<PAGE>

                                                                     EXHIBIT 4.3


                           CHORUM TECHNOLOGIES INC.

               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT

                               SEPTEMBER 5, 2000
<PAGE>

                               TABLE OF CONTENTS
                               -----------------

<TABLE>
<CAPTION>
                                                                          Page
                                                                          ----
<S>                                                                       <C>
1. Registration Rights 1
     1.1   Definitions...................................................   1
     1.2   Request for Registration......................................   3
     1.3   Company Registration..........................................   4
     1.4   Obligations of the Company....................................   5
     1.5   Furnish Information...........................................   6
     1.6   Expenses of Demand Registration...............................   6
     1.7   Expenses of Company Registration..............................   7
     1.8   Underwriting Requirements.....................................   7
     1.9   Delay of Registration.........................................   8
     1.10  Indemnification...............................................   8
     1.11  Reports Under Securities Exchange Act of 1934.................  10
     1.12  Form S-3 Registration.........................................  10
     1.13  Assignment of Registration Rights.............................  11
     1.14  Market Stand-Off Agreement....................................  12
     1.15  Termination of Registration Rights............................  12

2. Covenants of the Company 13
     2.1   Delivery of Financial Statements..............................  13
     2.2   Inspection....................................................  13
     2.3   Confidentiality, Assignment and Termination of Covenants......  14
     2.4   Right of First Offer..........................................  15
     2.5   Board of Directors............................................  16
     2.6   Observer Rights...............................................  17
     2.7   Proprietary Information and Inventions Agreement..............  18
     2.8   Key-Man Insurance.............................................  18
     2.9   Assignments of Rights of First Refusal........................  18
     2.10  Indemnification Agreement.....................................  19

3. Miscellaneous.........................................................  19
     3.1   Successors and Assigns........................................  19
     3.2   Governing Law.................................................  19
     3.3   Counterparts..................................................  19
     3.4   Titles and Subtitles..........................................  19
     3.5   Notices.......................................................  19
     3.6   Expenses......................................................  19
     3.7   Amendments and Waivers........................................  20
     3.8   Severability..................................................  20
     3.9   Aggregation of Stock..........................................  20
     3.10  Additional Parties............................................  20
     3.11  Delays or Omissions...........................................  20
     3.12  Entire Agreement..............................................  21
     3.13  Prior Agreement...............................................  21
</TABLE>

Schedule A   Schedule of Investors and Founders
<PAGE>

               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT


          THIS AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT is made as of
September 5, 2000, by and among Chorum Technologies Inc., a Delaware corporation
(the "Company"), and the investors and the founders of the Company listed on
Schedule A hereto (the "Investors" and the "Founders," respectively).
----------

                                   RECITALS

          WHEREAS, certain of the Investors (the "Prior Investors") and the
Founders possess registration rights and certain of the Investors possess other
investor rights granted pursuant to that certain Amended and Restated Investors'
Rights Agreement, dated November 10, 1999, among the Company, the Founders and
the persons listed on Schedule A thereto (the "Prior Agreement");
                      ----------

          WHEREAS, certain of the Investors (the "Series E Investors") are
parties to that certain Series E Preferred Stock Purchase Agreement of even date
herewith (the "Series E Agreement") among the Company and the investors listed
on the Schedule of Investors attached thereto, pursuant to which the Series E
Investors are purchasing shares of the Company's Series E Preferred Stock;

          WHEREAS, in order to induce the Company to enter into the Series E
Agreement and to induce the Series E Investors to invest funds in the Company
pursuant to the Series E Agreement, the Prior Investors and the Founders hereby
agree to waive their rights under the Prior Agreement, and the Investors, the
Founders and the Company hereby agree that this Agreement shall govern the
rights of the Investors and the Founders to cause the Company to register shares
of Common Stock issued or issuable to such persons, and certain other matters as
set forth herein; and

          WHEREAS, the Series E Investors and the Company have agreed, pursuant
to the Series E Agreement, to enter into this Agreement;

          NOW, THEREFORE, in consideration of the mutual promises and covenants
set forth herein, the parties hereto hereby agree as follows:

          1.   Registration Rights. The Company covenants and agrees as follows:
               -------------------

               1.1  Definitions.  For purposes of this Agreement:
                    -----------

                    (a)  The term "Act" means the Securities Act of 1933, as
amended.

                    (b)  The term "Form S-3" means such form under the Act as in
effect on the date hereof or any registration form under the Act subsequently
adopted by the SEC that permits inclusion or incorporation of substantial
information by reference to other documents filed by the Company with the SEC.
<PAGE>

                    (c)  The term "Holder" means any person owning or having the
right to acquire Registrable Securities or any assignee thereof in accordance
with Section 1.13 hereof.

                    (d)  The term "1934 Act" shall mean the Securities Exchange
Act of 1934, as amended.

                    (e)  The term "register", "registered," and "registration"
refer to a registration effected by preparing and filing a registration
statement or similar document in compliance with the Act, and the declaration or
ordering of effectiveness of such registration statement or document.

                    (f)  The term "Registrable Securities" means (i) the Common
Stock issuable or issued upon conversion of the Series B-1 Preferred Stock,
Series B-2 Preferred Stock, Series C Preferred Stock, Series D Preferred Stock
and Series E Preferred Stock, (ii) for purposes of Section 1.3 only (and other
portions of this Section 1, only to the extent they relate to rights or
registration under Section 1.3), the term "Registrable Securities" shall also
include shares of Series A Preferred Stock or Common Stock of the Company held
by the Founders, and (iii) any Common Stock of the Company issued as (or
issuable upon the conversion or exercise of any warrant, right or other security
that is issued as) a dividend or other distribution with respect to, or in
exchange for or in replacement of the shares referenced in (i) and (ii) above;
provided, however, that the term "Registrable Securities" shall exclude in all
cases any Registrable Securities sold by a person in a transaction in which his
rights under this Section 1 are not assigned and any shares of Common Stock
("Special Mandatory Conversion Shares") issued upon conversion of Series B-1
Preferred Stock, Series B-2 Preferred Stock, Series C Preferred Stock, Series D
Preferred Stock or Series E Preferred Stock, pursuant to Article IV, Section
(B), Paragraph 4(l) of the Company's Restated Certificate of Incorporation (as
amended, the "Certificate of Incorporation") (which Paragraph is entitled
"Special Mandatory Conversion"), as such provision may be amended from time to
time; provided further, that Registrable Securities shall only be treated as
Registrable Securities if and so long as, they have not been (A) sold to or
through a broker or dealer or underwriter in a public distribution or a public
securities transaction or (B) sold in a transaction exempt from the registration
and prospectus delivery requirements of the Act under Section 4(1) thereof so
that all transfer restrictions and restrictive legends with respect thereto are
removed upon the consummation of such sale.

                    (g)  The number of shares of "Registrable Securities then
outstanding" shall be determined by the number of shares of Common Stock
outstanding that are, and the number of shares of Common Stock issuable pursuant
to then exercisable or convertible securities that are, Registrable Securities.

                    (h)  The term "SEC" shall mean the Securities and Exchange
Commission or any other federal agency at the time administering the Act.

               1.2  Request for Registration.
                    ------------------------

                    (a)  If the Company shall receive at any time after the
earlier of (i) October 1, 2002, or (ii) twelve (12) months after the effective
date of the first registration

                                       2
<PAGE>

statement for a firmly underwritten public offering of securities of the Company
under the Act, the public offering of which is for aggregate proceeds to the
Company, net of underwriters' discounts and commissions, of more than
$15,000,000 (other than a registration statement relating either to the sale of
securities to employees of the Company pursuant to a stock option, stock
purchase or similar plan or a SEC Rule 145 transaction), a written request from
either (x) the Holders of fifty-one percent (51%) of the Registrable Securities
then outstanding or (y) in the case of a request made after a registration
requested pursuant to this Section 1.2 has already been declared effective, the
Holders of at least twenty-five percent (25%) of the Registrable Securities then
outstanding, that the Company file a registration statement under the Act
covering the registration of at least twenty percent (20%) of the Registrable
Securities then outstanding and having a reasonably anticipated aggregate
offering price, net of underwriting discounts and commissions, greater than
$7,500,000, then the Company shall:

                         (i)  within twenty (20) days of the receipt thereof,
give written notice of such request to all Holders; and

                         (ii) use its best efforts to cause to be registered
under the Act as soon as practicable, all Registrable Securities that the
Holders request to be registered and to cause the registration statement
pertaining thereto to be declared effective, subject to the limitations of
subsection 1.2(b), within twenty (20) days of the mailing of such notice by the
Company in accordance with Section 3.5.

                    (b)  If the Holders initiating the registration request
hereunder ("Initiating Holders") intend to distribute the Registrable Securities
covered by their request by means of an underwriting, they shall so advise the
Company as a part of their request made pursuant to subsection 1.2(a) and the
Company shall include such information in the written notice referred to in
subsection 1.2(a). The underwriter will be selected by the Company and shall be
reasonably acceptable to a majority in interest of the Initiating Holders. In
such event, the right of any Holder to include his Registrable Securities in
such registration shall be conditioned upon such Holder's participation in such
underwriting and the inclusion of such Holder's Registrable Securities in the
underwriting (unless otherwise mutually agreed by a majority in interest of the
Initiating Holders and such Holder) to the extent provided herein. All Holders
proposing to distribute their securities through such underwriting shall
(together with the Company as provided in subsection 1.4(e)) enter into an
underwriting agreement in customary form with the underwriter or underwriters
selected for such underwriting; provided, however, that in no event shall the
Holders be obligated to provide indemnification on claims arising out of facts
not provided by the Holders or in an amount exceeding the net proceeds realized
by the Holders. Notwithstanding any other provision of this Section 1.2, if the
underwriter advises the Initiating Holders in writing that marketing factors
require a limitation of the number of shares to be underwritten, then the
Initiating Holders shall so advise all Holders of Registrable Securities that
would otherwise be underwritten pursuant hereto, and the number of shares of
Registrable Securities that may be included in the underwriting shall be
allocated among all Holders electing to include shares in the offering,
including the Initiating Holders, in proportion (as nearly as practicable) to
the amount of Registrable Securities of the Company owned by each Holder;
provided, however, that the number of shares of Registrable Securities to be
included in such underwriting shall not be reduced unless and until all other
securities, including securities of the Founders under subsection 1.2(e) below,
are first entirely excluded from the underwriting.

                                       3
<PAGE>

                    (c)  Notwithstanding the foregoing, if the Company shall
furnish to Holders requesting a registration statement pursuant to this Section
1.2, a certificate signed by the Chief Executive Officer (or, if there is no
Chief Executive Officer, the President) of the Company stating that in the good
faith judgment of the Board of Directors of the Company, it would be seriously
detrimental to the Company and its stockholders for such registration statement
to be filed and it is therefore essential to defer the filing of such
registration statement, the Company shall have the right to defer taking action
with respect to such filing for a period of not more than one hundred twenty
(120) days after receipt of the request of the Initiating Holders; provided,
however, that the Company may not utilize this right more than once in any
twelve (12) month period.

                    (d)  In addition, the Company shall not be obligated to
effect, or to take any action to effect, any registration pursuant to this
Section 1.2:

                         (i)   In any particular jurisdiction in which the
Company would be required to execute a general consent to service of process in
effecting such registration, unless the Company is already subject to service in
such jurisdiction and may be required under the Act;

                         (ii)  After the Company has effected two (2)
registrations pursuant to this Section 1.2 and such registrations have been
declared or ordered effective;

                         (iii) During the period starting with the date sixty
(60) days prior to the Company's good faith estimate of the date of filing of,
and ending on a date one hundred eighty (180) days after the effective date of,
a registration subject to Section 1.3 hereof; provided that the Company is
actively employing its best efforts to cause such registration statement,
subject to Section 1.3 hereof, to be declared effective; or

                         (iv)  If the Initiating Holders propose to dispose of
shares of Registrable Securities that may be immediately registered on Form S-3
pursuant to a request made pursuant to Section 1.12 below.

                    (e)  In connection with a registration under this Section
1.2, the Founders shall be entitled to include any of their shares of Common
Stock in any registration by the Company under this Section 1.2, provided that
(A) such person's rights under this Section 1.2 shall be subordinate to the
rights of the Investors and (B) such persons agree to be bound by all other
provisions of this Agreement and participate in any such registration on the
same basis as each Holder in accordance with all applicable provisions of this
Agreement.

               1.3  Company Registration.
                    --------------------

                    (a)  If at any time or from time to time (but without any
obligation to do so), the Company proposes to register (including for this
purpose a registration effected by the Company for stockholders other than the
Holders) any of its stock or other securities under the Act in connection with
the public offering of such securities solely for cash (other than a
registration relating solely to the sale of securities to participants in a
Company stock plan or a registration on any form that does not include
substantially the same information

                                       4
<PAGE>

as would be required to be included in a registration statement covering the
sale of the Registrable Securities), the Company shall, at such time, promptly
give each Holder written notice of such registration. Upon the written request
of each Holder given within twenty (20) days after mailing of such notice by the
Company in accordance with Section 3.5, the Company shall, subject to the
provisions of Section 1.8, include in the registration statement all of the
Registrable Securities that each such Holder has requested to be registered.

                    (b)  Upon any sale by the Company of shares of its Common
Stock or other securities to the public in a firmly underwritten public offering
solely for cash, the Founders shall be entitled to include any of their
Registrable Securities in any registration by the Company under this Section
1.3, if such persons agree to be bound by all other provisions of this Agreement
and participate in any such registration on the same basis as each Holder
(except as specifically set forth in Section 1.8 below) in accordance with all
applicable provisions of this Agreement.

               1.4  Obligations of the Company.  Whenever required under this
                    --------------------------
Section 1 to effect the registration of any Registrable Securities, the Company
shall, as expeditiously as reasonably possible:

                    (a)  Prepare and file with the SEC a registration statement
with respect to such Registrable Securities and use its best efforts to cause
such registration statement to become effective, and, upon the request of the
Holders of a majority of the Registrable Securities registered thereunder, keep
such registration statement effective for a period of up to one hundred twenty
(120) days or, if earlier, until the distribution contemplated in the
registration statement has been completed.

                    (b)  Prepare and file with the SEC such amendments and
supplements to such registration statement and the prospectus used in connection
with such registration statement as may be necessary to comply with the
provisions of the Act with respect to the disposition of all securities covered
by such registration statement.

                    (c)  Furnish to the Holders such numbers of copies of a
prospectus, including a preliminary prospectus, in conformity with the
requirements of the Act, and such other documents as they may reasonably request
in order to facilitate the disposition of Registrable Securities owned by them.

                    (d)  Use its best efforts to register and qualify the
securities covered by such registration statement under such other securities or
blue sky laws of such jurisdictions as shall be reasonably requested by the
Holders; provided that the Company shall not be required in connection therewith
or as a condition thereto to qualify to do business or to file a general consent
to service of process in any such states or jurisdictions.

                    (e)  In the event of any underwritten public offering, enter
into and perform its obligations under an underwriting agreement, in usual and
customary form, with the managing underwriter of such offering. Each Holder
participating in such underwriting shall also enter into and perform its
obligations under such an agreement; provided, however, that in

                                       5
<PAGE>

no event shall the Holders be obligated to provide indemnification on claims
arising out of facts not provided by the Holders or in an amount exceeding the
net proceeds realized by the Holders.

                    (f)  Notify each Holder of Registrable Securities covered by
such registration statement at any time when a prospectus relating thereto is
required to be delivered under the Act of the happening of any event as a result
of which the prospectus included in such registration statement, as then in
effect, includes an untrue statement of a material fact or omits to state a
material fact required to be stated therein or necessary to make the statements
therein not misleading in the light of the circumstances then existing, and at
the request of any such seller, prepare and furnish to such seller a reasonable
number of copies of a supplement to or an amendment of such prospectus as may be
necessary so that, as thereafter delivered to the purchasers of such shares,
such prospectus shall not include an untrue statement of a material fact or omit
to state a material fact required to be stated therein or necessary to make the
statements therein not misleading or incomplete in light of the circumstances
then existing.

                    (g)  Cause all such Registrable Securities registered
pursuant hereunder to be listed on each securities exchange or nationally
recognized quotation system on which similar securities issued by the Company
are then listed.

                    (h)  Provide a transfer agent and registrar for all
Registrable Securities registered pursuant to such registration statement and a
CUSIP number for all such Registrable Securities, in each case not later than
the effective date of such registration.

               1.5  Furnish Information.
                    -------------------

                    (a)  It shall be a condition precedent to the obligations of
the Company to take any action pursuant to this Section 1 with respect to the
Registrable Securities of any selling Holder that such Holder shall furnish to
the Company such information regarding itself, the Registrable Securities held
by it, and the intended method of disposition of such securities as shall be
required to effect the registration of such Holder's Registrable Securities.

                    (b)  The Company shall have no obligation with respect to
any registration requested pursuant to Section 1.2 or Section 1.12 if, due to
the operation of subsection 1.5(a), the number of shares or the anticipated
aggregate offering price of the Registrable Securities to be included in the
registration does not equal or exceed the number of shares or the anticipated
aggregate offering price required to originally trigger the Company's obligation
to initiate such registration as specified in subsection 1.2(a) or subsection
1.12(b)(ii), whichever is applicable.

               1.6  Expenses of Demand Registration.  All expenses other than
                    -------------------------------
(i) underwriting discounts and commissions incurred in connection with
registrations, filings or qualifications pursuant to Section 1.2 and (ii) fees
and expenses of counsel for any selling Holder (if other than counsel to the
Company), including (without limitation) all registration, filing and
qualification fees, printers' and accounting fees, and fees and disbursements of
counsel for the Company shall be borne by the Company; provided, however, that
the Company shall not be required to pay for any expenses of any registration
proceeding begun pursuant to Section 1.2 if

                                       6
<PAGE>

the registration request is subsequently withdrawn at the request of the Holders
of a majority of the Registrable Securities to be registered (in which case, all
participating Holders shall either, at the election of the Initiating Holders,
bear such expenses or such withdrawn registration shall be counted as one (1) of
the two (2) registrations effectuated pursuant to Section 1.2), provided
further, however, that if at the time of such withdrawal, the Holders have
learned of a material adverse change in the condition, business, or prospects of
the Company from that known to the Holders at the time of their request and have
withdrawn the request with reasonable promptness following disclosure by the
Company of such material adverse change, then the Holders shall not be required
to pay any of such expenses and shall retain their rights pursuant to Section
1.2.

               1.7  Expenses of Company Registration.  The Company shall bear
                    --------------------------------
and pay all expenses incurred in connection with any registration, filing or
qualification of Registrable Securities with respect to the registrations
pursuant to Section 1.3 for each Holder (which right may be assigned as provided
in Section 1.13), including (without limitation) all registration, filing, and
qualification fees, printers' and accounting fees relating or apportionable
thereto and the fees and disbursements of counsel for the Company, but excluding
underwriting discounts and commissions relating to Registrable Securities and
fees and expenses of counsel for any selling Holder (if other than counsel to
the Company).

               1.8  Underwriting Requirements.  In connection with any offering
                    -------------------------
involving an underwriting of shares of the Company's capital stock, the Company
shall not be required under Section 1.3 to include any of the Holders'
securities in such underwriting unless they accept the terms of the underwriting
as agreed upon between the Company and the underwriters selected by it (or by
other persons entitled to select the underwriters) (provided, however, that in
no event shall the Holders be obligated to provide indemnification on claims
arising out of facts not provided by the Holders or in an amount exceeding the
net proceeds realized by the Holders), and then only in such quantity as the
underwriters determine in their sole discretion will not jeopardize the success
of the offering by the Company. If the total amount of securities, including
Registrable Securities, requested by stockholders to be included in such
offering exceeds the amount of securities sold other than by the Company that
the underwriters determine in their sole discretion is compatible with the
success of the offering, then the Company shall be required to include in the
offering only that number of such securities, including Registrable Securities,
which the underwriters determine in their sole discretion will not jeopardize
the success of the offering (the securities so included to be apportioned pro
rata among the selling stockholders according to the total amount of securities
entitled to be included therein owned by each selling stockholder or in such
other proportions as shall mutually be agreed to by such selling stockholders),
but in no event shall any shares being sold by a stockholder exercising a demand
registration right similar to that granted in Section 1.2 or Section 1.12 be
excluded from such offering. For purposes of the preceding parenthetical
concerning apportionment, for any selling stockholder that is a holder of
Registrable Securities and that is a partnership or corporation, the affiliated
partnerships, the partners, retired partners and stockholders of such holder, or
the estates and family members of any such partners and retired partners and any
trusts for the benefit of any of the foregoing persons shall be deemed to be a
single "selling stockholder," and any pro-rata reduction with respect to such
"selling stockholder" shall be based upon the aggregate amount of shares
carrying registration rights owned by all entities and individuals included in
such "selling stockholder," as defined in this sentence.

                                       7
<PAGE>

               1.9  Delay of Registration.  No Holder shall have any right to
                    ---------------------
obtain or seek an injunction restraining or otherwise delaying any such
registration as the result of any controversy that might arise with respect to
the interpretation or implementation of this Section 1.

               1.10 Indemnification.
                    ---------------

                    (a)  To the extent permitted by law, the Company will
indemnify and hold harmless each Holder (including its officers, directors,
managers, members, partners and employees), Founder, any underwriter (as defined
in the Act) for such Holder, Founder and each person, if any, who controls such
Holder, Founder or underwriter within the meaning of the Act or the 1934 Act,
against any expenses, losses, claims, damages, or liabilities (joint or
several), or actions in respect thereof or any of the foregoing incurred in
settlement of any litigation, commenced or threatened, to which they may become
subject under the Act, or the 1934 Act or other federal or state securities law,
insofar as such expenses, losses, claims, damages, or liabilities (or actions in
respect thereof) arise out of or are based upon any of the following statements,
omissions or violations (collectively a "Violation"): (i) any untrue statement
or alleged untrue statement of a material fact contained in such registration
statement, including any preliminary prospectus or final prospectus contained
therein or any amendments or supplements thereto, (ii) the omission or alleged
omission to state therein a material fact required to be stated therein, or
necessary to make the statements therein not misleading, or (iii) any violation
or alleged violation by the Company of the Act, the 1934 Act, any state
securities law or any rule or regulation promulgated under the Act, or the 1934
Act or any state securities law; and the Company will pay to each such Holder
(including its officers, directors, managers, members, partners and employees),
Founder, underwriter or controlling person, as incurred, any legal or other
expenses reasonably incurred by them in connection with investigating or
defending any such expense, loss, claim, damage, liability, or action; provided,
however, that the indemnity agreement contained in this subsection 1.10(a) shall
not apply to amounts paid in settlement of any such expense, loss, claim,
damage, liability, or action if such settlement is effected without the consent
of the Company (which consent shall not be unreasonably withheld), nor shall the
Company be liable in any such case for any such loss, claim, damage, liability,
or action to the extent that it arises out of or is based upon a Violation that
occurs in reliance upon and in conformity with written information furnished
expressly for use in connection with such registration by any such Holder,
Founder, underwriter or controlling person.

                    (b)  To the extent permitted by law, each selling Holder
(and any selling Founder) will indemnify and hold harmless the Company, each of
its directors, each of its officers who has signed the registration statement,
each person, if any, who controls the Company within the meaning of the Act, any
underwriter, any other Holder or Founder selling securities in such registration
statement and any controlling person of any such underwriter or other Holder or
Founder, against any expenses, losses, claims, damages, or liabilities (joint or
several), or actions in respect thereof or any of the foregoing incurred in
settlement of any litigation, commenced or threatened to which any of the
foregoing persons may become subject, under the Act, the 1934 Act or other
federal or state securities law, insofar as such expenses, losses, claims,
damages, or liabilities (or actions in respect thereto) arise out of or are
based upon any Violation, in each case to the extent (and only to the extent)
that such Violation occurs in

                                       8
<PAGE>

reliance upon and in conformity with written information furnished by such
Holder (or selling Founder, if applicable) expressly for use in connection with
such registration; and each such Holder (or selling Founder, if applicable) will
pay, as incurred, any legal or other expenses reasonably incurred by any person
intended to be indemnified pursuant to this subsection 1.10(b), in connection
with investigating or defending any such expense, loss, claim, damage,
liability, or action; provided, however, that the indemnity agreement contained
in this subsection 1.10(b) shall not apply to amounts paid in settlement of any
such expense, loss, claim, damage, liability or action if such settlement is
effected without the consent of the Holder (or selling Founder, if applicable),
which consent shall not be unreasonably withheld; provided, that, in no event
shall any indemnity under this subsection 1.10(b) exceed the net proceeds from
the offering received by such Holder (or selling Founder, if applicable).

                    (c)  Promptly after receipt by an indemnified party under
this Section 1.10 of notice of the commencement of any action (including any
governmental action), such indemnified party will, if a claim in respect thereof
is to be made against any indemnifying party under this Section 1.10, deliver to
the indemnifying party a written notice of the commencement thereof and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party
similarly noticed, to assume the defense thereof with counsel mutually
satisfactory to the parties; provided, however, that an indemnified party
(together with all other indemnified parties that may be represented without
conflict by one counsel) shall have the right to retain one separate counsel,
with the fees and expenses to be paid by the indemnifying party, if
representation of such indemnified party by the counsel retained by the
indemnifying party would be inappropriate due to an actual or potential conflict
of interest between such indemnified party and any other party represented by
such counsel in such proceeding. The failure to deliver written notice to the
indemnifying party within a reasonable time of the commencement of any such
action, if materially prejudicial to its ability to defend such action, shall
relieve such indemnifying party of any liability to the indemnified party under
this Section 1.10, but the omission so to deliver written notice to the
indemnifying party will not relieve it of any liability that it may have to any
indemnified party otherwise than under this Section 1.10. 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 to such claim or litigation. No indemnifying party shall be
required to indemnify any indemnified party with respect to any settlement
entered into without such indemnifying party's prior consent.

                    (d)  Notwithstanding the foregoing, to the extent that the
provisions on indemnification contained in the underwriting agreement entered
into in connection with the underwritten public offering are in conflict with
the foregoing provisions, the provisions in the underwriting agreement shall
control.

                    (e)  The obligations of the Company and Holders and Founders
under this Section 1.10 shall survive the completion of any offering of
Registrable Securities in a registration statement under this Section 1, and
otherwise.

                                       9
<PAGE>

               1.11 Reports Under Securities Exchange Act of 1934.  With a view
                    ---------------------------------------------
to making available to the Holders the benefits of Rule 144 promulgated under
the Act and any other rule or regulation of the SEC that may at any time permit
a Holder to sell securities of the Company to the public without registration or
pursuant to a registration on Form S-3, the Company agrees to:

                    (a)  make and keep public information available, as those
terms are understood and defined in SEC Rule 144, at all times after the
effective date of the first registration statement filed by the Company for the
offering of its securities to the general public;

                    (b)  file with the SEC in a timely manner all reports and
other documents required of the Company under the Act and the 1934 Act; and

                    (c)  furnish to any Holder, so long as the Holder owns any
Registrable Securities, forthwith upon request (i) a written statement by the
Company that it has complied with the reporting requirements of SEC Rule 144 (at
any time after ninety (90) days after the effective date of the first
registration statement filed by the Company), the Act and the 1934 Act (at any
time after it has become subject to such reporting requirements), or that it
qualifies as a registrant whose securities may be resold pursuant to Form S-3
(at any time after it so qualifies), (ii) a copy of the most recent annual or
quarterly report of the Company and such other reports and documents so filed by
the Company, and (iii) such other information as may be reasonably requested in
availing any Holder of any rule or regulation of the SEC that permits the
selling of any such securities without registration or pursuant to such form.

               1.12 Form S-3 Registration.  In case the Company shall receive
                    ---------------------
from any Holder or Holders a written request or requests that the Company effect
a registration on Form S-3 and any related qualification or compliance with
respect to all or a part of the Registrable Securities owned by such Holder or
Holders, the Company will:

                    (a)  promptly give written notice of the proposed
registration, and any related qualification or compliance, to all other Holders;
and

                    (b)  use its best efforts to effect, as soon as practicable,
such registration and all such qualifications and compliances as may be so
requested and as would permit or facilitate the sale and distribution of all or
such portion of such Holder's or Holders' Registrable Securities as are
specified in such request, together with all or such portion of the Registrable
Securities of any other Holder or Holders joining in such request as are
specified in a written request given within ten (10) days after receipt of such
written notice from the Company; provided, however, that the Company shall not
be obligated to effect any such registration, qualification or compliance,
pursuant to this section 1.12: (i) if Form S-3 is not available for such
offering by the Holders; (ii) if the Holders, together with the holders of any
other securities of the Company entitled to inclusion in such registration,
propose to sell Registrable Securities and such other securities (if any) at a
reasonably anticipated aggregate price to the public (net of any underwriters'
discounts or commissions) of less than $500,000; (iii) if the Company shall
furnish to the Holders a certificate signed by the Chief Executive Officer of
the Company (or, if there is no Chief Executive Officer of the Company, the
President) stating that in the good faith judgment of the Board of Directors of
the Company, it would be seriously detrimental to the

                                      10
<PAGE>

Company and its stockholders for such Form S-3 Registration to be effected at
such time, in which event the Company shall have the right to defer the filing
of the Form S-3 registration statement for a period of not more than ninety (90)
days after receipt of the request of the Holder or Holders under this Section
1.12; provided, however, that the Company shall not utilize this right more than
once in any twelve (12) month period; (iv) if the Company has, within the six
(6) month period preceding the date of such request, already effected one (1)
registration on Form S-3 for the Holders pursuant to this Section 1.12; (v) if
the Company has already effected a total of six (6) registrations on Form S-3
for the Holders pursuant to this Section 1.12; or (vi) in any particular
jurisdiction in which the Company would be required to qualify to do business or
to execute a general consent to service of process in effecting such
registration, qualification or compliance.

                    (c)  Subject to the foregoing, the Company shall file a
registration statement covering the Registrable Securities and other securities
so requested to be registered as soon as practicable after receipt of the
request or requests of the Holders. All expenses incurred in connection with a
registration requested pursuant to Section 1.12, including (without limitation)
all registration, filing, qualification, printers' and accounting fees and the
reasonable fees and disbursements of counsel for the Company, but excluding (i)
any underwriters' discounts or commissions associated with Registrable
Securities and (ii) fees and expenses of counsel for any selling Holder (if
other than counsel for the Company), shall be borne by the Company.
Registrations effected pursuant to this Section 1.12 shall not be counted as
demands for registration or registrations effected pursuant to Sections 1.2 or
1.3, respectively.

               1.13 Assignment of Registration Rights.  The rights to cause the
                    ---------------------------------
Company to register Registrable Securities pursuant to this Section 1 may be
assigned (but only with all related obligations) by a Holder to a transferee or
assignee of such securities who (i) after such assignment or transfer, holds at
least 1,005,000 shares of Registrable Securities (subject to appropriate
adjustment for stock splits, stock dividends, combinations and other
recapitalizations), (ii) is a partnership or other similar entity that is
affiliated with the transferring Holder that is also a partnership or other
similar entity or (iii) is a majority-owned subsidiary of the transferring
Holder; provided (a) the Company is, within thirty (30) days after such
transfer, furnished with written notice of the name and address of such
transferee or assignee and the securities with respect to which such
registration rights are being assigned; (b) such transferee or assignee agrees
in writing to be bound by and subject to the terms and conditions of this
Agreement, including without limitation the provisions of Section 1.14 below;
and (c) such assignment shall be effective only if immediately following such
transfer the further disposition of such securities by the transferee or
assignee is restricted under the Act. For the purposes of determining the number
of shares of Registrable Securities held by a transferee or assignee, the
holdings of transferees and assignees of a partnership or other similar entity
who are partners, retired partners, constituent members, or retired members of
such partnership or other similar entity (including spouses and ancestors,
lineal descendants and siblings of such partners or spouses who acquire
Registrable Securities by gift, will or intestate succession) shall be
aggregated together and with the partnership or other similar entity; provided
that all assignees and transferees who would not qualify individually for
assignment of registration rights shall have a single attorney-in-fact for the
purpose of exercising any rights, receiving notices or taking any action under
this Section 1.13.

                                      11
<PAGE>

               1.14 "Market Stand-Off" Agreement.  Each Investor and Founder
                     ---------------------------
hereby agrees that it will not, without the prior written consent of the
managing underwriter, during the period commencing on the date of the final
prospectus relating to the Company's initial public offering and ending on the
date specified by the Company and the managing underwriter (such period not to
exceed one hundred eighty (l80) days) (i) lend, offer, pledge, sell, contract to
sell, sell any option or contract to purchase, purchase any option or contract
to sell, grant any option, right or warrant to purchase, or otherwise transfer
or dispose of, directly or indirectly, any shares of Common Stock or any
securities convertible into or exercisable or exchangeable for Common Stock held
by it immediately prior to the date of the final prospectus, or (ii) enter into
any swap or other arrangement that transfers to another, in whole or in part,
any of the economic consequences of ownership of the Common Stock held by it
immediately prior to the date of the final prospectus, whether any such
transaction described in clause (i) or (ii) above is to be settled by delivery
of Common Stock or such other securities, in cash or otherwise. The underwriters
in connection with the Company's initial public offering are intended third
party beneficiaries of this Section 1.14 and shall have the right, power and
authority to enforce the provisions hereof as though they were a party hereto.
In order to enforce the foregoing covenant, the Company may impose stop-transfer
instructions with respect to such securities of each Investor and Founder (and
the shares or securities of every other person subject to the foregoing
restriction) until the end of such period. The foregoing provisions of this
Section 1.14 shall not apply to the sale of any shares to an underwriter
pursuant to an underwriting agreement and shall only be applicable to the
Holders if all officers and directors and greater than one percent (1%)
stockholders of the Company enter into similar agreements. Any waiver by the
underwriters of the "market stand-off" obligations set forth herein as to any
greater than one percent (1%) stockholder who is not an employee, officer or
director shall be effective as to the other Holders. In order for an amendment
of clauses (i) or (ii) above to be effective against any Holder that is an
Investment Company within the meaning of the Investment Company Act of 1940, as
amended (the "Investment Company Act"), such Holder or Holders must consent to
such amendment.

               1.15 Termination of Registration Rights.  No Holder shall be
                    ----------------------------------
entitled to exercise any right provided for in this Section 1 after the earlier
of (i) five (5) years following the consummation of the sale of securities
pursuant to a registration statement filed by the Company under the Act in
connection with the initial firm commitment underwritten offering of its
securities to the general public (the "IPO") or (ii) as to any Holder, such time
after the IPO at which all Registrable Securities held by such Holder can be
sold in any three-month period without registration in compliance with Rule 144
of the Act. Notwithstanding the foregoing, no Holder shall be entitled to
exercise any right provided for in this Section 1 with respect to any Special
Mandatory Conversion Shares.

          2.   Covenants of the Company.
               ------------------------

               2.1  Delivery of Financial Statements.  The Company shall
                    --------------------------------
deliver to each Investor that holds at least 1,005,000 shares (as adjusted for
subsequent stock splits, stock dividends, combinations and other
recapitalizations) of Preferred Stock (or Common Stock issued upon conversion
thereof) of the Company:

                                      12
<PAGE>

                    (a)  as soon as practicable, but in any event within ninety
(90) days after the end of each fiscal year of the Company, an income statement
for such fiscal year, a balance sheet of the Company and statement of
stockholder's equity as of the end of such fiscal year, and a statement of cash
flows for such fiscal year, such year-end financial reports to be in reasonable
detail, prepared in accordance with generally accepted accounting principles
consistently applied ("GAAP") and audited and certified by independent public
accountants of nationally recognized standing selected by the Company;

                    (b)  as soon as practicable, but in any event within forty-
five (45) days after the end of each of the first three (3) quarters of each
fiscal year of the Company, an unaudited income statement for such quarter,
statement of cash flows for such quarter and an unaudited balance sheet and
comparison to budget as of the end of such quarter;

                    (c)  within thirty (30) days of the end of each month, an
unaudited income statement and statement of cash flows for such month, and a
balance sheet and comparison to budget for and as of the end of such month, in
reasonable detail;

                    (d)  as soon as practicable, but in any event thirty (30)
days prior to the end of each fiscal year, a budget and business plan for the
next fiscal year, prepared on a monthly basis, including balance sheets and
income statements for such months and, as soon as prepared, any other budgets or
revised budgets prepared by the Company;

                    (e)  with respect to the financial statements called for in
subsections (b) and (c) of this Section 2.1, an instrument executed by the Chief
Financial Officer or President of the Company and certifying that such financial
statements were prepared in accordance with GAAP consistently applied with prior
practice for earlier periods (with the exception of footnotes that may be
required by GAAP) and fairly present the financial condition of the Company and
its results of operation for the period specified, subject to year-end audit
adjustment; and

                    (f)  such other information relating to the financial
condition, business, prospects or corporate affairs of the Company as the
Investor or any assignee of the Investor may from time to time request,
provided, however, that the Company shall not be obligated under this subsection
(f) or any other subsection of Section 2.1 to provide information that it deems
in good faith to be a trade secret or similar confidential information.

               2.2  Inspection.  The Company shall permit each Investor that
                    ----------
holds at least 1,005,000 shares (as adjusted for subsequent stock splits, stock
dividends, combinations and other recapitalizations) of Preferred Stock (or
Common Stock issued upon conversion thereof) of the Company, at such Investor's
expense, to visit and inspect the Company's properties, to examine its books of
account and records and to discuss the Company's affairs, finances and accounts
with its officers, all at such reasonable times as may be requested by the
Investor; provided, however, that the Company shall not be obligated pursuant to
this Section 2.2 to provide access to any information that it reasonably
considers to be a trade secret or similar confidential information.

                                      13
<PAGE>

               2.3  Confidentiality, Assignment and Termination of Covenants.
                    --------------------------------------------------------

                    (a)  Each Investor receiving information under the covenants
set forth in Section 2.1 and Section 2.2 hereby agrees to hold in confidence and
trust and to act in a fiduciary manner with respect to all information so
provided; provided, however, that notwithstanding the foregoing, (i) the
Investors may include summary financial information concerning the Company and
general statements concerning the nature and progress of the Company's business
in their reports to their limited partners or other constituent members; (ii)
the Investors may disclose such information to their respective board of
directors, investment advisers, attorneys, accountants, consultants and other
professionals to the extent necessary to obtain their services in connection
with their respective investments in the Company, provided that such persons
agree to hold such information confidential as provided in this Agreement; (iii)
with Company's prior written consent, the Investors may disclose such
information to any prospective purchaser of any shares of the Company owned by
such Investors as long as such prospective purchaser agrees in writing to be
bound by the confidentiality provisions as provided in this Agreement; (iv) the
Investors may disclose such information to any of their respective Affiliates
(as defined in the Investment Company Act), provided that such Affiliates agree
to hold such information confidential as provided in this Agreement; or (v) the
Investors may disclose such information as required by applicable law or
regulation, regulatory body, stock exchange, court or administrative order, or
with Company's prior written consent, any listing or trading agreement
concerning an Investor or the Company.

                    (b)  The covenants set forth in Section 2.1, Section 2.2,
Section 2.4, Section 2.5, Section 2.6, Section 2.7, Section 2.8 and Section 2.9
shall terminate as to Investors and be of no further force or effect when the
sale of securities pursuant to a registration statement filed by the Company
under the Act in connection with the firm commitment underwritten offering of
its securities to the general public is consummated or when the Company first
becomes subject to the periodic reporting requirements of Section 12(g) or 15(d)
of the 1934 Act, whichever event shall occur first.

                    (c)  It is agreed that for purposes of the covenants set
forth in Section 2.1 and Section 2.2 above, if Investment Companies (as defined
in the Investment Company Act) with the same Investment Advisors (as defined in
the Investment Company Act) hold in the aggregate greater than 1,005,000 shares
(as adjusted for subsequent stock splits, stock dividends, combinations and
other recapitalizations) of Preferred Stock (or Common Stock issued upon
conversion thereof) of the Company, such Investment Advisor shall be entitled to
exercise the rights set forth in Section 2.1 and Section 2.2 above for the
respective Investment Companies.

               2.4  Right of First Offer.  Subject to the terms and conditions
                    --------------------
specified in this paragraph 2.4, the Company hereby grants to each Investor a
right of first offer to purchase its Pro Rata Share (as hereinafter defined) (in
whole or in part) with respect to future sales by the Company of its Shares (as
hereinafter defined). Each Investor shall be entitled to assign or apportion the
right of first offer hereby granted it among itself and its partners and
affiliates (including in the case of a venture capital fund other venture
capital funds affiliated with such fund) in such proportions, as it deems
appropriate. For purposes of this Section 2.4, an Investor's "Pro Rata Share" of
Shares shall mean that number of Shares that equals the proportion that (x) the
number of shares of Common Stock issued and held, or issuable upon conversion of
the Preferred Stock then held, by such Investor (excluding any Special Mandatory

                                      14
<PAGE>

Conversion Shares) bears to (y) the total number of shares of Common Stock of
the Company then outstanding (assuming full conversion and exercise of all
convertible or exercisable securities then outstanding).

          Each time the Company proposes to offer any shares of, or securities
convertible into or exercisable for any shares of, any class of its capital
stock ("Shares"), the Company shall first make an offering of such Shares to
each Investor in accordance with the following provisions:

                    (a)  The Company shall deliver a notice by confirmed
facsimile transmission, certified mail or a nationally recognized overnight
courier service ("Notice") to each of the Investors stating (i) its bona fide
intention to offer such Shares, (ii) the number of such Shares to be offered,
and (iii) the price and a summary of the terms, if any, upon which it proposes
to offer such Shares.

                    (b)  By written notification received by the Company within
ten (10) calendar days after receipt of the Notice, each Investor may elect to
purchase or obtain, at the price and on the terms specified in the Notice, up to
its Pro Rata Share of such Shares. The Company shall promptly, in writing,
inform each Major Investor (as defined below) that elects to purchase all the
shares available to it (a "Fully-Exercising Investor") of any other Major
Investor's failure to do likewise. During the ten (10) day period commencing
after such information is given, each Fully-Exercising Investor may elect to
purchase that portion of the Shares for which Major Investors were entitled to
subscribe but which were not subscribed for by the Major Investors that is equal
to the proportion that the number of shares of Common Stock issued and held, or
issuable upon conversion of Preferred Stock then held, by such Fully-Exercising
Investor bears to the total number of shares of Common Stock issued and held, or
issuable upon conversion of the Preferred Stock then held, by all Fully-
Exercising Investors who wish to purchase some of the unsubscribed shares. For
purposes of this Section 2.4, a Major Investor shall mean any Investor or
transferee that holds at least three hundred thousand (300,000) shares of
Preferred Stock or the Common Stock issued upon conversion thereof (as adjusted
for stock splits, stock dividends, combinations and other recapitalizations).

                    (c)  If all Shares that the Investors are entitled to obtain
pursuant to subsection 2.4(b) are not elected to be obtained as provided in
subsection 2.4(b) hereof, the Company may, during the sixty (60) day period
following the expiration of the period provided in subsection 2.4(b) hereof,
offer the remaining unsubscribed portion of such Shares to any person or persons
at a price not less than, and upon terms no more favorable to the offeree than
those specified in the Notice. If the Company does not enter into an agreement
for the sale of the Shares within such period, or if such agreement is not
consummated within thirty (30) days of the execution thereof, the right provided
hereunder shall be deemed to be revived and such Shares shall not be offered
unless first reoffered to the Investors in accordance herewith.

                    (d)  The right of first offer in this Section 2.4 shall not
be applicable (i) to shares of Common Stock issuable or issued to employees,
consultants or directors pursuant to a stock option plan or restricted stock
plan approved by the Board of Directors or pursuant to a stock purchase
agreement unanimously approved by the Board of Directors, (ii) to shares of
Common Stock issued or issuable in a firm commitment underwritten

                                      15
<PAGE>

public offering, (iii) to shares of Common Stock issued or issuable upon
conversion of shares of Preferred Stock or as a dividend or distribution on the
shares of Preferred Stock, (iv) to securities issued or issuable to banks or
equipment lessors, provided that the principal purpose of such issuances is not
equity financing and such issuances are approved by the Board of Directors, (v)
to securities issued in connection with business combinations or corporate
partnering agreements approved by the Board of Directors, or (vi) to securities
sold pursuant to the Series E Agreement.

               2.5  Board of Directors.
                    ------------------

                    (a)  With respect to those three (3) members of the
Company's Board of Directors that the Restated Certificate of Incorporation
provides are to be elected by the holders of Series B-1 Preferred Stock and
Series B-2 Preferred Stock, voting together as a single class and not as
separate series, and on an as converted basis (the "Series B Board Members"),
the Investors hereby agree to vote all of their shares of Series B-1 Preferred
Stock and Series B-2 Preferred Stock now owned or hereafter acquired in favor of
the election of one designee of each of Sevin Rosen Fund V L.P., CenterPoint
Venture Partners, L.P. and InterWest Partners, collectively the ("Venture
Investors"); provided, however, that:

                         (i)   upon such time as one of the Venture Investors (a
"Force-Converted Venture Investor") has greater than fifty percent (50%) of the
shares of Series B-1 Preferred Stock and Series B-2 Preferred Stock originally
issued to it converted into Common Stock pursuant to Article IV, Section (B),
Paragraph 4(l) of the Restated Certificate of Incorporation (which Paragraph is
entitled "Special Mandatory Conversion"), as such provision may be amended from
time to time, the Investors hereby agree to vote all of their shares of Series
B-1 Preferred Stock and Series B-2 Preferred Stock now owned or hereafter
acquired in favor of the election of three (3) designees designated in the
following manner: one designee of each of the remaining Venture Investors who
are not Force-Converted Venture Investors of the Company (the "Remaining Venture
Investors") and one designee to be mutually agreed upon by the Remaining Venture
Investors.

                         (ii)  upon such time as two (2) of the Venture
Investors are Force-Converted Venture Investors, the Investors hereby agree to
vote all of their shares of Series B-1 Preferred Stock and Series B-2 Preferred
Stock now owned or hereafter acquired in favor of the election of three (3)
designees designated by the Remaining Venture Investor.

                         (iii) upon such time as all of the Venture Investors
are Force-Converted Venture Investors, the Investors hereby agree to vote all of
their shares of Series B-1 Preferred Stock and Series B-2 Preferred Stock now
owned or hereafter acquired in favor of the election of three (3) designees,
such designees to be designated by the holders of at least a majority of the
Preferred Stock and the Common Stock of the Company (voting together as a single
class and not as a separate series, and on an as-converted basis).

                    (b)  With respect to the member of the Company's Board of
Directors that the Restated Certificate of Incorporation provides is to be
elected by the holders of Series C Preferred Stock (the "Series C Board
Member"), the Investors hereby agree to vote all of their shares of Series C
Preferred Stock now owned or hereafter acquired in favor of the election of one
designee of Austin Ventures (collectively with its affiliated entities, "Austin

                                      16
<PAGE>

Ventures"); provided however, that upon such time as Rho Management Trust I
shall own a greater number of shares of Series C Preferred Stock than Austin
Ventures, with respect to the Series C Board Member, the Investors hereby agree
to vote all of their shares of Series C Preferred Stock now owned or hereafter
acquired in favor of the election of one designee acceptable to Rho Management
Trust I or Austin Ventures, whichever shall then own a greater number of shares
of Series C Preferred Stock. Notwithstanding the foregoing, with respect to the
Series C Board Member, the Investors hereby agree to vote all of their shares of
Series C Preferred Stock now owned or hereafter acquired in favor of the
election of an independent industry representative (an "Industry
Representative") reasonably acceptable to each of (i) the holders of sixty
percent (60%) of the shares of the Company's Series C Preferred Stock and (ii)
the Company's Chief Executive Officer (or, if there is no Chief Executive
Officer, the President), in the event that such parties shall agree to elect an
Industry Representative. Upon such time as each of Austin Ventures and Rho
Management Trust I has greater than fifty percent (50%) of the shares of Series
C Preferred Stock originally issued to it converted into Common Stock pursuant
to Article IV, Section (B), Paragraph 4(l) of the Restated Certificate of
Incorporation (which Paragraph is entitled "Special Mandatory Conversion"), as
such provision may be amended from time to time, with respect to the Series C
Board Member, the Investors hereby agree to vote all of their shares of Series C
Preferred Stock now owned or hereafter acquired in favor of the election of the
designee designated by the holders of at least a majority of the Preferred Stock
and the Common Stock of the Company (voting together as a single class and not
as a separate series, and on an as-converted basis).

                    (c)  With respect to the remaining members of the Company's
Board of Directors, that the Restated Certificate of Incorporation provides are
to be elected by the holders of Series A Preferred Stock, Series B-1 Preferred
Stock, Series B-2 Preferred Stock, Series C Preferred Stock, Series D Preferred
Stock, Series E Preferred Stock and Common Stock, voting together as a single
class and not as separate series, and on an as converted basis (the "Remaining
Board Members"), the Founders and the Investors hereby agree to vote all of
their shares of Series A Preferred Stock, Series B-1 Preferred Stock, Series B-2
Preferred Stock, Series C Preferred Stock, Series D Preferred Stock, Series E
Preferred Stock and Common Stock now owned or hereafter acquired so that the
first of the Remaining Board Members to be elected is the Company's Chief
Executive Officer (or, if there is no Chief Executive Officer, the President).

               2.6  Observer Rights. As long as Azure Capital Partners ("Azure")
                    ---------------
owns not less than fifty percent (50%) of the shares of the Series E Preferred
Stock it is purchasing hereunder (or an equivalent amount of Common Stock issued
upon conversion thereof), the Company shall invite a representative of Azure to
attend all meetings of its Board of Directors in a nonvoting observer capacity
and, in this respect, shall give such representative copies of all notices,
minutes, consents, and other materials that it provides to its directors;
provided, however, that such representative shall agree to hold in confidence
and trust and to act in a fiduciary manner with respect to all information so
provided; and, provided further, that the Company reserves the right to withhold
any information and to exclude such representative from any meeting or portion
thereof if access to such information or attendance at such meeting could
adversely affect the attorney-client privilege between the Company and its
counsel or would result in disclosure of trade secrets to such representative or
if such Investor or its representative is a direct competitor of the Company.

                                      17
<PAGE>

               2.7  Proprietary Information and Inventions Agreement.  The
                    ------------------------------------------------
Company agrees to cause each employee and officer of the Company to enter into a
proprietary information and inventions agreement in a form approved by the
Company's Board of Directors, which form shall include, among other things, a
provision prohibiting such person from soliciting any employees of the Company
for one year after the termination of such person's service with the Company.
The Company agrees to cause each director of the Company to enter into an
agreement prohibiting such director from disclosing any of the Company's
confidential information or from soliciting any employees or customers of the
Company for one year after the termination of such director's service on the
Company's Board of Directors.

               2.8  Key-Man Insurance.  At the discretion of the Company's Board
                    -----------------
of Directors, the Company agrees to use its best efforts to obtain from
financially sound and reputable insurers term life insurance on such employees
and in such amounts as shall be determined by the Board of Directors provided
that such insurance is available at normally prevailing rates for persons in
good health.  Such policies shall name the Company as sole loss payee and shall
not be cancelable by the Company without prior approval of the Board of
Directors.

               2.9  Assignments of Rights of First Refusal.  So long as any
                    --------------------------------------
shares of Preferred Stock are outstanding, the Company agrees that, as a
condition to issuing any shares of Common Stock to any employee or director or
other service provider of the Company under the Company's then applicable stock
option plan, such employee or director shall be required to enter into an
agreement with the Company that shall provide the Company, or any assignee or
assignees of the Company, with a right of first refusal, in the form set forth
in the Company's 1997 Stock Plan, to purchase any shares that such employee or
director proposes to sell or transfer to a person other than the Company.  The
Company further covenants and agrees that, in the event (i) an employee or
director or other service provider proposes to sell or transfer such shares to a
person other than the Company, (ii) the Company has not sold shares of the
Company's capital stock in an offering registered under the Act and (iii) the
Company has determined not to elect to exercise its right of first refusal to
purchase all of the shares that are proposed to be sold or transferred by such
employee or director or service provider (such balance of the shares not elected
to be purchased by the Company being the "Available Shares"), then the Company
agrees that it shall assign its right of first refusal to purchase the Available
Shares to the Investors by notice to the Investors made at least ten (10)
business days prior to the expiration of the Company's right of first refusal,
and each Investor shall thereafter have the right to elect to exercise such
right of first refusal to purchase its proportionate share of the Available
Shares based on the proportion that the number of shares of Registrable
Securities then held by such Investor bears to the aggregate number of shares of
Registrable Securities then held by all Investors (excluding the Founders).  The
exercise of such right of first refusal by the Investors shall be made subject
to and in compliance with the terms applicable to the right of first refusal in
favor of the Company as set forth in the applicable agreements used under the
applicable plan.

               2.10 Indemnification Agreement.  The Company shall enter into its
                    -------------------------
form of Indemnification Agreement with the Series C Board Member on or as
promptly as possible after the date hereof and with any successor Series C Board
Member as soon as then practicable.

                                      18
<PAGE>

          3.   Miscellaneous.
               -------------

               3.1  Successors and Assigns.  Except as otherwise provided
                    ----------------------
herein, the terms and conditions of this Agreement shall inure to the benefit of
and be binding upon the respective successors and assigns of the parties
(including transferees of any shares of Registrable Securities). Nothing in this
Agreement, express or implied, is intended to confer upon any party other than
the parties hereto or their respective successors and assigns any rights,
remedies, obligations, or liabilities under or by reason of this Agreement,
except as expressly provided in this Agreement.

               3.2  Governing Law.  This Agreement shall be governed by and
                    -------------
construed under the laws of the State of Delaware.

               3.3  Counterparts.  This Agreement may be executed in two or more
                    ------------
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

               3.4  Titles and Subtitles.  The titles and subtitles used in this
                    --------------------
Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement.

               3.5  Notices.  Unless otherwise provided, any notice required or
                    -------
permitted under this Agreement shall be given in writing and shall be deemed
effectively given upon personal delivery to the party to be notified or upon
delivery by confirmed facsimile transmission or nationally recognized overnight
courier service or upon deposit with the United States Post Office, by
registered or certified mail, postage prepaid and addressed to the party to be
notified at the address indicated for such party on the signature page hereof,
or at such other address as such party may designate by ten (10) days' advance
written notice to the other parties.

               3.6  Expenses. If any action at law or in equity is necessary to
                    --------
enforce or interpret the terms of this Agreement, the prevailing party shall be
entitled to reasonable attorneys' fees, costs and necessary disbursements in
addition to any other relief to which such party may be entitled.

               3.7  Amendments and Waivers. Any term of this Agreement may be
                    ----------------------
amended and the observance of any term of this Agreement may be waived (either
generally or in a particular instance and either retroactively or
prospectively), only with the written consent of the Company and the holders of
at least a majority of the Registrable Securities then outstanding (excluding
any Registrable Securities held by the Founders); provided, however, that in the
event such amendment or waiver adversely affects the rights and/or obligations
of the Founders under Section 1 of this Agreement in a different manner than all
other Holders, such amendment or waiver shall also require the written consent
of the holders of at least a majority of the Common Stock (assuming the
conversion of all outstanding shares of Series A Preferred Stock) then held by
the Founders.

                                      19
<PAGE>

               3.8  Severability.  If one or more provisions of this Agreement
                    ------------
are held to be unenforceable under applicable law, such provision shall be
excluded from this Agreement, and the balance of the Agreement shall be
interpreted as if such provision were so excluded, and shall be enforceable in
accordance with its terms.

               3.9  Aggregation of Stock.  All shares of Registrable Securities
                    --------------------
of the Company held or acquired by a stockholder and its affiliates shall be
aggregated together for the purpose of determining the availability of any
rights under this Agreement.  For purposes of the foregoing, the shares held by
any stockholder that (i) is a partnership, corporation or other similar entity
shall be deemed to include shares held by affiliated partnerships or the
partners, retired partners, stockholders and constituent members of such holder
or members of the "immediate family" (as defined below) of any such partners,
retired partners, stockholders and constituent members, and any custodian or
trustee for the benefit of any of the foregoing persons; (ii) is an  Investment
Company (as defined in the Investment Company Act) shall be deemed to include
shares held by any other Investment Company with the same Investment Advisors
(as defined in the Investment Company Act); and (iii) is an individual shall be
deemed to include shares held by any members of the stockholder's immediate
family ("immediate family" shall include any spouse, father, mother, brother,
sister, lineal descendant of spouse or lineal descendant) or to any custodian or
trustee for the benefit of any of the foregoing persons.

               3.10 Additional Parties. In the event of a subsequent closing
                    ------------------
with a purchaser as provided for in Section 1.3 of the Series E Agreement, such
purchaser shall become a party to this Agreement as an "Investor" upon receipt
from such purchaser of a fully executed signature page hereto.

               3.11 Delays or Omissions.  No delay or omission to exercise any
                    -------------------
right, power or remedy accruing to any party to this Agreement, upon any breach
or default of the other party, shall impair any such right, power or remedy of
such non-breaching party nor shall it be construed to be a waiver of any such
breach or default, or an acquiescence therein, or of any similar breach or
default thereafter occurring; nor shall any waiver of any single breach or
default be deemed a waiver of any other breach or default theretofore or
thereafter occurring. Any waiver, permit, consent or approval of any kind or
character on the part of any party of any breach or default under this
Agreement, or any waiver on the part of any party of any provisions or
conditions of this Agreement, must be made in writing and shall be effective
only to the extent specifically set forth in such writing.  All remedies, either
under this Agreement, or by law or otherwise afforded to any Holder, shall be
cumulative and not alternative.

               3.12 Entire Agreement.  This Agreement and the documents referred
                    ----------------
to herein constitute the entire agreement between the parties hereto pertaining
to the subject matter hereof and any other written or oral agreements between
the parties hereto are expressly canceled.

               3.13 Prior Agreement.  The Prior Agreement is hereby superseded
                    ---------------
in its entirety and shall be of no further force or effect.

                                      20
<PAGE>

          IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first above written.


                                             CHORUM TECHNOLOGIES INC.



                                             By:     /s/ Scott Grout
                                                ------------------------------
                                                Scott Grout
                                                President

                                   Address:  1303 East Arapaho Road
                                             Richardson, Texas 75081



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                            INVESTORS:

                                            AZURE CAPITAL PARTNERS



                                            By:     /s/ Martin Brusco
                                               -----------------------------
                                            Name:  Martin Brusco
                                            Title: Chief Financial Officer

                                Address:    c/o Paul Weinstein
                                            650 California Street, 11/th/ Floor
                                            San Francisco, CA 94108

                                Telephone:  (415) 276-5510
                                Facsimile:  (415) 276-5590



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        OCTANE CAPITAL FUND I, L.P.


                                        By:  Octane Capital Management, LLC,
                                              Its General Partner


                                        By:      /s/ Emeric J. McDonald
                                           -----------------------------------
                                           Emeric J. McDonald, Manager


                            Address:    c/o Tim Murphy
                                        1 Maritime Plaza, Suite 2555
                                        San Francisco, CA 94111

                            Telephone:  (415) 277-2978
                            Facsimile:  (415) 986-1312



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        TAILWIND CAPITAL PARTNERS 2000, L.P.


                                        By: Thomas Weisel Capital Partners LLC
                                            Its General Partner


                                        By:       /s/ Marianne Winkler
                                           ------------------------------------
                                        Name:   Marianne Winkler
                                        Title:  Chief Financial Officer

                            Address:    Pacific Telesis Tower
                                        One Montgomery Street
                                        San Francisco, CA 94104

                            Telephone:  (415) 364-2500
                            Facsimile:  (415) 364-5980



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                              /s/ David Welch
                                        -----------------------------
                                        David Welch

                            Address:    1845 White Oak Drive
                                        Menlo Park, CA  94025

                            Telephone:  (650) 326-9339
                            Facsimile:  (650) 328-5938



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   BOWMAN CAPITAL CLIPPER FUND, L.P.,

                                   By:  Bowman Capital Management, LLC,
                                        its General Partner


                                   By:        /s/ Eric Moore
                                      ----------------------------------
                                        Eric Moore
                                        Controller

                       Address:    1875 South Grant Street, Suite 600
                                   San Mateo, California  94402
                       Telephone:  (650) 287-2277
                       Facsimile:  (650) 522-8497


                                   BOWMAN CAPITAL CROSSOVER FUND, L.P.,

                                   By:  Bowman Capital Management, LLC,
                                        its General Partner


                                   By:        /s/ Eric Moore
                                      ----------------------------------
                                        Eric Moore
                                        Controller

                       Address:    1875 South Grant Street, Suite 600
                                   San Mateo, California  94402
                       Telephone:  (650) 287-2277
                       Facsimile:  (650) 522-8497


                                   BOWMAN CAPITAL CROSSOVER FUND "A", L.P.,

                                   By:  Bowman Capital Management, LLC,
                                        its General Partner


                                   By:        /s/ Eric Moore
                                      ----------------------------------
                                        Eric Moore
                                        Controller

                       Address:    1875 South Grant Street, Suite 600
                                   San Mateo, California  94402
                       Telephone:  (650) 287-2277
                       Facsimile:  (650) 522-8497



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        AUSTIN VENTURES V, L.P.

                                        By:  AV Partners V, L.P.,
                                        Its: General Partner


                                        By:         /s/ Ed Olkkola
                                           ---------------------------------
                                             Ed Olkkola
                                             General Partner

                            Address:    114 West 7/th/ Street, Suite 1300
                                        Austin, Texas  78701
                            Telephone:  (512) 479-0055
                            Facsimile:  (512) 485-1995


                                        AUSTIN VENTURES V AFFILIATES
                                        FUND, L.P.

                                        By:  AV Partners V, L.P.
                                        Its: General Partner


                                        By:         /s/ Ed Olkkola
                                           ---------------------------------
                                             Ed Olkkola
                                             General Partner

                            Address:    114 West 7/th/ Street, Suite 1300
                                        Austin, Texas  78701
                            Telephone:  (512) 479-0055
                            Facsimile:  (512) 485-1995



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        PARSON FINANCE LIMITED

                                        By:  Crown Advisors International Ltd.,
                                             Investment Management


                                        By:       /s/ David F. Bellet
                                           ------------------------------
                                           David F. Bellet
                                           Chairman

                            Address:    Lincoln Building
                                        Suite 3405
                                        60 East 42/nd/ Street
                                        New York, New York 10165
                            Telephone:  (212) 808-5278
                            Facsimile:  (212) 808-9073



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        CROWN GROWTH PARTNERS, LP

                                        By: Crown Growth Partners, LLC


                                        By:      /s/ David F. Bellet
                                           ------------------------------
                                           David F. Bellet
                                           General Partner


                            Address:    Lincoln Building
                                        Suite 3405
                                        60 East 42/nd/ Street
                                        New York, New York 10165
                            Telephone:  (212) 808-5278
                            Facsimile:  (212) 808-9073



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        /s/ David F. Bellet
                                        ------------------------------
                                        David F. Bellet

                            Address:    Lincoln Building
                                        Suite 3405
                                        60 East 42/nd/ Street
                                        New York, New York 10165
                            Telephone:  (212) 808-5278
                            Facsimile:  (212) 808-9073



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        ROUNDTABLE ASSOCIATES LLC



                                        By:       /s/ David F. Bellet
                                           -------------------------------
                                           David F. Bellet
                                           Managing Member

                            Address:    Lincoln Building
                                        Suite 3405
                                        60 East 42/nd/ Street
                                        New York, New York 10165
                            Telephone:  (212) 808-5278
                            Facsimile:  (212) 808-9073



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        RHO MANAGEMENT TRUST I


                                        By:  Rho Management Company, Inc.,
                                               as investment advisor


                                        By:        /s/ Peter Kalkanis
                                           --------------------------------
                                           Name:  Peter Kalkanis
                                           Title: CFO

                            Address:    c/o Peter Kalkanis
                                        152 West 57th Street, 23rd Floor
                                        New York, New York 10019
                            Telephone:  (212) 751-6677
                            Facsimile:  (212) 826-3692



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        CG ASIAN-AMERICAN FUND, L.P.

                                        By:  CG Asian-American Partners, L.P.
                                        Its: General Partner


                                        By:  Sycamore Management Corporation
                                        Its: General Partner


                                        By:     /s/ Simon Wong
                                           -----------------------------
                                           Simon Wong
                                           Vice President and Partner
                            Address:    Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                              /s/ Kit C. Wong
                                        --------------------------------
                                        Kit C. Wong
                                        Attorney-In-Fact

                            Address:    c/o Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                                  /s/ Kilin To
                                        -------------------------------
                                        Kilin To
                                        Attorney-in-Fact

                            Address:    c/o Kit C. Wong
                                        Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                                /s/ Simon Wong
                                        -------------------------------
                                        Simon Wong

                            Address:    c/o Kit C. Wong
                                        Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                                  /s/ Subir Ray
                                        -------------------------------
                                        Subir Ray
                                        Attorney-In-Fact

                              Address:  c/o Kit C. Wong
                                        Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                                /s/ John Witman
                                        ------------------------------
                                        John Whitman
                                        Attorney-In-Fact

                            Address:    c/o Kit C. Wong
                                        Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                                /s/ Peter Jerry
                                        --------------------------------
                                        Peter Gerry
                                        Attorney-In-Fact

                            Address:    c/o Kit C. Wong
                                        Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101




                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             /s/ David Lichtenstein
                                        ---------------------------------
                                        David Lichtenstein
                                        Attorney-In-Fact

                            Address:    c/o Kit C. Wong
                                        Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                              /s/ Richard Chong
                                        --------------------------------
                                        Richard Chong
                                        Attorney-In-Fact

                            Address:    c/o Kit C. Wong
                                        Sycamore Ventures
                                        989 Lenox Drive, Suite 208
                                        Lawrenceville, New Jersey 08648
                            Telephone:  (609) 219-0100
                            Facsimile:  (609) 219-0101



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                               /s/ Dennis Gorman
                                        -----------------------------
                                        Dennis Gorman

                            Address:    3540 Ranchero Road
                                        Plano, Texas 75093

                            Telephone:  (972) 378-1514
                            Facsimile:  (972) 378-1271



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        SEVIN ROSEN FUND V L.P.

                                        By:  SRB Associates V L.P.
                                             Its General Partner


                                        By:        /s/ Jon W. Bayless
                                           ----------------------------------
                                             Jon W. Bayless
                                             General Partner


                                        SEVIN ROSEN V AFFILIATES FUND L.P.

                                        By:  SRB Associates V L.P.
                                             Its General Partner


                                        By:        /s/ Jon W. Bayless
                                           ----------------------------------
                                             Jon W. Bayless
                                             General Partner


                                        SEVIN ROSEN FUND VI L.P.

                                        By:  SRB Associates VI L.P.
                                             Its General Partner


                                        By:        /s/ Jon W. Bayless
                                           ----------------------------------
                                             Jon W. Bayless
                                             General Partner


                                        SEVIN ROSEN VI AFFILIATES FUND L.P

                                        By:  SRB Associates VI L.P.
                                             Its General Partner


                                        By:        /s/ Jon W. Bayless
                                           ----------------------------------
                                             Jon W. Bayless
                                             General Partner



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                        SEVIN ROSEN FUND VII L.P.

                                        By:  SRB Associates VII L.P.
                                             Its General Partner


                                        By:      /s/ Jon W. Bayless
                                           -------------------------------
                                             Jon W. Bayless
                                             General Partner


                                        SEVIN ROSEN VII AFFILIATES FUND L.P.

                                        By:  SRB Associates VII L.P.
                                             Its General Partner


                                        By:  /s/ Jon W. Bayless
                                           -------------------------------
                                             Jon W. Bayless
                                             General Partner

                            Address:    c/o The Sevin Rosen Funds
                                        13455 Noel Road, Suite 1670
                                        Dallas, Texas 75240
                            Telephone:  (972) 702-1100
                            Facsimile:  (972) 702-1103



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                          SEVIN ROSEN BAYLESS MANAGEMENT COMPANY


                                          By: /s/ Jon W. Bayless
                                             ------------------
                                          Name: Jon W. Bayless
                                          Title: General Partner

                              Address:    c/o Jon W. Bayless
                                          The Sevin Rosen Funds
                                          13455 Noel Road, Suite 1670
                                          Dallas, Texas 75240
                              Telephone:  (972) 702-1100
                              Facsimile:  (972) 702-1103

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                          THE L.J. SEVIN TRUST FUND


                                          By: /s/ L.J. Sevin
                                             ---------------
                                          Name: L.J. Sevin
                                          Title: Trustee

                              Address:    c/o The Sevin Rosen Funds
                                          13455 Noel Road, Suite 1670
                                          Dallas, Texas 75240
                              Telephone:  (972) 702-1100
                              Facsimile:  (972) 702-1103

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                           /s/ Jean Denzel, Attorney-in-Fact
                                          ----------------------------------
                                          Benjamin M. Rosen

                              Address:    One Central Park West, Suite 43A
                                          New York, NY 10023
                              Telephone:  (212) 315-0383
                              Facsimile:  (212) 315-0610


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                          CENTERPOINT VENTURE PARTNERS L.P.
                                          A Delaware limited partnership

                                          By: Paluck Associates L.P.,
                                              A Delaware limited partnership
                                              General Partner

                                          By: /s/ Cam McMartin
                                             ------------------------
                                             Cam McMartin
                                             General Partner and CFO

                              Address:    Two Galleria Tower
                                          13455 Noel Road, Suite 1670
                                          Dallas, Texas 75240
                              Telephone:  (972) 702-1101
                              Facsimile:  (972) 702-1103

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                          CENTERPOINT VENTURE FUND II, L.P.

                                          By:  Centerpoint Associates II, L.P.,
                                               Its: General Partner

                                          By:  Centerpoint Associates Management
                                               II, L.L.C.
                                          Its: General Partner



                                          By: /s/ Cam McMartin
                                             ------------------------
                                             Cam McMartin
                                             General Partner and CFO

                              Address:    Two Galleria Tower
                                          13455 Noel Road, Suite 1670
                                          Dallas, Texas 75240
                              Telephone:  (972) 702-1101
                              Facsimile:  (972) 702-1103

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                      INTERWEST PARTNERS VI, L.P.,
                                      a California limited partnership

                                      By: InterWest Management Partners VI, LLC,
                                          a California limited liability
                                          company, its general partner



                                      By:  /s/ Philip T. Gianos
                                          --------------------
                                          Philip T. Gianos
                                          Managing Member



                                      INTERWEST INVESTORS VI, LP,
                                      a California limited partnership

                                      By: Interwest Management Partners VI, LLC,
                                          a California limited liability
                                          company, its general partner


                                      By:   /s/ Philip T. Gianos
                                          -----------------------
                                          Philip T. Gianos
                                          Managing Member

                           Address:   3000 Sand Hill Road, #3-255
                                      Menlo Park, California 94025
                           Telephone: (650) 854-8585
                           Facsimile: (650) 854-4706

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                      INTERWEST PARTNERS VII L.P.,
                                      a California limited partnership

                                      By: InterWest Management Partners VI, LLC,
                                          a California limited liability
                                          company, its general partner



                                      By:   /s/ Philip T. Gianos
                                          ----------------------
                                           Philip T. Gianos
                                           Managing Member



                                      INTERWEST INVESTORS VII, LP,
                                      a California limited partnership

                                      By: Interwest Management Partners VI, LLC,
                                          a California limited liability
                                          company, its general partner


                                      By:   /s/ Philip T. Gianos
                                          ------------------------
                                           Philip T. Gianos
                                           Managing Member


                           Address:   3000 Sand Hill Road, #3-255
                                      Menlo Park, California 94025
                           Telephone: (650) 854-8585
                           Facsimile: (650) 854-4706

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                      G & H PARTNERS

                                      By: /s/ Kirk E. Lundburg
                                          ---------------------------
                                          Kirk E. Lundburg
                                          Partner

                           Address:   2700 Via Fortuna, Suite 300
                                      Austin, TX 78746
                           Telephone: (512) 732-8400
                           Facsmile:  (512) 732-0081

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                       /s/ Dietrich R. Erdmann
                                      ------------------------
                                      Dietrich R. Erdmann

                            Address:  Rengestrasse 36
                                      6052 Hergiswil
                                      Switzerland
            Telephone and Facsimile:  41 41 6301372

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                       /s/ Steven J. Wallach
                                      ----------------------
                                      Steven J. Wallach

                            Address:  7314 Wester Way
                                      Dallas, Texas 75248
                          Telephone:  (972) 644-4111 x206
                          Facsimile:  (972) 644-4101

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                                         /s/ Frank J. Kozel
                                                        ------------------
                                                        Frank J. Kozel

                                             Address:   6024 Oakcrest Road
                                                        Dallas, Texas 75248
                                             Telephone: (972) 788-5332
                                             Facsimile: (972) 788-5291

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                    DAIN RAUSCHER WESSELS INVESTORS, L.L.C.

                                    By: Dain Rauscher Incorporated, its managing
                                        member

                                    By: /s/ Mary Zimmer
                                        ---------------
                                        Mary Zimmer
                                        Director of Finance and Administration,
                                        Dain Rauscher Wessels, A Division
                                        of Dain Rauscher Incorporated

                              Address:  Dain Rauscher Wessels
                                        60 South 6/th/ Street
                                        Minneapolis, Minnesota 55402

                            Telephone:  (612) 371-2708
                            Facsimile:  (612) 373-1610

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                         PANTHEON PARTNERS LP


                                          By: /s/ Kelly Pan
                                             -----------------
                                            Kelly Pan
                                            Managing Principal

                               Address:   c/o Kelly Pan
                                          Pantheon Capital Management
                                          880 Third Avenue, 16/th/ floor
                                          New York, New York 10022

                               Telephone: (212) 350-7208
                               Facsimile: (212) 350-7296

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                       JAMES STABLEFORD


                                       By:  /s/ James Stableford
                                          ----------------------
                                       Name: James Stableford

                           Address:    c/o Amerindo Advisors (UK) Ltd.
                                       43 Upper Grosvenor Street
                                       London, England WIX 9PG

                           Telephone:  011-44207-499-5410
                           Facsimile:  011-44207-499-5166

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT

<PAGE>

                                           JOAQUIN GARCIA-LARRIEU


                                           By: /s/ Joaquin Garcia-Larrieu
                                               --------------------------
                                           Name: Joaquin Garcia-Larrieu

                                Address:   c/o Amerindo Investment Advisors Inc.
                                           399 Park Avenue, 22/nd/ Floor
                                           New York, NY 10022

                                Telephone: (212) 371-6360
                                Facsimile: (212) 317-9382

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                           MARC WEISS


                                           By: /s/ Marc Weiss
                                               --------------
                                           Name:  Marc Weiss

                                Address:   c/o Amerindo Investment Advisors Inc.
                                           399 Park Avenue, 22/nd/ Floor
                                           New York, NY 10022

                                Telephone: (212) 371-6360
                                Facsimile: (212) 656-1396

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                           DANIEL CHAPEY


                                           By: /s/ Daniel Chapey
                                              -------------------
                                           Name: Daniel Chapey

                                Address:   c/o Amerindo Investment Advisors Inc.
                                           One Embarcadero Center, Suite 2300
                                           San Francisco, CA 94111-3162

                                Telephone: (415) 249-1528
                                Facsimile: (415) 249-1552

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                               /s/ Matthew O.Fitzmaurice
                                              ---------------------------
                                              Matthew O. Fitzmaurice

                                   Address:   150 West Lake Street, Suite 106
                                              Wayzata, MN 55391

                                   Telephone: (612) 249-0104
                                   Facsimile: (612) 249-0105

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                          VERTEX CAPITAL II, LLC



                                          By: /s/ Matthew Fitzmaurice
                                             -----------------------
                                             Name: Matthew Fitzmaurice
                                             Title: Manager

                               Address:   c/o Amerindo Investment Advisors, Inc.
                                          150 West Lake Street
                                          Wayzata, MN 55391

                               Telephone: (612) 249-0104
                               Facsimile: (612) 249-0105

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                          VERTEX CAPITAL III, LLC



                                          By: /s/ Matthew Fitzmaurice
                                              -----------------------
                                          Name: Matthew Fitzmaurice
                                          Title: Manager

                               Address:   c/o Amerindo Investment Advisors, Inc.
                                          50 West Lake Street
                                          Wayzata, MN 55391

                               Telephone: (612) 249-0104
                               Facsimile: (612) 249-0105

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                     AMERINDO TECHNOLOGY GROWTH FUND II INC.



                                     By:       /s/ Gary Tanaka
                                        --------------------------------------
                                     Name:  Gary Tanaka
                                     Title:  Director

                          Address:   c/o Amerindo Investment Advisors, Inc.
                                     Attn:  Jessica Caruso
                                     399 Park Avenue, 22/nd/ Floor
                                     New York, NY 10022

                          Telephone: (212) 371-6360
                          Facsimile: (212) 980-5118

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                AMERINDO INTERNET FUND PLC



                                By:       /s/ Gary Tanaka
                                   --------------------------------------
                                Name:  Gary Tanaka
                                Title:  Manager

                     Address:   c/o Amerindo Investment Advisors, Inc.
                                399 Park Avenue, 22/nd/ Floor
                                New York, NY 10022

                     Telephone: (212) 371-6360
                     Facsimile: (212) 980-5118

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                EMERGING TECHNOLOGY PORTFOLIO



                                By:       /s/ Gary Tanaka
                                   --------------------------------------
                                Name:  Gary Tanaka
                                Title:  Attorney-in-Fact

                     Address:   c/o Amerindo Investment Advisors, Inc.
                                Attn:  Jessica Caruso
                                399 Park Avenue, 22/nd/ Floor
                                New York, NY 10022

                     Telephone: (212) 371-6360
                     Facsimile: (212) 980-5118

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                    LITTON MASTER TRUST



                                    By:       /s/ Gary Tanaka
                                       --------------------------------------
                                    Name:  Gary Tanaka
                                    Title:  Attorney-in-Fact

                         Address:   c/o Amerindo Investment Advisors, Inc.
                                    Attn:  Jessica Caruso
                                    399 Park Avenue, 22/nd/ Floor
                                    New York, NY 10022

                         Telephone: (212) 371-6360
                         Facsimile: (212) 980-5118

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                    MITCHELL BARTLETT



                                    By:       /s/ Mitchell Bartlett
                                       --------------------------------------
                                    Name:  Mitchell Bartlett

                         Address:   c/o Amerindo Investment Advisors Inc.
                                    150 West Lake Street
                                    Wayzata, MN 55391

                         Telephone: (612) 249-0864
                         Facsimile: (612) 249-0105

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                    WILLIAM F. HARTFIEL, III



                                    By:       /s/ William F. Hartfiel, III
                                       --------------------------------------
                                    Name:  William F. Hartfiel, III

                         Address:   c/o Amerindo Investment Advisors Inc.
                                    150 West Lake Street
                                    Wayzata, MN 55391

                         Telephone: (612) 249-0104
                         Facsimile: (612) 249-0105

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                             STAR GROWTH ENTERPRISE, A GERMAN
                             CIVIL LAW PARTNERSHIP
                             (with limitation of liability)


                             By:      /s/ Dr. Meir Barel
                                ---------------------------------------------
                             By: SVM Star Ventures
                                 Managementgesellschaft mbH Nr. 3
                                 Managing Partner
                                 By:  Dr. Meir Barel - Managing Director

                  Address:   c/o Star Ventures Management
                             Possartstrasse 9
                             D-81679 Munich
                             Germany

                  Telephone: 011+49-89-4194-30-0
                  Facsimile: 011+49-89-4194-30-30


                             SVM STAR VENTURES
                             MANAGEMENTGESELLSCHAFT MBH NR. 3


                             By:   /s/ Dr. Meir Barel
                                ---------------------------------------------
                                      By:  Dr. Meir Barel - Managing Director

                  Address:   c/o Star Ventures Management
                             Possartstrasse 9
                             D-81679 Munich
                             Germany

                  Telephone: 011+49-89-4194-30-0
                  Facsimile: 011+49-89-4194-30-30

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                         /s/ John Ryan
                                    ----------------------------------------
                                    John Ryan

                         Address:   836 Flaxberry Lane
                                    San Rafael, CA 94903

                         Telephone: (415) 491-0559
                         Facsimile: (415)

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   /s/ Peter Hankin
                              -----------------------------------------
                              Peter Hankin

                   Address:   813 Melville Avenue
                              Palo Alto, CA 94301

                 Telephone:   (650) 325-0732
                 Facsimile:   (650)

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                         /s/ Linda W. Seale
                                    ----------------------------------------
                                    Linda Seale

                         Address:   813 Melville Avenue
                                    Palo Alto, CA 94301

                         Telephone: (650) 325-0732
                         Facsimile: (650)

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                         /s/ James Kent
                                    ----------------------------------------
                                    James Kent

                         Address:   790 Vespucci Lane
                                    Foster City, CA 94404

                         Telephone: (650) 341-8665
                         Facsimile: (650) 561-9767

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                     /s/ James Kent
                               ----------------------------------------
                               San Francisco Investment Partners

                    Address:   30 Sunrise Court, Suite 100
                               Menlo Park, CA  94025

                    Telephone: (650) 341-8665
                    Facsimile: (650) 561-9767

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                     /s/ Joel Jakubson
                                 ----------------------------------------
                                 Joel Jakubson

                      Address:   30 Sunrise Court, Suite 100
                                 Menlo Park, CA 94025

                      Telephone: (650) 341-8665
                      Facsimile: (650) 561-9767

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                               LIU-WANG PARTNERS, LTD.



                               By:           /s/ Jian-Yu Liu
                                  -------------------------------------
                               Name:  Jian-Yu Liu
                               Title:

                    Address:   2721 Woods Lane
                               Garland, Texas 75044

                    Telephone: (972) 495-2418
                    Facsimile:

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             /s/ Thomas H. Aschenbrenner
                                    ----------------------------------------
                                    Thomas H. Aschenbrenner

                         Address:   6016 Oakcrest
                                    Dallas, Texas 75248

                         Telephone: (972) 392-0840
                         Facsimile: (972) 702-1103

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                          /s/ Kuang-Yi Wu
                                 ----------------------------------------
                                 Kuang-Yi Wu

                      Address:   c/o Chorum Technologies
                                 1303 E. Arapaho Road
                                 Richardson, Texas 75081

                      Telephone: (972)
                      Facsimile: (972)

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                 FOUNDERS:

                                 LIU-WANG PARTNERS, LTD.

                                          /s/ Pei-Lan Wang
                                 ----------------------------------------
                                 Pei-Lan Wang

                      Address:   1155 East Collins Boulevard
                                 Suite 200
                                 Richardson, Texas 75081
                      Telephone: (972) 238-1770
                      Facsimile: (972) 238-1477

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                    LIU-WANG PARTNERS, LTD.


                                             /s/ Jian-Yu Liu
                                    ----------------------------------------
                                    Jian-Yu Liu

                         Address:   1155 East Collins Boulevard
                                    Suite 200
                                    Richardson, Texas 75081
                         Telephone: (972) 238-1770
                         Facsimile: (972) 238-1477

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                           /s/ Kuang-Yi Wu
                                           -------------------------------
                                           Kuang-Yi Wu

                                 Address:  1155 East Collins Boulevard
                                           Suite 200
                                           Richardson, Texas 75081

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   DRW VENTURE PARTNERS, LP

                                   By:  Dain Rauscher Corporation, its General
                                        Partner


                                   By: /s/ Mary Zimmer
                                      ------------------------------------
                                      Mary Zimmer
                                      Director of Finance and Administration,
                                      Dain Rauscher Wessels, A Division
                                      of Dain Rauscher Incorporated


                         Address:  Dain Rauscher Wessels
                                   60 South 6/th/ Street
                                   Minneapolis, Minnesota 55402

                       Telephone:  (612) 371-2708
                       Facsimile:  (612) 373-1610

                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Securities Fund:
                                   Fidelity Blue Chip Growth Fund


                                   By: /s/ John H. Costello
                                      ---------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617)476-7774


             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Capital Trust:
                                   Fidelity Capital Appreciation Fund


                                   By: /s/ John H. Costello
                                      --------------------------------------
                                   Name: John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Advisor Series VII:
                                   Fidelity Advisor Technology Fund


                                   By: /s/ John H. Costello
                                      ---------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Advisor Series I:
                                   Fidelity Advisor Equity Growth Fund


                                   By: /s/ John H. Costello
                                      --------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Advisor Series I:
                                   Fidelity Advisor Retirement Growth Fund


                                   By: /s/ John H. Costello
                                      ---------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Advisor Series I:
                                   Fidelity Advisor Small Cap Fund


                                   By: /s/ John H. Costello
                                      --------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Destiny Portfolios:
                                   Destiny II


                                   By: /s/ John H. Costello
                                      ------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Financial Trust:
                                   Fidelity Retirement Growth Fund


                                   By: /s/ John H. Costello
                                      --------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Hastings Street Trust:
                                   Fidelity Contrafund II


                                   By: /s/ John H. Costello
                                      ---------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Mt. Vernon Street Trust:
                                   Fidelity Aggressive Growth Fund


                                   By: /s/ John H. Costello
                                      --------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Select Portfolios:
                                   Technology Portfolio


                                   By: /s/ John H. Costello
                                      -------------------------------------
                                   Name: John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Advisor Series I:
                                   Fidelity Advisor Value Strategies Fund


                                   By: /s/ John H. Costello
                                      --------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                   INVESTORS:


                                   Fidelity Mt. Vernon Street Trust:
                                   Fidelity New Millennium Fund


                                   By: /s/ John H. Costello
                                      ---------------------------------------
                                   Name:  John H. Costello
                                   Title: Assistant Treasurer

                         Address:  82 Devonshire Street, E20E
                                   Boston, MA 02109
                                   Attn: Travis Rhodes, Esq.

                       Telephone:  (617) 563-2599
                       Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:


                                             Variable Insurance Products Fund:
                                             Growth Portfolio


                                             By:     /s/ John H. Costello
                                                ------------------------------
                                             Name:  John H. Costello
                                             Title: Assistant Treasurer

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn:  Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:

                                             Fidelity Focus Technology Fund:


                                             By:     /s/ John H. Costello
                                                -------------------------
                                             Name: John H. Costello
                                             Title: Authorized Signatory

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn: Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (617) 476-7774

             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:


                                             Edward C. Johnson Fund



                                             By:   /s/ Richard Weidmann
                                                ------------------------------
                                             Name: Richard Weidmann
                                             Title: Treasurer

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn: Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:


                                             Fidelity Foundation


                                             By:   /s/ Richard Weidmann
                                                ------------------------------
                                             Name: Richard Weidmann
                                             Title: Treasurer

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn: Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (6170 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:


                                             Fidelity Non-Profit Management
                                             Foundation


                                             By:   /s/ Richard Weidmann
                                                ------------------------------
                                             Name: Richard Weidmann
                                             Title: Treasurer

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn: Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:


                                             AHIMSA Foundation



                                             By:   /s/ Edith D. Murphy
                                                ------------------------------
                                             Name: Edith D. Murphy
                                             Title: Treasurer

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn: Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:


                                             The Northern Trust Company as
                                             Trustee for the Ford Motor Company
                                             Master Trust Fund


                                             By:   /s/ Susan J. Wallace
                                                ------------------------------
                                             Name: Susan J. Wallace
                                             Title: Second Vice President

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn: Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


The Northern Trust Company executes this
instrument as Trustee of aforesaid, and is not
to be held liable in its individual capacity in
any way by reason of this instrument


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                             INVESTORS:


                                             Mellon Bank N.A. as Trustee for the
                                             MCI Communications Corporation
                                             Employees Pension Trust, solely in
                                             its capacity as Trustee and not in
                                             its individual capacity, (as
                                             directed by Fidelity Investments)

                                             By:  /s/ Carole Bruno
                                                --------------------------------
                                             Name: Carole Bruno
                                             Title: Authorized Signatory

                                 Address:    82 Devonshire Street, E20E
                                             Boston, MA 02109
                                             Attn: Travis Rhodes, Esq.

                                 Telephone:  (617) 563-2599
                                 Fascimile:  (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------


The decision to participate in the investment, any
representations made herein by the participant, and any
actions taken hereunder by the participant has/have been made
solely at the direction of the investment fiduciary who has
sole investment discretion with respect to this investment.


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                 INVESTORS:


                                 Fidelity Small Cap American Fund



                                 By:  /s/ John Costello
                                    ---------------------------------------
                                 Name: John Costello
                                 Title: Authorized Signatory

                    Address:     82 Devonshire Street, E20E
                                 Boston, MA 02109
                                 Attn: Travis Rhodes, Esq.

                    Telephone:   (617) 563-2599
                    Fascimile:   (617) 476-7774



             PLEASE PROVIDE ALL OF THE ABOVE-REQUESTED INFORMATION
             -----------------------------------------------------



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

                                  SCHEDULE A
                                  ----------

                      Schedule of Investors and Founders

Investors
---------

Spinnaker Crossover Institutional Fund, LP
Spinnaker Crossover Fund, LP
Spinnaker Clipper Fund, LP
c/o Spencer Punter
Bowman Capital Management
1875 South Grant Street, Suite 600
San Mateo, California  94402

James Stableford
Anthony Ciulla
Joaquin Garcia-Larrieu
Ralph Cechettini
William Slattery
Marc Weiss
Dana Smith
Dan Chapey
Matthew Fitzmaurice
Vertex Capital II

Emeric McDonald

ATGF II
c/o Marc Weiss
Amerindo Investment Advisors, Inc.
399 Park Avenue, 22/nd/ Floor
New York, New York  10022

Seligman Communications and Information Fund, Inc.
Seligman New Technologies Fund, Inc.
Seligman Investment Opportunities (Master ) Fund - NTV
 Portfolio
c/o Mike Guthrie
J&W Seligman
125 University Avenue
Palo Alto, California  94301
Sevin Rosen Fund V L.P.
Sevin Rosen V Affiliates Fund L.P.
Sevin Rosen Fund VI L.P.
Sevin Rosen VI Affiliates Fund L.P
Sevin Rosen Fund VII L.P.
Sevin Rosen VII Affiliates Fund L.P.
Marianne Radwan
c/o Jon Bayless
    John Jaggers
13455 Noel Road, Suite 1670
Dallas, Texas 75240


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               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

L.J. Sevin
13455 Noel Road, Suite 1670
Dallas, Texas 75240

Benjamin M. Rosen
One Central Park West
Apartment 43A
New York, New York 10023

InterWest Partners VI, L.P.
InterWest Investors VI, LP
InterWest Partners VII, L.P.
InterWest Investors VII, LP
c/o Philip Gianos
3000 Sand Hill Road, #3-255
Menlo Park, California  94025
CenterPoint Venture Fund II, L.P.

Two Galleria Tower
13455 Noel Road, Suite 1670
Dallas, Texas 75240

Austin Ventures V, L.P.
Austin Ventures V Affiliates Fund, L.P.
c/o Ed Olkkola
114 West 7/th/ Street, Suite 1300
Austin, Texas 78701

Rho Management Trust I
c/o Danielle Bodor/Akiva Katz
152 West 57th Street, 23rd Floor
New York, New York 10019

CG Asian-American Fund, L.P.
Kit C. Wong
Kilin To
Simon Wong
Subir Ray
John Whitman
Peter Gerry
David Lichtenstein
Richard Chong
c/o Sycamore Ventures
989 Lenox Drive, Suite 208
Lawrenceville, New Jersey 08648

Dain Rauscher Wessels Investors, L.L.C.
c/o Scott Malloy
Dain Rauscher Wessels
60 South 6/th/ Street
Minneapolis, Minnesota 55402


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

Pantheon Partners L.P.
c/o Kelly Pan
Pantheon Capital Management
880 Third Avenue, 16/th/ floor
New York, New York 10022

Bayview Investors, Ltd.
c/o Jennifer Sherrill
555 California Street, Suite 2600
San Francisco, California 94104

Crown Growth Partners, LP
Parson Finance Limited
c/o David Bellet
Lincoln Building
Suite 3405
60 East 42/nd/ Street
New York, New York 10165

Roundtable Associates LLC
Lincoln Building
Suite 3405
60 East 42/nd/ Street
New York, New York 10165

David Bellet
c/o Crown Advisors International Ltd.
Lincoln Building, Ste. 3405
60 East 42/nd/ Street
New York, New York 10165

Star Growth Enterprise,
a German Civil Law Partnership
SVM Star Ventures
Managementgesellschaft mbH Nr. 3
c/o Star Ventures Management
Possartstrasse 9
D-81679 Munich
Germany

Mary B. Cash 1992 Grandchildren Trust,
Harry B. Cash Trustee
Two Galleria Tower
13455 Noel Road, Suite 1670
Dallas, Texas 75240

G & H Partners
155 Constitution Drive
Menlo Park, California 94025

Dietrich R. Erdmann
Rengestrasse 36
6052 Hergiswil
Switzerland

Steven J. Wallach
7314 Wester Way
Dallas, Texas 75248



                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

Dennis Gorman
3540 Ranchero Road
Plano, Texas 75093

San Francisco Investment Partners
601 Gateway Boulevard, Suite 550
San Francisco, California 94080

The Subhedar Revocable Trust Dated 2/5/93
88 Spencer Lane
Atherton, California 94027

Richard M. Moley
19910 Robin Way
Saratoga, California 95070

Frank J. Kozel
6024 Oakcrest Road
Dallas, Texas 75248

Shen-Chia Wong
No. 10, Lane 121, Li-Der Road
Peitou District, Taipei, Taiwan
R. O. C.

Cheng Sun Lan
No. 10, Lane 121, Li-Der Road
Peitou District, Taipei, Taiwan
R. O. C.

Li Hung Yang
No. 10, Lane 121, Li-Der Road
Peitou District, Taipei, Taiwan
R. O. C.

Thomas H. Aschenbrenner
6016 Oakcrest
Dallas, Texas 75248


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
<PAGE>

Founders
--------

Pei-Lan Wang (Liu-Wang Partners, Ltd.)
1155 East Collins Boulevard, Suite 200
Richardson, Texas 75081

Jian-Yu Liu (Liu-Wang Partners, Ltd.)
1155 East Collins Boulevard, Suite 200
Richardson, Texas 75081

Kuang-Yi Wu
1155 East Collins Boulevard, Suite 200
Richardson, Texas 75081

Shen-Chia Wong
No. 10, Lane 121, Li-Der Road
Peitou District, Taipei, Taiwan
R. O. C.

Cheng Sun Lan
No. 10, Lane 121, Li-Der Road
Peitou District, Taipei, Taiwan
R. O. C.

Li Hung Yang
No. 10, Lane 121, Li-Der Road
Peitou District, Taipei, Taiwan
R. O. C.


                  SIGNATURE PAGE TO CHORUM TECHNOLOGIES INC.
               AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT