<PAGE>   1
                                                                     Exhibit 4.2

                AMENDMENT NO. 2 TO REGISTRATION RIGHTS AGREEMENT

     This Amendment No. 2 (the "Amendment") to Registration Rights Agreement
dated as of January 12, 1998 (the "Rights Agreement") and amended by Amendment
No. 1 to Registration Rights Agreement dated as of January 12, 1999, is made and
entered into as of November 21, 1999 pursuant to Section 19 of the Rights
Agreement by and among Turnstone Systems, Inc., a Delaware corporation, and the
purchasers of the Company's Series A Preferred Stock pursuant to the Series A
Preferred Stock Purchase Agreement dated January 12, 1998 (the "Series A
Purchasers") and the purchasers of the Company's Series B Preferred Stock
pursuant to the Series B Preferred Stock Purchase Agreements dated January 12,
1999 and June 21, 1999 (the "Series B Purchasers" and together with the Series A
Purchasers, the "Preferred Holders"). Capitalized terms used in this Amendment
that are not otherwise defined herein shall have the respective meanings
assigned to them in the Rights Agreement.


                                    RECITALS

     WHEREAS, Section 6(a) of the Rights Agreement grants the Preferred Holders
the right to include their Registrable Securities in any registration by the
Company of its securities, for its own account or the account of a security
holder or holders exercising their demand registration rights, other than
registrations relating solely to employee benefit plans or registrations
relating solely to transactions pursuant to Rule 145 promulgated under the
Securities Act, and Section 19 of the Rights Agreement permits amendment thereof
by written consent of the Company and holders of 66-2/3% of the outstanding
shares of Registrable Securities; and

     WHEREAS, the Preferred Holders and the Company wish to enter this Amendment
to amend and waive their registration rights under Section 6(a).

     NOW, THEREFORE, the parties hereto hereby agree as follows:

                                    AGREEMENT

     1.   Section 6(a) of the Rights Agreement is hereby amended and restated to
read as follows:

     Notice of Registration. If the Company shall determine to register any of
its securities, either for its own account or the account of a security holder
or holders exercising their respective demand registration rights, other than
(i) a registration relating solely to employee benefit plans; (ii) a
registration relating solely to a transaction pursuant to Rule 145 promulgated
under the Securities Act; or (iii) the initial public offering of the Company's
common stock, the Company will:

     (x)  promptly give to each Holder written notice thereof; and

     (y)  include in such registration (and any related qualification under blue
sky laws or other compliance), and in any underwriting involved therein, all the
Registrable Securities specified in a written request or requests, made within
30 days after receipt of such written notice from the Company, by any Holder or
Holders.

<PAGE>   2

     2.   Section 6(b) of the Rights Agreement is hereby amended to delete the
following of phrase at the end of the first sentence (such that the first
sentence shall now end immediately before such phrase):

          ", provided that in the case of the Company's initial public offering,
          the managing underwriter shall include in such offering at least 25%
          of the Registrable Securities requested to be included in such
          offering"

     3.   Except as expressly set forth in this Amendment, the Rights Agreement
shall continue in full force and effect in accordance with its terms.

     4.   This Amendment may be executed in one or more counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument. This Amendment shall be governed by and construed
and enforced in accordance with the laws of the State of California, without
reference to the conflict of laws provisions thereof.



                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                                       2

<PAGE>   3

     IN WITNESS WHEREOF, the parties hereto have caused this AMENDMENT NO. 2 TO
REGISTRATION RIGHTS AGREEMENT to be executed as of the date first above written.

"COMPANY"                             TURNSTONE SYSTEMS
                                      A DELAWARE CORPORATION

                                      By: /s/ RICHARD N. TINSLEY
                                          -------------------------------------
                                      Name:   Richard N. Tinsley
                                            -----------------------------------
                                      Title:  CEO
                                            -----------------------------------


"PREFERRED HOLDERS"                   MATRIX PARTNERS V, L.P.

                                      By: Matrix V Management Company, L.L.C.
                                      Its General Partner

                                      /s/ ANDREW W. VERHALEN
                                      -----------------------------------------
                                      By: Andrew W. Verhalen, Member

                                      MATRIX PARTNERS
                                      ENTREPRENEURS FUND, L.P.

                                      By: Matrix V Management Company, L.L.C.
                                      Its General Partner

                                      /s/ ANDREW W. VERHALEN
                                      -----------------------------------------
                                      By: Andrew W. Verhalen, Member






      [SIGNATURE PAGE TO AMENDMENT NO. 2 TO REGISTRATION RIGHTS AGREEMENT]

                                       3

<PAGE>   4



"PREFERRED HOLDERS" (CONT'D)          INSTITUTIONAL VENTURE PARTNERS
                                      VII, L.P.,
                                      BY ITS GENERAL PARTNER

                                      INSTITUTIONAL VENTURE MANAGEMENT VII, L.P.

                                      /s/ GEOFFREY Y. YANG
                                      -----------------------------------------
                                      Geoffrey Y. Yang, General Partner

                                      INSTITUTIONAL VENTURE
                                      MANAGEMENT VII,  L.P.

                                      /s/ GEOFFREY Y. YANG
                                      -----------------------------------------
                                      Geoffrey Y. Yang, General Partner

                                      IVP FOUNDERS FUND I, L.P.,
                                      BY ITS GENERAL PARTNER

                                      INSTITUTIONAL VENTURE MANAGEMENT VI, L.P.

                                      /s/ GEOFFREY Y. YANG
                                      -----------------------------------------
                                      Geoffrey Y. Yang, General Partner

                                      IVP BROADBAND MANAGEMENT, LLC
                                      BY ITS MANAGING DIRECTOR,

                                      INSTITUTIONAL VENTURE MANAGEMENT VIII, LLC

                                      By: /s/ GEOFFREY Y. YANG
                                         --------------------------------------
                                         Geoffrey Y. Yang, Managing Director






      [SIGNATURE PAGE TO AMENDMENT NO. 2 TO REGISTRATION RIGHTS AGREEMENT]

                                       4

<PAGE>   5



"PREFERRED HOLDERS" (CONT'D)          BENCHMARK CAPITAL PARTNERS II, L.P.
                                      as nominee for


                                      BENCHMARK CAPITAL PARTNERS, II, L.P.

                                      BENCHMARK FOUNDERS' FUND II, L.P.

                                      BENCHMARK FOUNDERS' FUND II-A, L.P.

                                      BENCHMARK MEMBERS' FUND II, L.P.


                                      By: Benchmark Capital Management
                                          Co. II,  L.L.C.
                                      Its General Partner

                                      By:
                                          -------------------------------------
                                                              , Managing Member















      [SIGNATURE PAGE TO AMENDMENT NO. 2 TO REGISTRATION RIGHTS AGREEMENT]

                                       5
<PAGE>   6
                AMENDMENT NO. 1 TO REGISTRATION RIGHTS AGREEMENT

         This Amendment No. 1 (the "Amendment") to Registration Rights Agreement
dated as of January 12, 1998 (the "Rights Agreement") is made and entered into
as of January 12, 1999 pursuant to Section 19 of the Rights Agreement by and
among Turnstone Systems, Inc., a Delaware corporation, and the purchasers of the
Company's Series A Preferred Stock pursuant to the Series A Preferred Stock
Purchase Agreement dated January 12, 1998 (the "Series A Purchasers").
Capitalized terms used in this Amendment that are not otherwise defined herein
shall have the respective meanings assigned to them in the Rights Agreement.


                                    RECITALS

         WHEREAS, Section 20(b) of the Rights Agreement permits the Company to
grant additional registration rights on a pari passu basis with those granted to
the Series A Purchasers pursuant to the Rights Agreement, and Section 19 of the
Rights Agreement permits amendment thereof by written consent of the Company and
holders of 66-2/3% of the outstanding shares of Registrable Securities;

         WHEREAS, certain of the Series A Purchasers and certain other entities
have agreed to purchase shares of the Company's Series B Preferred Stock (the
"Series B Shares") pursuant to the Series B Preferred Stock Purchase Agreement
dated December 23, 1998;

         WHEREAS, the Company has agreed to grant the purchasers of the Series B
Shares registration rights on a pari passu basis with those currently held by
the Series A Purchasers.

         WHEREAS, the Series A Purchasers and the Company wish to enter this
Amendment to clarify the definition section of the Rights Agreement and to
include as Registrable Securities thereunder future series of Preferred Stock of
the Company (including the Series B Shares) that may be issued pursuant to stock
purchase agreements approved by the Board of Directors.

                                    AGREEMENT

         NOW, THEREFORE, the parties hereto hereby agree as follows:

         1. For purposes of the Rights Agreement, each reference to "Series A
Preferred" shall hereafter be deemed to refer to the "Preferred."

         2. For purposes of the Rights Agreement, "Preferred" shall mean shares
of any series of the Preferred Stock of the Company issued and sold by the
Company pursuant to a stock purchase agreement approved by the Company's Board
of Directors.

         3. The definition of "Series A Preferred" in Section 1 of the Rights
Agreement shall be deleted in its entirety.

         4. The definition of Purchaser in Section 1 of the Rights Agreements is
hereby amended and restated to read as follows:

<PAGE>   7

                  "Purchaser" shall mean each person or entity who has acquired
         shares of Preferred and who is a signatory to this Agreement or who
         holds Registrable Securities and may become a signatory pursuant to
         Section 20(b) hereof.

         5. This Amendment shall also be deemed to make each of the undersigned
a signatory to the Rights Agreement pursuant to Section 20(b) thereof.

         6. Except as expressly set forth in this Amendment, the Rights
Agreement shall continue in full force and effect in accordance with its terms.

         7. This Amendment may be executed in one or more counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument. This Amendment shall be governed by and construed
and enforced in accordance with the laws of the State of California, without
reference to the conflict of laws provisions thereof.



                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

<PAGE>   8

         IN WITNESS WHEREOF, the parties hereto have caused this AMENDMENT NO. 1
TO REGISTRATION RIGHTS AGREEMENT to be executed as of the date first above
written.

"COMPANY"                         TURNSTONE SYSTEMS
                                  A DELAWARE CORPORATION

                                  By:   /s/ Richard N. Tinsley
                                     --------------------------------
                                  Name: Richard N. Tinsely
                                       ------------------------------

                                  Title: CEO
                                        -----------------------------

"PURCHASERS"                      INSTITUTIONAL VENTURE PARTNERS
                                  VII, L.P.,
                                  BY ITS GENERAL PARTNER

                                  INSTITUTIONAL VENTURE MANAGEMENT VII, L.P.

                                  /s/ Geoffrey Yang
                                  -----------------------------------
                                  Geoffrey Y. Yang, General Partner

                                  INSTITUTIONAL VENTURE
                                  MANAGEMENT VII,  L.P.

                                  /s/ Geoffrey Yang
                                  -----------------------------------
                                  Geoffrey Y. Yang, General Partner

                                  IVP FOUNDERS FUND I, L.P.,
                                  BY ITS GENERAL PARTNER

                                  INSTITUTIONAL VENTURE MANAGEMENT VI, L.P.

                                  /s/ Geoffrey Yang
                                  -----------------------------------
                                  Geoffrey Y. Yang, General Partner







      [SIGNATURE PAGE TO AMENDMENT NO. 1 TO REGISTRATION RIGHTS AGREEMENT]


<PAGE>   9



"PURCHASERS" (CONT'D)             MATRIX PARTNERS V, L.P.

                                  By:  Matrix V Management Company, L.L.C.
                                  Its General Partner

                                  /s/ Andrew Verhalen
                                  -----------------------------------
                                  By:      Andrew W. Verhalen, Member





      [SIGNATURE PAGE TO AMENDMENT NO. 1 TO REGISTRATION RIGHTS AGREEMENT]

<PAGE>   10

"PURCHASERS" (CONT'D)             BENCHMARK CAPITAL PARTNERS II, L.P.
                                  as nominee for


                                  BENCHMARK CAPITAL PARTNERS, II, L.P.

                                  BENCHMARK FOUNDERS' FUND II, L.P.

                                  BENCHMARK FOUNDERS' FUND II-A, L.P.

                                  BENCHMARK MEMBERS' FUND II, L.P.


                                  By:      Benchmark Capital Management
                                  Co. II,  L.L.C.
                                  Its General Partner

                                  By: /s/ Andrew S. Rachleff
                                     -------------------------------
                                      Managing Member






      [SIGNATURE PAGE TO AMENDMENT NO. 1 TO REGISTRATION RIGHTS AGREEMENT]

<PAGE>   11

"PURCHASERS" (CONT'D)             WS INVESTMENT COMPANY 97B

                                  By: /s/ Thomas C. DeFilipps
                                     ----------------------------------
                                  Thomas C. DeFilipps, General Partner



                                  /s/ Thomas C. DeFilipps
                                  -------------------------------------
                                  Thomas C. DeFilipps



                                  COMDISCO, INC.

                                  By:  /s/ James P. Labe
                                      ---------------------------------

                                  Name:  James P. Labe
                                        -------------------------------

                                  Title: President, Comdisco Ventures Division
                                         -------------------------------------


                                  IVP BROADBAND FUND, L.P.,
                                  BY ITS GENERAL PARTNER

                                  IVP BROADBAND MANAGEMENT, LLC
                                  BY ITS MANAGING DIRECTOR,

                                  INSTITUTIONAL VENTURE MANAGEMENT VIII, LLC

                                  By: /s/ Geoffrey Yang
                                      ---------------------------------
                                  Geoffrey Y. Yang, Managing Director






      [SIGNATURE PAGE TO AMENDMENT NO. 1 TO REGISTRATION RIGHTS AGREEMENT]

<PAGE>   12

"PURCHASERS" (CONT'D)             STANFORD UNIVERSITY

                                  By:  /s/ H. A. Turner
                                     ---------------------------------
                                  Name: Harry A. Turner
                                       -------------------------------
                                  Title: Managing Director,
                                         Stanford Management Company
                                        ------------------------------






      [SIGNATURE PAGE TO AMENDMENT NO. 1 TO REGISTRATION RIGHTS AGREEMENT]

<PAGE>   13

         IN WITNESS WHEREOF, the parties hereto have caused this Addendum
Signature Page to Registration Rights Agreement to be executed as of the date
first above written.

"COMPANY"                         TURNSTONE SYSTEMS, INC.
                                  A DELAWARE CORPORATION

                                  By:  /s/ Richard N. Tinsley
                                     --------------------------------
                                  Name: Richard N. Tinsley
                                       ------------------------------
                                  Title: CEO
                                        -----------------------------


"ADDITIONAL PURCHASER"            CHARLES ROSS PARTNERS INVESTMENT
                                  FUND NUMBER 7

                                  By:  /s/ Gregory E. Lawler
                                     --------------------------------

                                  Name: Gregory E. Lawler
                                       ------------------------------

                                  Title: General Partner
                                        -----------------------------






          [SIGNATURE PAGE TO ADDENDUM TO REGISTRATION RIGHTS AGREEMENT]

<PAGE>   14

                             TURNSTONE SYSTEMS, INC.

                          REGISTRATION RIGHTS AGREEMENT

         This Registration Rights Agreement (the "Agreement") is made as of
January 12, 1998 by and among Turnstone Systems, Inc., a Delaware corporation
(the "Company"), and each of the persons and entities who have purchased shares
of the Company's Series A Preferred Stock (individually, a "Purchaser," and
collectively, the "Purchasers") pursuant to the Series A Preferred Stock
Purchase Agreement of even date herewith between the Company and the Purchasers
(the "Purchase Agreement").

         NOW, THEREFORE, the parties hereto agree as follows:

                                    AGREEMENT

      1. Certain Definitions. As used in this Agreement, the following terms
shall have the following respective meanings:

         "Commission" shall mean the Securities and Exchange Commission or any
successor agency.

         "Exchange Act" shall mean the Securities Exchange Act of 1934, as
amended, or any successor law thereto.

         "Holder" shall mean each Purchaser, and any transferee of Registrable
Securities who pursuant to Section 15 below is entitled to registration rights
hereunder.

         "Purchaser" shall mean each person or entity who has acquired shares of
Series A Preferred Company and who is a signatory to this Agreement or who holds
Registrable Securities and may become a signatory pursuant to Section 20(b)
hereof.

         "Registrable Securities" shall mean (i) shares of the Company's Common
Stock issued or issuable upon the conversion of the Series A Preferred; (ii) any
Common Stock of the Company issued or issuable in respect of shares of the
Series A Preferred; (iii) shares of the Company's Common Stock issued or
issuable upon any conversion of the Series A Preferred upon any stock split,
stock dividend, recapitalization, or similar event; and (iv) any shares of the
Company's Common Stock issued or issuable upon conversion or exercise of any
convertible security for which subsequent registration rights are granted in
accordance with Section 20(b); provided, however, that Registrable Securities
shall not include shares of Common Stock that have been sold to or though a
broker or dealer or underwriter in a public distribution or public securities
transaction, sold in a transaction exempt from the registration and prospectus
delivery requirements of the Securities Act under Section 4(1) thereof so that
all transfer restrictions and restrictive legends with respect thereto, if any,
are removed upon the consummation of such sale, or Registrable Securities sold
by a person in a transaction in which rights under this Agreement are not
assigned.

<PAGE>   15

         The terms "register," "registered" and "registration" refer to a
registration effected by preparing and filing a registration statement in
compliance with the Securities Act, and the declaration or ordering of the
effectiveness of such registration statement.

         "Registration Expenses" shall mean all expenses incurred by the Company
in complying with Sections 5, 6, and 7 hereof, including, without limitation,
all registration, qualification and filing fees, printing expenses, escrow fees,
fees and disbursements of counsel for the Company, fees and disbursements of one
special counsel to the Holders, blue sky fees and expenses, and the expense of
any special audits incident to or required by any such registration but
excluding all Selling Expenses.

         "Restricted Securities" shall mean the securities of the Company
required to bear the legend set forth in Section 3 hereof (or any similar
legend).

         "Securities Act" shall mean the Securities Act of 1933, as amended, or
any successor law thereto.

         "Selling Expenses" shall mean all underwriting discounts, selling
commissions, and stock transfer taxes applicable to the securities registered by
the Holders and any fees of counsel to any Holder.

         "Series A Preferred" shall mean shares of the Company's Series A
Preferred Stock issued and sold by the Company pursuant to the Purchase
Agreement.

      2. Restrictions on Transferability. The Restricted Securities shall not be
transferable except upon the conditions specified in this Agreement, which
conditions are intended to ensure compliance with the provisions of the
Securities Act. Until the Company's initial public offering pursuant to a
registration statement filed with and declared effective by the Commission under
the Securities Act, the Restricted Securities shall not, without the prior
written consent of the Company, be transferred to any entity (or any affiliate
thereof) which is engaged in the development, marketing or sale of products that
are, in the Company's judgment, the same or similar to those of the Company, and
any such attempted transfer shall be void ab initio. Each holder of Restricted
Securities will cause any proposed transferee of the Restricted Securities held
by such holder to agree to take and hold such Restricted Securities subject to
the provisions and upon the conditions specified in this Agreement.

      3. Restrictive Legend. Each certificate representing (i) the Series A
Preferred, (ii) shares of the Company's Common Stock issued upon conversion of
the Series A Preferred, and (iii) any other securities issued in respect of the
Series A Preferred (or Common Stock issued upon conversion of the Series A
Preferred) upon any stock split, stock dividend, recapitalization, merger,
consolidation or similar event, shall (unless otherwise permitted by the
provisions of Section 4 below) be stamped or otherwise imprinted with a legend
in substantially the following form (in addition to any legend required under
applicable state securities laws):


                                      -2-
<PAGE>   16

         THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
         INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
         1933, AS AMENDED (THE "SECURITIES ACT"). THESE SECURITIES MAY NOT BE
         OFFERED, SOLD, PLEDGED OR TRANSFERRED UNLESS (I) A REGISTRATION
         STATEMENT UNDER THE SECURITIES ACT IS EFFECTIVE COVERING SUCH TRANSFER
         OR (II) THERE IS AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY,
         THAT AN EXEMPTION THEREFROM IS AVAILABLE. COPIES OF THE AGREEMENT
         COVERING THE PURCHASE OF THESE SECURITIES AND RESTRICTING THEIR
         TRANSFER MAY BE OBTAINED AT NO COST BY WRITTEN REQUEST MADE BY THE
         HOLDER OF RECORD OF THIS CERTIFICATE TO THE SECRETARY OF THE COMPANY AT
         ITS PRINCIPAL EXECUTIVE OFFICES.

         Each Purchaser and Holder consents to the Company's making a notation
on its records and giving instructions to any transfer agent of the Series A
Preferred or the Common Stock in order to implement the restrictions on transfer
established in this Section.

      4. Notice of Proposed Transfers. The holder of each certificate
representing Restricted Securities by acceptance thereof agrees to comply in all
respects with the provisions of this Section. Prior to any proposed transfer of
any Restricted Securities, unless there is in effect a registration statement
under the Securities Act covering the proposed transfer, the holder thereof
shall give written notice to the Company of such Holder's intention to effect
such transfer. Each such notice shall describe the manner and circumstances of
the proposed transfer in sufficient detail, and shall be accompanied by either
(i) a written opinion of legal counsel, who shall be reasonably satisfactory to
the Company, addressed to the Company and reasonably satisfactory in form and
substance to the Company's counsel, to the effect that the proposed transfer of
the Restricted Securities may be effected without registration under the
Securities Act or (ii) a "No Action" letter from the Commission to the effect
that the transfer of such securities without registration will not result in a
recommendation by the staff of the Commission that action be taken with respect
thereto, whereupon the holder of such Restricted Securities shall be entitled to
transfer such Restricted Securities in accordance with the terms of the notice
delivered by the holder to the Company. Each certificate evidencing the
Restricted Securities transferred as above provided shall bear the appropriate
restrictive legends described above, except that such certificate shall not bear
any such restrictive legend if in the opinion of counsel for the Company such
legend is not required.

      5. Requested Registration. (a) Request for Registration. If at any time
after the expiration of six months following the Company's initial registered
public offering, the Company shall receive from any Holder or group of Holders
of Registrable Securities, representing not less than 66-2/3% of the Registrable
Securities then outstanding (assuming conversion of all shares of the Series A
Preferred), a written request that the Company effect any registration,
qualification or compliance with respect


                                      -3-
<PAGE>   17

to Registrable Securities representing at least 33-1/3% of the Registrable
Securities then outstanding, the Company will:

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

               (y) as soon as practicable, use its best efforts to effect within
120 days of the receipt of such request such registration, qualification or
compliance (including, without limitation, the execution of an undertaking to
file post-effective amendments, appropriate qualification under applicable blue
sky or other state securities laws and appropriate compliance with applicable
regulations issued under the Securities Act and any other governmental
requirements or regulations) as may be so requested and as would permit or
facilitate the sale and distribution of all or such portion of such Registrable
Securities as are specified in such request, together with all or such portion
of the Registrable Securities of any Holder or Holders joining in such request
as are specified in a written request received by the Company within 15 days
after receipt of such written notice from the Company;

provided, however, that the Company shall not be obligated to take any action to
effect any such registration, qualification or compliance pursuant to this
Section:

                    (A) in any particular jurisdiction in which the Company
would be required to execute a general consent to service of process in
effecting such registration, qualification or compliance unless the Company is
already subject to service in such jurisdiction and except as may be required by
the Securities Act;

                    (B) prior to 90 days immediately following the effective
date of any registration statement pertaining to securities of the Company
(other than a registration of securities in a Rule 145 transaction or with
respect to an employee benefit plan); or

                    (C) after the Company has effected two such registrations
pursuant to this Section and such registration has been declared or ordered
effective.

      Subject to the foregoing clauses, the Company shall file a registration
statement covering the Registrable Securities so requested to be registered as
soon as practicable after receipt of the request or requests of any Holder or
Holders. If, however, the Company shall furnish to the Holder or Holders
requesting a registration statement pursuant to this Section a certificate
signed by 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 such filing for a period of not more
than 120 days after receipt of the request of the Holder or Holders requesting
such registration; provided, however, that the Company may not utilize this
right more than once in any 12-month period.

         (b) Underwriting. If the 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


                                      -4-
<PAGE>   18

their request and the Company shall include such information in its written
notice to the other Holders. The right of any Holder to registration pursuant to
this Section shall be conditioned upon such Holder's participation in such
underwriting and the inclusion of such Holder's Registrable Securities in the
underwriting to the extent provided herein.

      The Company shall (together with all Holders proposing to distribute their
securities through such underwriting) enter into an underwriting agreement in
customary form with the managing underwriter selected for such underwriting by
the Holders of a majority of the Registrable Securities proposed by such Holders
to be distributed through such underwriting. Notwithstanding any other provision
of this Section, if the managing underwriter advises the Holders in writing that
marketing factors require a limitation of the number of shares to be
underwritten, then, subject to the provisions of Section 5(a), the Company shall
so advise all Holders and the number of shares of Registrable Securities that
may be included in the registration and underwriting shall be allocated among
all Holders requesting inclusion in the registration in proportion, as nearly as
practicable, to the respective amounts of Registrable Securities originally
requested by such Holders to be included in the registration statement. No
Registrable Securities excluded from the underwriting by reason of the managing
underwriter's marketing limitation shall be included in such registration.

      If the managing underwriter has not limited the number of Registrable
Securities to be underwritten, the Company may include securities for its own
account or for the account of others in such registration if the underwriter so
agrees and if the number of Registrable Securities which would otherwise have
been included in such registration and underwriting will not thereby be limited,
and provided that the Company or the other selling stockholders shall bear an
equitable share of the Registration Expenses in connection with such
registration and underwriting.

      If any Holder of Registrable Securities disapproves of the terms of the
underwriting, such person may elect to withdraw therefrom by written notice to
the Company, the managing underwriter and the other Holders. The Registrable
Securities and/or other securities so withdrawn shall also be withdrawn from
registration; provided, however, that if, by the withdrawal of such Registrable
Securities, a greater number of Registrable Securities held by other Holders may
be included in such registration (up to the maximum of any limitation imposed by
the underwriters), then the Company shall offer to all Holders who have included
Registrable Securities in the registration the right to include additional
Registrable Securities in the same proportion used in determining the
underwriter limitation in this Section. If the registration does not become
effective due to the withdrawal of Registrable Securities, then either (1) the
Holders requesting registration shall reimburse the Company for expenses
incurred in complying with the request or (2) the aborted registration shall be
treated as effected for purposes of Section 5(a)(C).

      6. Company Registration.


         (a) Notice of Registration. If the Company shall determine to register
any of its securities, either for its own account or the account of a security
holder or holders exercising their respective demand registration rights, other
than: (i) a registration relating solely to employee benefit


                                      -5-
<PAGE>   19

plans or (ii) a registration relating solely to a transaction pursuant to Rule
145 promulgated under the Securities Act, the Company will:

               (x) promptly give to each Holder written notice thereof; and

               (y) include in such registration (and any related qualification
under blue sky laws or other compliance), and in any underwriting involved
therein, all the Registrable Securities specified in a written request or
requests, made within 30 days after receipt of such written notice from the
Company, by any Holder or Holders.

         (b) Cut-back and Allocation. Notwithstanding any other provision of
this Section, if the managing underwriter determines that marketing factors
require a limitation of the number of shares to be underwritten, the managing
underwriter may limit the number of Registrable Securities to be included in the
registration and underwriting, provided that except in the case of the Company's
initial public offering, the managing underwriter shall include in such offering
at least 25% of the Registrable Securities requested to be included in such
offering. In such event, the Company shall so advise all Holders of Registrable
Securities which would otherwise be registered and underwritten pursuant hereto,
and the number of shares of Registrable Securities that may be included in the
registration and underwriting shall be allocated among the Holders in
proportion, as nearly as practicable, to the respective amounts of Registrable
Securities held by such Holders. If any Holder disapproves of the terms of any
such underwriting, such Holder may elect to withdraw therefrom by written notice
to the Company and the managing underwriter. Any Registrable Securities excluded
or withdrawn from such underwriting shall be withdrawn from such registration.

         (c) Right to Terminate Registration. The Company shall have the right
to terminate or withdraw any registration initiated by it under this Section
prior to the effectiveness of such registration whether or not any Holder has
elected to include securities in such registration.

      7. Registration on Form S-3. The Company shall use its best efforts to
qualify for registration on Form S-3, and to that end, the Company shall comply
with the reporting requirements of the Exchange Act. After the Company has
qualified for the use of Form S-3, each Holder shall have the right to request
an unlimited number of registrations on Form S-3 (such requests shall be in
writing and shall state the number of shares of Registrable Securities to be
disposed of and the intended method of disposition of such shares by each such
Holder), subject to the following limitations:

               (i) the Company shall not be obligated to cause a registration on
Form S-3 to become effective prior to 90 days following the effective date of a
Company-initiated registration (other than a registration effected solely to
qualify an employee benefit plan or to effect a business combination pursuant to
Rule 145);

               (ii) the Company shall not be obligated to cause a registration
on Form S-3 to become effective prior to expiration of 90 days following the
effective date of the most recent registration pursuant to a request under
Section 5 of this Agreement or pursuant to a request by a


                                      -6-
<PAGE>   20

holder of registration rights under any other agreement of the Company granting
Form S-3 demand registration rights;

 (iii) the Company shall not be required to
effect a registration on Form S-3 unless the Holder or Holders requesting
registration propose to dispose of shares of Registrable Securities having an
aggregate disposition price (before deduction of underwriting discounts and
expenses of sale) of at least $1,000,000; and

               (iv) the Company shall not be required to maintain and keep any
such registration on Form S-3 effective for a period greater than the period
equal to the shorter of (x) 90 days or (y) that time reasonably necessary to
permit the disposition of the Registrable Securities subject to such
registration. The Company shall give notice to all Holders of the receipt of a
request for registration pursuant to this Section and shall provide a reasonable
opportunity for all such other Holders, to participate in the registration.
Subject to the foregoing, the Company will use its best efforts to effect
promptly the registration of all shares of Registrable Securities on Form S-3 to
the extent requested by the Holder or Holders thereof for purposes of
disposition.

      8. Expenses of Registration. All Registration Expenses incurred in
connection with any registration, qualification or compliance pursuant to
Section 5, Section 6, or Section 7 shall be borne by the Company. All Selling
Expenses relating to securities registered by the Holders pursuant to either
Section 5, Section 6, or Section 7 shall be borne by the Holders of such
securities pro rata on the basis of the number of shares so registered.
Notwithstanding the foregoing, the Company shall not be required to pay for
Registration Expenses pursuant to Section 5 if the registration request is
subsequently withdrawn at the request of the Holders of a majority of the
Registrable Securities to be registered (which Holders shall bear such
expenses), unless the Holders of a majority of the Registrable Securities agree
to forfeit their right to demand registration pursuant to Section 5; provided,
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, then the Holders
shall not be required to pay any of such Registration Expenses and shall retain
their rights pursuant to Section 5.

      9. Registration Procedures. In the case of each registration,
qualification or compliance effected by the Company pursuant to this Agreement,
the Company will keep each Holder advised in writing as to the initiation of
each registration, qualification and compliance and as to the completion
thereof. At its expense the Company will furnish such number of prospectuses and
other documents incident thereto as a Holder from time to time may reasonably
request.

      10. Termination of Registration Rights. The registration rights granted
pursuant to this Agreement shall terminate (i) as to any Holder of less than
500,000 shares of Registrable Securities, at such time after the Company's
initial public offering as the Registrable Securities held by such Holder may be
sold within any three month period pursuant to Rule 144 and (ii) as to any
Holder of 500,000 or more shares of Registrable Securities (as adjusted for
stock splits, stock dividends or distributions, recapitalizations, and similar
events), on the earlier to occur of (A) the date such Registrable Securities
held by such Holder may be sold pursuant to Rule 144(k) and (B) the third


                                      -7-
<PAGE>   21

anniversary of the date on which the Company first becomes subject to the
reporting requirements under Section 12 of the Exchange Act.

      11. Lock-up Agreement. In consideration for the Company agreeing to its
obligations under this Agreement, each Holder of Registrable Securities and each
transferee pursuant to Section 15 hereof agrees, in connection with the first
registration of the Company's securities, upon request of the Company or the
underwriters managing any underwritten offering of the Company's securities, not
to sell, make any short sale of, loan, grant any option for the purchase of or
otherwise dispose of any securities of the Company (other than those included in
the registration) without the prior written consent of the Company or such
underwriters, as the case may be, for such period of time (not to exceed 180
days) from the effective date of such registration as the Company or the
underwriters may specify, provided that each officer and director of the Company
is similarly bound. Each Holder agrees that the Company may instruct its
transfer agent to place stop-transfer notations in its records to enforce the
provisions of this Section. This Section 11 shall supersede any conflicting
provision of Section 5 or Section 7 above. Notwithstanding any other provision
of this Agreement, the Company may assign each Holder's obligations under this
Section 11 to any underwriter of the Company's initial public offering of
securities.

      12. Indemnification.

          (a) The Company will indemnify each Holder, each of its officers,
directors and partners and such Holder's legal counsel and independent
accountants, and each person controlling such Holder within the meaning of
Section 15 of the Securities Act, with respect to which registration,
qualification or compliance has been effected pursuant to this Agreement, and
each underwriter, if any, and each person who controls any underwriter within
the meaning of Section 15 of the Securities Act, against all expenses, claims,
losses, damages and liabilities (or actions in respect thereof), including any
of the foregoing incurred in settlement of any litigation, commenced or
threatened, arising out of or based on any untrue statement (or alleged untrue
statement) of a material fact contained in any registration statement,
prospectus, offering circular or other document, or any amendment or supplement
thereto, incident to any such registration, qualification or compliance, or
based on any omission (or alleged omission) to state therein a material fact
required to be stated therein or necessary to make the statements therein, in
light of the circumstances in which they were made, not misleading, or any
violation by the Company of the Securities Act or the Exchange Act or securities
act of any state or any rule or regulation thereunder, and relating to action or
inaction required of the Company in connection with any such registration,
qualification or compliance, and will reimburse each such Holder, each of its
officers, directors and partners and such Holder's legal counsel and independent
accountants, and each person controlling such Holder, each such underwriter and
each person who controls any such underwriter, for any legal and any other
expenses reasonably incurred in connection with investigating, preparing or
defending any such claim, loss, damage, liability or action, provided that the
Company will not be liable in any such case to the extent that any such claim,
loss, damage, liability or expense arises out of or is based on any untrue
statement or omission or alleged untrue statement or omission, made in reliance
upon and in conformity with written information furnished to the Company by an
instrument duly executed by such Holder or underwriter and stated to be
specifically for use therein; and provided,


                                      -8-
<PAGE>   22

further, that the Company will not be liable to any such person or entity with
respect to any such untrue statement or omission or alleged untrue statement or
omission made in any preliminary prospectus that is corrected in the final
prospectus filed with the Commission pursuant to Rule 424(b) promulgated under
the Securities Act (or any amendment or supplement to such prospectus) if the
person asserting any such loss, claim, damage or liability purchased securities
but was not sent or given a copy of the prospectus (as amended or supplemented)
at or prior to the written confirmation of the sale of such securities to such
person in any case where such delivery of the prospectus (as amended or
supplemented) is required by the Securities Act, unless such failure to deliver
the prospectus (as amended or supplemented) was a result of the Company's
failure to provide such prospectus (as amended or supplemented).

          (b) Each Holder will, severally and not jointly, if Registrable
Securities held by such Holder are included in the securities as to which such
registration, qualification or compliance is being effected, indemnify the
Company, each of its directors and officers and its legal counsel and
independent accountants, each underwriter, if any, of the Company's securities
covered by such a registration statement, each person who controls the Company
or such underwriter within the meaning of Section 15 of the Securities Act, and
each other such Holder, each of its officers, directors and partners and each
person controlling such Holder within the meaning of Section 15 of the
Securities Act, against all claims, losses, damages and liabilities (or actions
in respect thereof) arising out of or based on any untrue statement (or alleged
untrue statement) of a material fact contained in any such registration
statement, prospectus, offering circular or other document, or any omission (or
alleged omission) to state therein a material fact required to be stated therein
or necessary to make the statements therein not misleading, and will reimburse
the Company, such Holders, such directors, officers, legal counsel, independent
accountants, underwriters or control persons for any legal or any other expenses
reasonably incurred in connection with investigating or defending any such
claim, loss, damage, liability or action, in each case to the extent, but only
to the extent, that such untrue statement (or alleged untrue statement) or
omission (or alleged omission) is made in such registration statement,
prospectus, offering circular or other document in reliance upon and in
conformity with written information furnished to the Company by an instrument
duly executed by such Holder and stated to be specifically for use therein;
provided, however, that the obligations of such Holders hereunder shall be
limited to an amount equal to the proceeds, net of underwriting discounts and
commissions but not expenses, to each such Holder of Registrable Securities sold
as contemplated herein.

          (c) Each party entitled to indemnification under this Section (the
"Indemnified Party") shall give notice to the party required to provide
indemnification (the "Indemnifying Party") promptly after such Indemnified Party
has actual knowledge of any claim as to which indemnity may be sought, and shall
permit the Indemnifying Party to assume the defense of any such claim or any
litigation resulting therefrom, provided that counsel for the Indemnifying
Party, who shall conduct the defense of such claim or litigation, shall be
approved by the Indemnified Party (whose approval shall not be unreasonably
withheld), and the Indemnified Party may participate in such defense at such
party's expense, and provided further that the failure of any Indemnified Party
to give notice as provided herein shall not relieve the Indemnifying Party of
its obligations under this Agreement, except to the extent, but only to the
extent, that the Indemnifying Party's ability to defend against


                                      -9-
<PAGE>   23

such claim or litigation is impaired as a result of such failure to give notice.
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.


          (d) If the indemnification provided for in this Section 12 is held by
a court of competent jurisdiction to be unavailable to an indemnified party with
respect to any loss, liability, claim, damage, or expense referred to therein,
then the indemnifying party, in lieu of indemnifying such indemnified party
hereunder, shall contribute to the amount paid or payable by such indemnified
party as a result of such loss, liability, claim, damage, or expense in such
proportion as is appropriate to reflect the relative fault of the indemnifying
party on the one hand and of the indemnified party on the other in connection
with the statements or omissions that resulted in such loss, liability, claim,
damage, or expense as well as any other relevant equitable considerations. The
relative fault of the indemnifying party and of the indemnified party shall be
determined by reference to, among other things, whether the untrue or alleged
untrue statement of a material fact or the omission to state a material fact
relates to information supplied by the indemnifying party or by the indemnified
party and the parties' relative intent, knowledge, access to information, and
opportunity to correct or prevent such statement or omission. In no event shall
any contribution by a Holder under this Section 12(d) exceed the proceeds, net
of underwriting discounts and commissions but not expenses, from the offering
received by such Holder.

      13. Information by Holder. The Holder or Holders of Registrable Securities
included in any registration shall furnish to the Company such information
regarding such Holder or Holders and the distribution proposed by such Holder or
Holders as the Company may request in writing and as shall be required in
connection with any registration, qualification or compliance referred to in
this Agreement.

      14. Rule 144 Reporting. With a view to making available the benefits of
certain rules and regulations of the Commission which may at any time permit the
sale of the Restricted Securities to the public without registration, after such
time as a public market exists for the Common Stock of the Company, the Company
agrees to use its best efforts to:

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

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

          (c) furnish to Holders upon request a written statement as to its
compliance with the reporting requirements of Rule 144 (at any time after 90
days after the effective date of the first registration statement filed by the
Company for an offering of its securities to the general public),


                                      -10-
<PAGE>   24

and of the Securities Act and the Exchange Act (at any time after it has become
subject to such reporting requirements), a copy of the most recent annual or
quarterly report of the Company, and such other reports and documents of the
Company as a Holder may reasonably request in availing itself of any rule or
regulation of the Commission allowing such Holder to sell any such securities
without registration.

      15. Transfer of Rights. Provided that the Company is given prior written
notice of such assignment, the rights granted hereunder to cause the Company to
register securities may be assigned to (i) a transferee or assignee who acquires
at least 500,000 shares of Registrable Securities (appropriately adjusted for
stock splits, recapitalizations and like after the date hereof) and (ii) any
affiliate or constituent partner, including limited partner, of a Purchaser.

      16. Governing Law. This Agreement shall be governed by and interpreted in
accordance with the laws of the State of California as applied to agreements
among California residents entered and to be performed entirely within
California. The parties hereto agree to submit to the jurisdiction of the
federal and state courts of the State of California with respect to the breach
or interpretation of this Agreement or the enforcement of any and all rights,
duties, liabilities, obligations, powers and other relations between the parties
arising under this Agreement.

      17. Entire Agreement. This Agreement constitutes the full and entire
understanding among the parties regarding the subject matter herein. Except as
otherwise expressly provided herein, the provisions hereof shall inure to the
benefit of, and be binding upon, the successors, assigns, heirs, executors and
administrators of the parties hereto.

      18. Notices, etc. All notices and other communications required or
permitted hereunder shall be in writing and shall be deemed effectively given
upon delivery to the party to be notified in person or by courier service or
five (5) days after deposit with the United States mail, by registered or
certified mail, postage prepaid, addressed (a) if to a holder of any Registrable
Securities, to such address as such holder shall have furnished the Company in
writing, or, until any such holder so furnishes an address to the Company, then
to and at the address of the last holder of such securities who has so furnished
an address to the Company, or (b) if to the Company, to its address set forth on
the first page of this Agreement and addressed to the attention of the Chief
Financial Officer, or at such other address as the Company shall have furnished
to the Holders in writing.

      19. Amendment. Any provision of this Agreement may be amended, waived or
modified upon the written consent of (i) the Company and (ii) holders of 66-2/3%
of the outstanding shares of Registrable Securities. Any Holder may waive any of
his or her rights or the Company's obligations hereunder without obtaining the
consent of any other person.

      20. Limitations on Subsequent Registration Rights.

          (a) From and after the date of this Agreement, the Company shall not
enter into any agreement granting any holder or prospective holder of any
securities of the Company registration rights with respect to such securities
without the prior written consent of 66-2/3% of the Registrable Securities then
outstanding unless (1) such new registration rights, including standoff


                                      -11-
<PAGE>   25

obligations, are on a pari passu basis with those rights of the Holders
hereunder or (2) such new registration rights, including standoff obligations,
are subordinate to the registration rights granted the Holders hereunder.

          (b) Where the Company determines to grant any holder or prospective
holder of any securities of the Company registration rights that are on a pari
passu basis with those rights of the Holders hereunder and determines that the
grant of such rights shall be made pursuant to this Agreement, then such grant
shall be evidenced by the execution of an additional signature page to this
Agreement by the Company and such holder, without any requirement on the part of
the Company to seek the consent or approval of the Holders.

      21. Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be an original and all of which together shall
constitute one instrument.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]


                                      -12-
<PAGE>   26

                         [REGISTRATION RIGHTS AGREEMENT]

         IN WITNESS WHEREOF, the undersigned have executed this Registration
Rights Agreement as of the date set forth above.

"COMPANY"                       TURNSTONE SYSTEMS, INC.,
                                A DELAWARE CORPORATION

                                By:  /s/ Richard N. Tinsley
                                   -------------------------------------
                                Name:   Richard N. Tinsley
                                     -----------------------------------
                                Title:   CEO
                                      ----------------------------------

"PURCHASERS"                    INSTITUTIONAL VENTURE PARTNERS VII, L.P.,
                                BY ITS GENERAL PARTNER

                                INSTITUTIONAL VENTURE MANAGEMENT VII, L.P.

                                /s/ Geoffrey Yang
                                ----------------------------------------
                                Geoffrey Y. Yang, General Partner

                                INSTITUTIONAL VENTURE MANAGEMENT VII, L.P.

                                 /s/ Geoffrey Yang
                                ----------------------------------------
                                Geoffrey Y. Yang, General Partner

                                IVP FOUNDERS FUND I, L.P.,
                                BY ITS GENERAL PARTNER

                                INSTITUTIONAL VENTURE MANAGEMENT VI, L.P.

                                /s/ Geoffrey Yang
                                ----------------------------------------
                                Geoffrey Y. Yang, General Partner


                                      -13-
<PAGE>   27

                         [REGISTRATION RIGHTS AGREEMENT]

"PURCHASERS" (cont'd)        MATRIX PARTNERS V, L.P.

                             By:  Matrix V Management Company, L.L.C.

                                  Its General Partner

                                  /s/ A. Verhalen
                                  -------------------------------------------
                                  By:   Andrew W. Verhalen, Member

                             BENCHMARK CAPITAL PARTNERS II, L.P.

                             By:  BENCHMARK CAPITAL MANAGEMENT CO. II, L.L.C.
                                  Its General Partner

                             By:  /s/ Andrew S. Rachleff
                                  -------------------------------------------
                                  Member

                             BENCHMARK FOUNDERS' FUND II, L.P.

                             By:  BENCHMARK CAPITAL MANAGEMENT CO. II, L.L.C.
                                  Its General Partner

                             By:  /s/ Andrew S. Rachleff
                                  -------------------------------------------
                                  Member

                             BENCHMARK FOUNDERS' FUND II-A, L.P.

                             By:  BENCHMARK CAPITAL MANAGEMENT CO. II, L.L.C.
                                  Its General Partner

                             By:  /s/ Andrew S. Rachleff
                                  -------------------------------------------
                                  Member


                                      -14-
<PAGE>   28

                         [REGISTRATION RIGHTS AGREEMENT]

"PURCHASERS" (cont'd)        BENCHMARK MEMBERS' FUND II, L.P.

                             By:  BENCHMARK CAPITAL MANAGEMENT CO. II, L.L.C.
                                  Its General Partner

                             By:  /s/ Andrew S. Rachleff
                                  -------------------------------------------
                                  Member

                             WS INVESTMENT COMPANY 97B

                             By:  /s/ Thomas C. DeFilipps
                                  -------------------------------------------
                                  Thomas C. DeFilipps, Esq., Member

                                  /s/ Thomas C. DeFilipps
                             ------------------------------------------------
                             Thomas C. DeFilipps, Esq. (individually)


                                      -15-
<PAGE>   29

        [REGISTRATION RIGHTS AGREEMENT SECOND CLOSING FEBRUARY 27, 1998]

         IN WITNESS WHEREOF, the undersigned have executed this Registration
Rights Agreement as of the date set forth above.

"COMPANY"               TURNSTONE SYSTEMS, INC.,
                        a Delaware corporation

                        By:  /s/ Richard N. Tinsley
                           --------------------------------------------------

                        Name: Richard N. Tinsley
                           --------------------------------------------------

                        Title: CEO
                           --------------------------------------------------

"PURCHASERS"            STANFORD UNIVERSITY

                        By:  /s/ Carol Gilmer
                           --------------------------------------------------

                        Name:    Carol Gilmer
                           --------------------------------------------------

                        Title: Assistant Secretary,  Board of Trustees of the
                               Leland Stanford Junior University
                               ----------------------------------------------

                             /s/ Tim Latchem
                        -----------------------------------------------------
                        Timothy Latchem

                             /s/ Patricia A. Crowe
                        -----------------------------------------------------
                        Patricia Crowe

                                      -16-