities of the Company, or (iv) any debt securities convertible into
capital stock of the Company (collectively, the "Offered Securities"), unless in
each such case the Company shall have first complied with Article IV of this
Agreement.  The Company shall deliver to each Investor a written notice of any
proposed or intended issuance, sale or exchange of Offered Securities (the
"Offer"), which Offer shall (i) identify and describe the Offered Securities,
(ii) describe the price and other terms upon which they are to be issued, sold
or exchanged, and the number or amount of the Offered Securities to be issued,
sold or exchanged, (iii) identify the persons or entities, if known, to which or
with which the Offered Securities are to be offered, issued, sold or exchanged,
and (iv) offer to issue and sell to or exchange with such Investor such portion
of the Offered Securities as is equal to the number of Offered Securities
multiplied by a fraction, the numerator of which is the aggregate number of
shares of Common Stock issued or

                                      -11-
<PAGE>

issuable upon conversion of the Shares held by such Investor and the denominator
of which is the total number of shares of Common Stock then outstanding (giving
effect to the assumed conversion of all outstanding shares of convertible
preferred stock) (the "Pro Rata Share"). Each Investor shall have the right, for
a period of 20 days following delivery of the Offer, to purchase or acquire, at
the price and upon the other terms specified in the Offer, the number or amount
of Offered Securities described above. The Offer by its term shall remain open
and irrevocable for such 20-day period.

          (b) To accept an Offer, in whole or in part, an Investor must deliver
a written notice to the Company prior to the end of the 20-day period of the
Offer, setting forth the portion of such Investor's Pro Rata Share that such
Investor elects to purchase (a "Notice of Acceptance").

          (c) The Company shall have 90 days from the expiration of the 20-day
period set forth in Section 1(a) to issue, sell or exchange all or any part of
such Offered Securities as to which a Notice of Acceptance has not been given by
the Investors (the "Available Securities"), but only to the offerees or
purchasers described in the Offer and only upon terms and conditions which are
not more favorable, in the aggregate, to the acquiring person or persons or less
favorable to the Company than those set forth in the Offer.

          (d) In the event the Company shall propose to sell less than all the
Available Securities (any such sale to be in the manner and on the terms
specified in Section 1(c)), then each Investor may, at its sole option and in
its sole discretion, reduce the number or amount of the Offered Securities
specified in its Notice of Acceptance to an amount that shall be not less than
the number or amount of the Offered Securities that the Investor elected to
purchase pursuant to Section 1(b) multiplied by a fraction, (i) the numerator of
which shall be the number or amount of Offered Securities the Company actually
proposes to issue, sell or exchange (including Offered Securities to be issued
or sold to Investors pursuant to Article IV, Section 1(b) prior to such
reduction) and (ii) the denominator of which shall be the amount of all Offered
Securities.  In the event that an Investor so elects to reduce the number or
amount of Offered Securities specified in its Notice of Acceptance, the Company
may not issue, sell or exchange more than the reduced number or amount of the
Offered Securities unless and until such securities have again been offered to
the Investors in accordance with Section 1(a).

          (e) Upon the closing of the issuance, sale or exchange of all or less
than all the Available Securities, the Investors shall acquire from the Company,
and the Company shall issue to the Investors, the number or amount of Offered
Securities specified in the Notices of Acceptance, as reduced pursuant to
Section 1(d) if the Investors have so elected, upon the terms and conditions
specified in the Offer.  The purchase by the Investors of any Offered Securities
is subject in all cases to the

                                      -12-
<PAGE>

preparation, execution and delivery by the Company and the Investors of a
purchase agreement relating to such Offered Securities reasonably satisfactory
in form and substance to the Investors and the Company.

         (f) Any Offered Securities not acquired by the Investors or other
persons in accordance with Section 1(c) may not be issued, sold or exchanged
until they are again offered to the Investors under the procedures specified in
this Article.

     2.  Excluded Issuances.  The rights of the Investors under this Article IV
         ------------------
shall not apply to:

         (a) Common Stock issued as a stock dividend to holders of Common Stock
or upon any subdivision or combination of shares of Common Stock;

         (b) the issuance of any shares of Common Stock upon conversion of
outstanding shares of convertible preferred stock;

         (c) shares of Series C Preferred Stock issued at Additional Closings,
as defined in the Purchase Agreement;

         (d) up to 5,995,604 shares of Common Stock, either issued in the form
of restricted stock awards or options exercisable for Common Stock (subject to
appropriate adjustment for stock split, stock dividends, combinations and other
similar recapitalizations affecting such shares), plus such additional number of
shares as may be approved by a majority of the non-employee directors of the
Company, issued or issuable to officers, directors, consultants and employees of
the Company or any subsidiary pursuant to any plan, agreement or arrangement
approved by the Board of Directors of the Company;

         (e) securities issued solely in consideration for the acquisition
(whether by merger or otherwise) by the Company or any of its subsidiaries of
all or substantially all of the stock or assets of any other entity;

         (f) shares of Common Stock sold by the Company in an underwritten
public offering pursuant to an effective registration statement under the
Securities Act; and

         (g) securities issued to equipment lessors, as approved by a majority
of non-employee directors of the Company.

                                      -13-
<PAGE>

ARTICLE V. GENERAL

     1.    Termination.  Article II and Article IV of this Agreement shall
           -----------
terminate in their entirety upon the earlier of (a) an Acquisition (as defined
below), or (b) the closing of an Initial Public Offering, or (c) the redemption
of all Shares.  An "Acquisition" shall mean any (i) merger or consolidation
which results in the voting securities of the Company outstanding immediately
prior thereto representing immediately thereafter (either by remaining
outstanding or by being converted into voting securities of the surviving or
acquiring entity) less than a majority of the combined voting power of the
voting securities of the Company or such surviving or acquiring entity
outstanding immediately after such merger or consolidation, (ii) sale of all or
substantially all the assets of the Company or (iii) sale of shares of capital
stock of the Company, in a single transaction or series of related transactions,
representing at least 80% of the voting power of the voting securities of the
Company.

     2.    Transfer of Rights.  This Agreement, and the rights and obligations
           ------------------
of an Investor hereunder, may be assigned by such Investor to any person or
entity to which at least 100,000 Shares, as adjusted for stock splits, stock
dividends, recapitalizations and similar events (or 100% of the Shares
originally purchased hereunder by such Investor, if less than 100,000 Shares),
are transferred by such Investor, and such transferee shall be deemed an
"Investor" for purposes of this Agreement; provided that the transferee provides
                                           --------
written notice of such assignment to the Company and agrees to be bound by the
terms and conditions set forth herein. The rights and obligations of a Founder
under Article II hereunder may be assigned by said Founder to any spouse, child,
grandchild or trust for his or her benefit and such transferee shall be deemed a
"Founder" for purposes of Article II, provided that the transferee provides
                                      --------
written notice of such assignment to the Company and agrees to be bound by the
terms and conditions set forth herein.

     3.    Severability.  The provisions of this Agreement are severable, so
           ------------
that the invalidity or unenforceability of any provision of this Agreement shall
not affect the validity or enforceability of any other term or provision of this
Agreement, which shall remain in full force and effect.

     4.    Specific Performance.  In addition to any and all other remedies that
           --------------------
may be available at law in the event of any breach of this Agreement, the
Investors and the Founders shall be entitled to specific performance of the
agreements and obligations of the other parties hereunder and to such other
injunctive or other equitable relief as may be granted by a court of competent
jurisdiction.

     5.    Governing Law.  This Agreement shall be governed by, and construed
           -------------
and enforced in accordance with, the laws of the State of Delaware (without
reference to the conflicts of law provisions thereof).

                                      -14-
<PAGE>

     6.  Notices.  All notices, requests, consents, and other communications
         -------
under this Agreement shall be in writing and shall be delivered by hand, sent
via a reputable nationwide overnight courier service or mailed by first class
certified or registered mail, return receipt requested, postage prepaid:

     If to the Company, at Sycamore Networks, Inc., 10 Elizabeth Drive,
Chelmsford, MA 01824, Attn: President, or at such other address or addresses as
may have been furnished in writing by the Company to the Purchasers, with a copy
to Hale and Dorr LLP, 60 State Street, Boston, MA 02109, Attn:  Mark G. Borden,
Esq.;

     If to an Investor, at its or his address as set forth on the signature page
hereto, or at such other address or addresses as may have been furnished in
writing by such Investor to the Company.

     If to a Founder, at his address as set forth on the signature page hereto,
or at such other address or addresses as may have been furnished by such Founder
to the Company and the Investors.

     Notices provided in accordance with this Section 6 shall be deemed
delivered upon personal delivery, one business day after being sent via a
reputable nationwide overnight courier service, or three business days after
deposit in the mail.

     7.  Complete Agreement; Amendments.
         ------------------------------

         (a)  This Agreement constitutes the full and complete agreement of the
parties hereto with respect to the subject matter hereof.

         (b)  This Agreement may be amended at any time by a written instrument
signed by the Company and Investors holding at least a majority of the shares of
Common Stock issued or issuable upon conversion of the Shares, provided that no
                                                               --------
consent shall be required for an amendment pursuant to Section 11 below and
provided further that Section 8 of Article III shall not be amended to include
-------- -------
any securities of the Company other than the Registrable Shares without the
consent of each Investor who would be adversely affected by such amendment.  No
waivers of or exceptions to any term, condition or provision of this Agreement,
in any one or more instances, shall be deemed to be, or construed as, a further
or continuing waiver of any such term, condition or provision.

         (c)  The Prior Agreement is hereby amended and restated and superseded
in all respects by this Agreement.  Each of the Investors waives any right it
may have had under Article IV of the Prior Agreement with respect to the
issuance and sale by Company of the shares of Series C Convertible Preferred
Stock pursuant to the Purchase Agreement.

                                      -15-
<PAGE>

     8.  Pronouns.  Whenever the content may require, any pronouns used in this
         --------
Agreement shall include the corresponding masculine, feminine or neuter forms,
and the singular form of nouns and pronouns shall include the plural, and vice
versa.

     9.  Counterparts.  This Agreement may be executed in any number of
         ------------
counterparts, each of which shall be deemed to be an original, and all of which
together shall constitute one Agreement binding on all the parties hereto.

     10. Captions.  Captions of sections have been added only for convenience
         --------
and shall not be deemed to be a part of this Agreement.

     11. Addition of Purchasers.  Each purchaser of shares of Series C
         ----------------------
Preferred Stock of the Company under Section 2.2 of the Purchase Agreement shall
become a party to and an Investor under this Agreement upon the closing of its
purchase of shares of Series C Preferred Stock thereunder and its execution of a
counterpart signature page to this Agreement.

                                      -16-
<PAGE>

     IN WITNESS WHEREOF, this Agreement has been executed as of the date first
written above.

                                    COMPANY:

                                    SYCAMORE NETWORKS, INC.


                                    By: /s/ Daniel Smith
                                       -------------------------------------
                                         Daniel Smith
                                         President

                                    INVESTORS:

                                    Matrix Partners V, L.P.
                                    Bay Colony Corporate Center
                                    1000 Winter Street, Suite 4500
                                    Waltham, MA 02154

                                    By:  Matrix V Management Co., L.L.C., its
                                         General Partner


                                         By: /s/ Timothy A. Barrows
                                            -------------------------------


                                    Matrix V Entrepreneurs Fund, L.P.
                                    Bay Colony Corporate Center
                                    1000 Winter Street, Suite 4500
                                    Waltham, MA 02154

                                    By:  Matrix V Management Co., L.L.C., its
                                         General Partner


                                         By: /s/ Timothy A. Barrows
                                            -------------------------------

                                      -17-
<PAGE>

                                    North Bridge Venture Partners II, L.P.
                                    404 Wyman Street, Suite 365
                                    Waltham, MA 02154

                                    By:  North Bridge Venture Partners II, L.P.
                                         its General Partner

                                         By: /s/ Edward T. Anderson
                                            -------------------------------


                                    Integral Capital Partners IV, L.P.
                                    2750 Sand Hill Road
                                    Menlo Park, CA 94025-7020

                                    By:  Integral Capital Management IV, LLC
                                         its General Partner


                                         By: /s/ Pamela K. Hagenah
                                            ------------------------------------
                                            Pamela K. Hagenah
                                            a Manager

                                    Integral Capital Partners IV MS Side
                                    Fund, L.P.
                                    2750 Sand Hill Road
                                    Menlo Park, CA 94025-7020

                                    By:  ICP MS Management, LLC
                                         its General Partner


                                         By: /s/ Pamela K. Hagenah
                                            ------------------------------------
                                            Pamela K. Hagenah
                                            a Manager

                                    Pequot Private Equity Fund, L.P.
                                    500 Nyala Farm Road
                                    Westport, CT 06880

                                      -18-
<PAGE>

                                    By: /s/ David J. Malat
                                       ------------------------------------

                                    Pequot Offshore Private Equity Fund, Inc.
                                    500 Nyala Farm Road
                                    Westport, CT 06880


                                    By: /s/ David J. Malat
                                       ------------------------------------


                                    Pequot Venture Partners, L.P.
                                    500 Nyala Farm Road
                                    Westport, CT 06880


                                    By: /s/ David J. Malat
                                       ------------------------------------


                                    Spinnaker Founders Fund, L.P.
                                    1875 South Grant Street
                                    San Mateo, CA 94402

                                    By:  Bowman Capital Management, L.L.C.
                                         its General Partner


                                         By:  /s/ William J. Haggerty
                                           -----------------------------------
                                           William J. Haggerty,
                                           Managing Director of
                                           Operations of Bowman
                                           Capital Management, L.L.C.


                                    Spinnaker Offshore Founders Fund,
                                    1875 South Grant Street
                                    San Mateo, CA 94402

                                      -19-
<PAGE>

                                    By:  Bowman Capital Management, L.L.C.,
                                         its Investment Adviser and
                                         Attorney-in-Fact



                                         By:  /s/ William J. Haggerty
                                            -----------------------------------
                                            William J. Haggerty,
                                            Managing Director of
                                            Operations of Bowman
                                            Capital Management,
                                            L.L.C.


                                    Spinnaker Clipper Fund, L.P.
                                    1875 South Grant Street
                                    San Mateo, CA 94402

                                    By:  Bowman Capital Management,
                                         L.L.C., its General Partner


                                         By:  /s/ William J. Haggerty
                                            -----------------------------------
                                            William J. Haggerty,
                                            Managing Director of
                                            Operations of Bowman
                                            Capital Management,
                                            L.L.C.



                                    ATGF II, a Panamanian corporation
                                    SUCRE Building Calle 48 Este
                                    Bella Vista, P.O. Box 5168
                                    Panama S, Panama


                                    By: /s/ illegible
                                       ------------------------------------
                                          Director


                                    /s/ Ralph H. Cechettini
                                    ----------------------------------------
                                    The Ralph H. Cechettini 1995 Trust

                                      -20-
<PAGE>

                                         /s/ James Stableford
                                      ------------------------------------------
                                      James Stableford


                                         /s/ Anthony Ciulla
                                      ------------------------------------------
                                      Anthony Ciulla


                                         /s/ William Slattery
                                      ------------------------------------------
                                      William Slattery



                                         /s/ Marc Weiss
                                      ------------------------------------------
                                      Marc Weiss


                                         /s/ Gururaj Deshpande
                                      ------------------------------------------
                                      Gururaj Deshpande


                                         /s/ Daniel Smith
                                      ------------------------------------------
                                      Daniel Smith


                                         /s/ Chikong Shue
                                      ------------------------------------------
                                      Chikong Shue


                                         /s/ Siu Wing Li
                                      ------------------------------------------
                                      Siu Wing Li


                                      FOUNDERS:


                                         /s/ Gururaj Deshpande
                                      ------------------------------------------
                                      Gururaj Deshpande

                                      Address:  9 Sparta Way
                                                Andover, MA 01810

                                      -21-
<PAGE>

                                         /s/ Richard Barry
                                      ------------------------------------------
                                      Richard Barry

                                      Address:  1284 Beacon Street, #815
                                                Brookline, MA 02138

                                      -22-
<PAGE>

                                  Schedule I
                                  ----------

                                   Investors
                                   ---------


Matrix Partners V, L.P.
Matrix V Entrepreneurs Fund, L.P.
North Bridge Venture Partners II, L.P.
Integral Capital Partners IV, L.P.
Integral Capital Partners IV MS Side Fund, L.P.
Pequot Private Equity Fund, L.P.
Pequot Venture Partners, L.P.
Pequot Offshore Private Equity Fund, Inc.
ATGF II, a Panamanian corporation
The Ralph H. Cechettini 1995 Trust
James Stableford
Anthony Ciulla
William Slattery
Marc Weiss
Spinnaker Founders Fund, L.P.
Spinnaker Offshore Founders Fund, Cayman Limited
Spinnaker Clipper Fund, L.P.
Gururaj Deshpande
Daniel Smith
Chikong Shue
Siu Wing Li
Michael Viren
Steven Finn
Eric MacDonald
John Dowling
Leif Uptegrove
Scott Baker
Jeannette Slaff

                                      -23-
<PAGE>

                                  Schedule II
                                  -----------


                                   Founders
                                   --------

Gururaj Deshpande
Richard Barry

                                      -24-
<PAGE>

                                AMENDMENT NO. 1
                                      TO
             SECOND AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT
             -----------------------------------------------------


     This Amendment dated as of July 23, 1999 is entered into by and among
Sycamore Networks, Inc., a Delaware corporation (the "Company"), Siemens
Information and Communication Networks, Inc., a Delaware corporation
("Siemens"), the Investors (as defined below) and the Founders (as defined
below).

     WHEREAS, the Company has entered into a Second Amended and Restated
Investor Rights Agreement dated as of February 26, 1999 (the "Agreement") with
the persons and entities listed on Schedule I thereto under the heading
"Investors" (individually, an "Investor" and collectively, the "Investors") and
the persons listed on Schedule II thereto under the heading "Founders"
(individually, a "Founder" and collectively, the "Founders");

     WHEREAS, the Company and Siemens have entered into a Series D Preferred
Stock Purchase Agreement of even date herewith (the "Series D Stock Purchase
Agreement"); and

     WHEREAS, the parties hereto desire to amend the Agreement pursuant to this
Amendment No. 1.

     NOW THEREFORE, in consideration of the mutual covenants contained herein
and for other valuable consideration, the receipt and sufficiency of which is
hereby acknowledged, the parties hereto agree as follows:

     1.  The definition of "Initial Public Offering" contained in Article I of
the Agreement shall be deleted in its entirety and the following substituted in
its place:

         "Initial Public Offering" means the sale of shares of Common Stock in
          -----------------------
         a firm commitment underwritten public offering pursuant to a
         Registration Statement at a price to the public of at least $29.00 per
         share (adjusted for stock splits, stock dividends and similar events)
         resulting in proceeds (net of the underwriting discounts or commissions
         and offering expenses) to the Company of at least $10,000,000.

     2.  The definition of "Shares" contained in Article I  of the Agreement
shall be deleted in its entirety and the following substituted in its place:

         "Shares" means the shares of Series A Convertible Preferred Stock,
          ------
         Series B Convertible Preferred Stock, Series C Convertible
<PAGE>

         Preferred Stock and Series D Convertible Preferred Stock held by the
         Stockholders.

     3.  Section 2 to Article IV of the Agreement shall be deleted in its
entirety and the following substituted in its place:

         2.   Excluded Issuances.  The rights of the Investors under this
              ------------------
         Article IV shall not apply to:

         (a)  Common Stock issued as a stock dividend to holders of Common Stock
         or upon any subdivision or combination of shares of Common Stock;

         (b)  the issuance of any shares of Common Stock upon conversion of
         outstanding shares of convertible preferred stock;

         (c)  shares of Series C Preferred Stock issued at Additional Closings,
         as defined in the Purchase Agreement;

         (d)  shares of Series D Preferred Stock issued pursuant to the Series D
         Stock Purchase Agreement;

         (e)  up to 9,000,000 shares of Common Stock, either issued in the form
         of restricted stock awards or options exercisable for Common Stock
         (subject to appropriate adjustment for stock split, stock dividends,
         combinations and other similar recapitalizations affecting such
         shares), plus such additional number of shares as may be approved by a
         majority of the non-employee directors of the Company, issued or
         issuable to officers, directors, consultants and employees of the
         Company or any subsidiary pursuant to any plan, agreement or
         arrangement approved by the Board of Directors of the Company;

         (f)  securities issued solely in consideration for the acquisition
         (whether by merger or otherwise) by the Company or any of its
         subsidiaries of all or substantially all of the stock or assets of any
         other entity;

         (g)  shares of Common Stock sold by the Company in an underwritten
         public offering pursuant to an effective registration statement under
         the Securities Act; and

         (h)  securities issued to equipment lessors, as approved by a majority
         of non-employee directors of the Company.

                                      -2-
<PAGE>

     4.  Schedule I attached to the Agreement shall be deleted in its entirety
         ----------
and Schedule I attached hereto shall be substituted in its place.
    ----------

     5.  The Agreement, as supplemented and modified by this Amendment, together
with the other writings referred to in the Agreement or delivered pursuant
thereto which form a part thereof, contain the entire agreement among the
parties with respect to the subject matter thereof and amend, restate and
supersede all prior and contemporaneous arrangements or understandings with
respect thereto.  Siemens shall become a party to and an Investor under the
Agreement, as amended, upon its execution of this Amendment.

     6.  Upon the effectiveness of this Amendment, on and after the date hereof,
each reference in the Agreement to "this Agreement," "hereunder," "hereof,"
"herein" or words of like import, and each reference in the other documents
entered into in connection with the Agreement, shall mean and be a reference to
the Agreement, as amended hereby.  Except as specifically amended above, the
Agreement shall remain in full force and effect and is hereby ratified and
confirmed.

     7.  This Amendment shall be governed by the laws of the State of Delaware,
notwithstanding the conflict-of-law doctrines of Delaware or any other
jurisdiction to the contrary.

     8.  This Amendment may be executed in any number of counterparts, and each
such counterpart shall be deemed to be an original instrument, but all such
counterparts together shall constitute but one agreement.

     9.  This Amendment shall be binding on all parties to the Agreement as and
when executed by the Company and Investors holding at least a majority of the
shares of Common Stock issued or issuable upon conversion of the Shares (as
defined in the Agreement).

                                      -3-
<PAGE>

     IN WITNESS WHEREOF the parties hereto have executed this Amendment on the
date first above written.

                                    COMPANY:

                                    SYCAMORE NETWORKS, INC.


                                    By:  /s/ Daniel Smith
                                        -------------------------------------
                                        Daniel Smith
                                        President

                                    SIEMENS

                                    SIEMENS INFORMATION AND COMMUNICATION
                                    NETWORKS, INC.


                                    By:  /s/ Illegible
                                        -------------------------------------
                                    Address:     900 Broken Sound Parkway
                                                 Boca Raton, FL 33487


                                    For Notices: P.O. Box 58075
                                                 Santa Clara, CA 95052-8075
                                                 Attn:  Bjoern Christensen
                                                 Fax:  (408) 492-4821

                                    With copies to  Siemens Corporation
                                                    1301 Avenue of the Americas
                                                    New York, NY 10019

                                                    Attn:  General Counsel
                                                    Fax:   (212) 258-4490

                                      -4-
<PAGE>

                                    Matrix Partners V, L.P.
                                    Bay Colony Corporate Center
                                    1000 Winter Street, Suite 4500
                                    Waltham, MA 02154

                                    By:  Matrix V Management Co., L.L.C.,
                                         its General Partner


                                    By:  /s/ Timothy A. Barrows
                                       -------------------------------------


                                    Matrix V Entrepreneurs Fund, L.P.
                                    Bay Colony Corporate Center
                                    1000 Winter Street, Suite 4500
                                    Waltham, MA 02154

                                    By:  Matrix V Management Co., L.L.C.
                                         its General Partner


                                    By:  /s/ Timothy A. Barrows
                                       -------------------------------------

                                      -5-
<PAGE>

                                    North Bridge Venture Partners II, L.P.
                                    404 Wyman Street, Suite 365
                                    Waltham, MA 02154

                                    By:  North Bridge Venture Partners II, L.P.
                                         its General Partner


                                    By:  /s/ Edward T. Anderson
                                       -------------------------------------


                                    Integral Capital Partners IV, L.P.
                                    2750 Sand Hill Road
                                    Menlo Park, CA 94025-7020

                                    By:  Integral Capital Management IV, LLC
                                         its General Partner


                                    By:  /s/ Pamela K. Hagenah
                                       -------------------------------------
                                          Pamela K. Hagenah
                                          a Manager

                                    Integral Capital Partners IV MS Side
                                      Fund, L.P.
                                    2750 Sand Hill Road
                                    Menlo Park, CA 94025-7020

                                    By:  ICP MS Management, LLC
                                         its General Partner


                                    By:  /s/ Pamela K. Hagenah
                                       -------------------------------------
                                          Pamela K. Hagenah
                                          a Manager

                                    Pequot Private Equity Fund, L.P.
                                    500 Nyala Farm Road
                                    Westport, CT 06880


                                    By:  /s/ David J. Malat
                                       -------------------------------------

                                      -6-
<PAGE>



                                    Pequot Offshore Private Equity Fund, Inc.
                                    500 Nyala Farm Road
                                    Westport, CT 06880


                                    By: /s/ David J. Malat
                                       ------------------------------------


                                    Pequot Venture Partners, L.P.
                                    500 Nyala Farm Road
                                    Westport, CT 06880


                                    By: /s/ David J. Malat
                                       ------------------------------------

                                    Spinnaker Founders Fund, L.P.
                                    1875 South Grant Street
                                    San Mateo, CA 94402

                                    By:  Bowman Capital Management, L.L.C.,
                                         its General Partner

                                    By:
                                       ------------------------------------
                                         William J. Haggerty, Managing
                                         Director of Operations of Bowman
                                         Capital Management, L.L.C.

                                    Spinnaker Offshore Founders Fund,
                                    Cayman Limited
                                    1875 South Grant Street
                                    San Mateo, CA 94402

                                    By:  Bowman Capital Management, L.L.C.,
                                         its Investment Adviser and
                                         Attorney-in-Fact


                                    By:
                                       ------------------------------------
                                          William J. Haggerty, Managing
                                          Director of Operations of Bowman
                                          Capital Management, L.L.C.


                                      -7-
<PAGE>

                                    Spinnaker Clipper Fund, L.P.
                                    1875 South Grant Street
                                    San Mateo, CA 94402

                                    By:  Bowman Capital Management, L.L.C.,
                                         its General Partner

                                    By:
                                       ------------------------------------
                                         William J. Haggerty, Managing
                                         Director of Operations of Bowman
                                         Capital Management, L.L.C.


                                    ATGF II, a Panamanian corporation
                                    SUCRE Building Calle 48 Este
                                    Bella Vista, P.O. Box 5168
                                    Panama S, Panama


                                    By: /s/ Gary A. Tanaka
                                       -------------------------------------
                                      Director


                                    /s/ Ralph H. Cechettini
                                    ---------------------------------------
                                    The Ralph H. Cechettini 1995 Trust


                                    /s/ James Stableford
                                    ----------------------------------------
                                    James Stableford


                                    /s/ Anthony Ciulla
                                    ---------------------------------------
                                    Anthony Ciulla



                                    ---------------------------------------
                                    William Slattery



                                    ---------------------------------------
                                    Marc Weiss

                                      -8-
<PAGE>

                                    ---------------------------------------
                                    Chikong Shue


                                    ---------------------------------------
                                    Siu Wing Li


                                    /s/ Gururaj Deshpande
                                    ---------------------------------------
                                    Gururaj Deshpande


                                    /s/ Daniel Smith
                                    ---------------------------------------
                                    Daniel Smith




                                    FOUNDERS:


                                    /s/ Gururaj Deshpande
                                    ----------------------------------------
                                    Gururaj Deshpande

                                    Address:  9 Sparta Way
                                              Andover, MA 01810


                                    ---------------------------------------
                                    Richard Barry

                                    Address:  1284 Beacon Street, #815
                                              Brookline, MA 02138

                                      -9-
<PAGE>

                                  Schedule I
                                  ----------

                                   Investors
                                   ---------


Matrix Partners V, L.P.
Matrix V Entrepreneurs Fund, L.P.
North Bridge Venture Partners II, L.P.
Integral Capital Partners IV, L.P.
Integral Capital Partners IV MS Side Fund, L.P.
Pequot Private Equity Fund, L.P.
Pequot Venture Partners, L.P.
Pequot Offshore Private Equity Fund, Inc.
ATGF II, a Panamanian corporation
The Ralph H. Cechettini 1995 Trust
James Stableford
Anthony Ciulla
William Slattery
Marc Weiss
Spinnaker Founders Fund, L.P.
Spinnaker Offshore Founders Fund, Cayman Limited
Spinnaker Clipper Fund, L.P.
Gururaj Deshpande
Daniel Smith
Chikong Shue
Siu Wing Li
Michael Viren
Steven Finn
Eric MacDonald
John Dowling
Leif Uptegrove
Scott Baker
Jeannette Slaff
Siemens Information and Communication Networks, Inc.

                                     -10-