1
                                                                Exhibit 10.13


               AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT

                                December 28, 1995


To each of the several Purchasers named in Schedule I to the 
Series B Convertible Preferred Stock Purchase Agreement of 
even date herewith

Dear Sirs:

     This will confirm that in consideration of your agreement on the date
hereof to purchase an aggregate of up to 4,479,613 shares (the "Series B
Preferred Shares") of Series B Convertible Preferred Stock, $.01 par value
("Series B Preferred Stock"), of Concord Communications, Inc., a Massachusetts
corporation (the "Company"), pursuant to the Series B Convertible Preferred
Stock Purchase Agreement of even date herewith (the "Purchase Agreement")
between the Company and you and as an inducement to you to consummate the
transactions contemplated by the Purchase Agreement, the Company covenants and
agrees with each of you as follows:

      1. TERMINATION OF 1993 AGREEMENT. Certain of the parties hereto agree to
amend and restate in its entirety the Registration Rights Agreement dated
November 8, 1993, as amended by an Amendment No. 1 dated December 9, 1994 (the
"1993 Agreement") with this Agreement.

      1A.   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 other federal agency at the time administering the Securities Act.

            "COMMON STOCK" shall mean the Common Stock, $.01 par value, of the
      Company, as constituted as of the date of this Agreement.

            "CONVERSION SHARES" shall mean shares of Common Stock issued upon
      conversion of the Preferred Shares.

            "EXCHANGE ACT" shall mean the Securities Exchange Act of 1934, as
      amended, or any similar federal statute, and the rules and regulations of
      the Commission thereunder, all as the same shall be in effect at the time.

            "PREFERRED SHARES" shall mean the Preferred Stock held by the
      Purchasers and any parties to the 1993 Agreement who are not Purchasers.

            "PREFERRED STOCK" shall mean the Company's Series A Convertible
      Preferred Stock, $.01 par value, Series A-1 Convertible Preferred Stock,
      $.01 par value, and Series B Preferred Stock.

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            "REGISTRATION EXPENSES" shall mean the expenses so described in
      Section 8.

            "RESTRICTED STOCK" shall mean the Conversion Shares, excluding
      Conversion Shares which have been (a) registered under the Securities Act
      pursuant to an effective registration statement filed thereunder and
      disposed of in accordance with the registration statement covering them or
      (b) publicly sold pursuant to Rule 144 under the Securities Act.

            "SECURITIES ACT" shall mean the Securities Act of 1933, as amended,
      or any similar federal statute, and the rules and regulations of the
      Commission thereunder, all as the same shall be in effect at the time.

            "SELLING EXPENSES" shall mean the expenses so described in
      Section 8.

      2.    RESTRICTIVE LEGEND.  Each certificate representing Preferred
Shares or Conversion Shares shall, except as otherwise provided in this
Section 2 or in Section 3, be stamped or otherwise imprinted with a legend
substantially in the following form:

            "THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
            1933 OR ANY STATE SECURITIES LAWS AND MAY NOT BE TRANSFERRED OR
            OTHERWISE DISPOSED OF UNLESS IT HAS BEEN REGISTERED UNDER SUCH ACT
            AND ALL SUCH APPLICABLE LAWS OR AN EXEMPTION FROM REGISTRATION IS
            AVAILABLE."

A certificate shall not bear such legend if in the opinion of counsel
satisfactory to the Company (it being agreed that Testa, Hurwitz & Thibeault
shall be satisfactory) the securities being sold thereby may be publicly sold
without registration under the Securities Act and any applicable state
securities laws.

      3. NOTICE OF PROPOSED TRANSFER. Prior to any proposed transfer of any
Preferred Shares or Conversion Shares (other than under the circumstances
described in Sections 4, 5 or 6), the holder thereof shall give written notice
to the Company of its intention to effect such transfer. Each such notice shall
describe the manner of the proposed transfer and, if requested by the Company,
shall be accompanied by an opinion of counsel satisfactory to the Company (it
being agreed that Testa, Hurwitz & Thibeault shall be satisfactory) to the
effect that the proposed transfer may be effected without registration under the
Securities Act and any applicable state securities laws, whereupon the holder of
such stock shall be entitled to transfer such stock in accordance with the terms
of its notice; PROVIDED, HOWEVER, that no such opinion of counsel shall be
required for a transfer to one or more partners of the transferor (in the case
of a transferor that is a partnership) or to an affiliated corporation (in the
case of a transferor that is a corporation). Each certificate for Preferred
Shares or Conversion Shares transferred as above provided shall bear the legend
set forth in Section 2, except that such certificate shall not bear such legend
if (i) such transfer is in accordance with the provisions of Rule 144 or Rule
144A (or any other rule

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permitting public sale without registration under the Securities Act) or (ii)
the opinion of counsel referred to above is to the further effect that the
transferee and any subsequent transferee (other than an affiliate of the
Company) would be entitled to transfer such securities in a public sale without
registration under the Securities Act. The restrictions provided for in this
Section 3 shall not apply to securities which are not required to bear the
legend prescribed by Section 2 in accordance with the provisions of that
Section.

      4. REQUIRED REGISTRATION. (a) At any time after the earlier of (i) six
months after any registration statement covering a public offering of securities
of the Company under the Securities Act shall have become effective, and (ii)
November 8, 1996, the holders of Restricted Stock constituting at least 40% of
the total shares of Restricted Stock then outstanding may request the Company to
register under the Securities Act all or any portion of the shares of Restricted
Stock held by such requesting holder or holders for sale in the manner specified
in such notice, PROVIDED that the shares of Restricted Stock for which
registration has been requested shall constitute at least 20% of the total
shares of Restricted Stock originally issued if such holder or holders shall
request the registration of less than all shares of Restricted Stock then held
by such holder or holders (or any lesser percentage if the reasonably
anticipated aggregate price to the public of such public offering would exceed
$5,000,000). For purposes of this Section 4 and Sections 5, 6, 13(a) and 13(d),
the term "Restricted Stock" shall be deemed to include the number of shares of
Restricted Stock which would be issuable to a holder of Preferred Shares upon
conversion of all shares of Preferred Stock held by such holder at such time,
PROVIDED, HOWEVER, that the only securities which the Company shall be required
to register pursuant hereto shall be shares of Common Stock, and PROVIDED,
FURTHER, HOWEVER, that, in any underwritten public offering contemplated by this
Section 4 or Sections 5 and 6, the holders of Preferred Shares shall be entitled
to sell such Preferred Shares to the underwriters for conversion and sale of the
shares of Common Stock issued upon conversion thereof. Notwithstanding anything
to the contrary contained herein, no request may be made under this Section 4
within 120 days after the effective date of a registration statement filed by
the Company covering a firm commitment underwritten public offering in which the
holders of Restricted Stock shall have been entitled to join pursuant to
Sections 5 or 6 and in which there shall have been effectively registered all
shares of Restricted Stock as to which registration shall have been requested.

            (b) Following receipt of any notice under this Section 4, the
Company shall immediately notify all holders of Restricted Stock from whom
notice has not been received and shall use its best efforts to register under
the Securities Act, for public sale in accordance with the method of disposition
specified in such notice from requesting holders, the number of shares of
Restricted Stock specified in such notice (and in all notices received by the
Company from other holders within 30 days after the giving of such notice by the
Company). If such method of disposition shall be an underwritten public
offering, the holders of a majority of the shares of Restricted Stock to be sold
in such offering may designate the managing underwriter of such offering,
subject to the approval of the Company, which approval shall not be unreasonably
withheld or delayed. The Company shall be obligated to register Restricted Stock
pursuant to this Section 4 on two occasions only, PROVIDED, HOWEVER, that such
obligation shall be deemed satisfied only when a registration statement covering
all shares of Restricted Stock specified in notices received as aforesaid, for
sale in accordance with the method of disposition specified by

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the requesting holders, shall have become effective and, if such method of
disposition is a firm commitment underwritten public offering, all such shares
shall have been sold pursuant thereto.

            (c) The Company shall be entitled to include in any registration
statement referred to in this Section 4, for sale in accordance with the method
of disposition specified by the requesting holders, shares of Common Stock to be
sold by the Company for its own account, except as and to the extent that, in
the opinion of the managing underwriter (if such method of disposition shall be
an underwritten public offering), such inclusion would adversely affect the
marketing of the Restricted Stock to be sold. Except for registration statements
on Form S-4, S-8 or any successor thereto, the Company will not file with the
Commission any other registration statement with respect to its Common Stock,
whether for its own account or that of other stockholders, from the date of
receipt of a notice from requesting holders pursuant to this Section 4 until the
completion of the period of distribution of the registration contemplated
thereby.

      5. INCIDENTAL REGISTRATION. If the Company at any time (other than
pursuant to Section 4 or Section 6) proposes to register any of its securities
under the Securities Act for sale to the public, whether for its own account or
for the account of other security holders or both (except with respect to
registration statements on Forms S-4, S-8 or another form not available for
registering the Restricted Stock for sale to the public), each such time it will
give written notice to all holders of outstanding Restricted Stock of its
intention so to do. Upon the written request of any such holder, received by the
Company within 30 days after the giving of any such notice by the Company, to
register any of its Restricted Stock, the Company will use its best efforts to
cause the Restricted Stock as to which registration shall have been so requested
to be included in the securities to be covered by the registration statement
proposed to be filed by the Company, all to the extent requisite to permit the
sale or other disposition by the holder of such Restricted Stock so registered.
In the event that any registration pursuant to this Section 5 shall be, in whole
or in part, an underwritten public offering of Common Stock, the number of
shares of Restricted Stock to be included in such an underwriting may be reduced
(pro rata among the requesting holders based upon the number of shares of
Restricted Stock owned by such holders) if and to the extent that the managing
underwriter shall be of the opinion that such inclusion would adversely affect
the marketing of the securities to be sold by the Company therein, PROVIDED,
HOWEVER, that such number of shares of Restricted Stock shall not be reduced if
any shares are to be included in such underwriting for the account of any person
other than the Company or requesting holders of Restricted Stock except that if
any shares which have the benefit of a certain Registration Rights Agreement
dated August 7, 1986 are to be included in such underwriting, then the number of
shares of Restricted Stock to be included may be reduced on a pro rata basis
among the requesting holders of Restricted Stock and the holders of such other
shares based upon the aggregate number of shares of Conversion Shares and Common
Stock owned by such persons, and PROVIDED, FURTHER, HOWEVER, that in no event
may less than one-third of the total number of shares of Common Stock to be
included in such underwriting be made available for shares of Restricted Stock.
Notwithstanding the foregoing provisions, the Company may withdraw any
registration statement referred to in this Section 5 without thereby incurring
any liability to the holders of Restricted Stock.


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      6. REGISTRATION ON FORM S-3. If at any time (i) a holder or holders of
Preferred Shares or Restricted Stock request that the Company file a
registration statement on Form S-3 or any successor thereto for a public
offering of all or any portion of the shares of Restricted Stock held by such
requesting holder or holders, the reasonably anticipated aggregate price to the
public of which would exceed $1,000,000, and (ii) the Company is a registrant
entitled to use Form S-3 or any successor thereto to register such shares, then
the Company shall use its best efforts to register under the Securities Act on
Form S-3 or any successor thereto, for public sale in accordance with the method
of disposition specified in such notice, the number of shares of Restricted
Stock specified in such notice. Whenever the Company is required by this Section
6 to use its best efforts to effect the registration of Restricted Stock, each
of the procedures and requirements of Section 4 (including but not limited to
the requirement that the Company notify all holders of Restricted Stock from
whom notice has not been received and provide them with the opportunity to
participate in the offering) shall apply to such registration, PROVIDED,
HOWEVER, that there shall be no limitation on the number of registrations on
Form S-3 which may be requested and obtained under this Section 6, and PROVIDED,
FURTHER, HOWEVER, that the requirements contained in the first sentence of
Section 4(a) shall not apply to any registration on Form S-3 which may be
requested and obtained under this Section 6.

      7. REGISTRATION PROCEDURES. If and whenever the Company is required
by the provisions of Sections 4, 5 or 6 to use its best efforts to effect the
registration of any shares of Restricted Stock under the Securities Act, the
Company will, as expeditiously as possible:

            (a) prepare and file with the Commission a registration statement
(which, in the case of an underwritten public offering pursuant to Section 4,
shall be on Form S-1 or other form of general applicability satisfactory to the
managing underwriter selected as therein provided) with respect to such
securities and use its best efforts to cause such registration statement to
become and remain effective for the period of the distribution contemplated
thereby (determined as hereinafter provided);

            (b) prepare and file with the Commission such amendments and
supplements to such registration statement and the prospectus used in connection
therewith as may be necessary to keep such registration statement effective for
the period specified in paragraph (a) above and comply with the provisions of
the Securities Act with respect to the disposition of all Restricted Stock
covered by such registration statement in accordance with the sellers' intended
method of disposition set forth in such registration statement for such period;

            (c) furnish to each seller of Restricted Stock and to each
underwriter such number of copies of the registration statement and the
prospectus included therein (including each preliminary prospectus) as such
persons reasonably may request in order to facilitate the public sale or other
disposition of the Restricted Stock covered by such registration statement;

            (d) use its best efforts to register or qualify the Restricted Stock
covered by such registration statement under the securities or "blue sky" laws
of such jurisdictions as the sellers of Restricted Stock or, in the case of an
underwritten public offering, the managing underwriter reasonably shall request,
PROVIDED, HOWEVER, that the Company shall not for any such 

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purpose be required to qualify generally to transact business as a foreign
corporation in any jurisdiction where it is not so qualified or to consent to
general service of process in any such jurisdiction;

            (e) use its best efforts to list the Restricted Stock covered by
such registration statement with any securities exchange on which the Common
Stock of the Company is then listed;

            (f) immediately notify each seller of Restricted Stock and each
underwriter under such registration statement, at any time when a prospectus
relating thereto is required to be delivered under the Securities Act, of the
happening of any event of which the Company has knowledge as a result of which
the prospectus contained 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 light of the circumstances then existing;

            (g) if the offering is underwritten and at the request of any seller
of Restricted Stock, use its best efforts to furnish on the date that Restricted
Stock is delivered to the underwriters for sale pursuant to such registration:
(i) an opinion dated such date of counsel representing the Company for the
purposes of such registration, addressed to the underwriters and to such seller,
stating that such registration statement has become effective under the
Securities Act and that (A) to the best knowledge of such counsel, no stop order
suspending the effectiveness thereof has been issued and no proceedings for that
purpose have been instituted or are pending or contemplated under the Securities
Act, (B) the registration statement, the related prospectus and each amendment
or supplement thereof comply as to form in all material respects with the
requirements of the Securities Act (except that such counsel need not express
any opinion as to financial statements contained therein) and (C) to such other
effects as reasonably may be requested by counsel for the underwriters or by
such seller or its counsel and (ii) a letter dated such date from the
independent public accountants retained by the Company, addressed to the
underwriters and to such seller, stating that they are independent public
accountants within the meaning of the Securities Act and that, in the opinion of
such accountants, the financial statements of the Company included in the
registration statement or the prospectus, or any amendment or supplement
thereof, comply as to form in all material respects with the applicable
accounting requirements of the Securities Act, and such letter shall
additionally cover such other financial matters (including information as to the
period ending no more than five business days prior to the date of such letter)
with respect to such registration as such underwriters reasonably may request;
and

            (h) make available for inspection by each seller of Restricted
Stock, any underwriter participating in any distribution pursuant to such
registration statement, and any attorney, accountant or other agent retained by
such seller or underwriter, all financial and other records, pertinent corporate
documents and properties of the Company, and cause the Company's officers,
directors and employees to supply all information reasonably requested by any
such seller, underwriter, attorney, accountant or agent in connection with such
registration statement.

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      For purposes of Section 7(a) and 7(b) and of Section 4(c), the period of
distribution of Restricted Stock in a firm commitment underwritten public
offering shall be deemed to extend until each underwriter has completed the
distribution of all securities purchased by it, and the period of distribution
of Restricted Stock in any other registration shall be deemed to extend until
the earlier of the sale of all Restricted Stock covered thereby and 120 days
after the effective date thereof.

      In connection with each registration hereunder, the sellers of Restricted
Stock will furnish to the Company in writing such information with respect to
themselves and the proposed distribution by them as reasonably shall be
necessary in order to assure compliance with federal and applicable state
securities laws.

      In connection with each registration pursuant to Sections 4, 5 or 6
covering an underwritten public offering, the Company and each seller agree to
enter into a written agreement with the managing underwriter selected in the
manner herein provided in such form and containing such provisions as are
customary in the securities business for such an arrangement between such
underwriter and companies of the Company's size and investment stature.

      8. EXPENSES. All expenses incurred by the Company in complying with
Sections 4, 5 and 6, including, without limitation, all registration and filing
fees, printing expenses, fees and disbursements of counsel and independent
public accountants for the Company, fees and expenses (including counsel fees)
incurred in connection with complying with state securities or "blue sky" laws,
fees of the National Association of Securities Dealers, Inc., transfer taxes,
fees of transfer agents and registrars, costs of insurance and fees and
disbursements of one counsel for the sellers of Restricted Stock, but excluding
any Selling Expenses, are called "Registration Expenses". All underwriting
discounts and selling commissions applicable to the sale of Restricted Stock are
called "Selling Expenses".

      The Company will pay all Registration Expenses in connection with each
registration statement under Sections 4, 5 or 6. All Selling Expenses in
connection with each registration statement under Sections 4, 5 or 6 shall be
borne by the participating sellers in proportion to the number of shares sold by
each, or by such participating sellers other than the Company (except to the
extent the Company shall be a seller) as they may agree.

      9. INDEMNIFICATION AND CONTRIBUTION. (a) In the event of a registration of
any of the Restricted Stock under the Securities Act pursuant to Sections 4, 5
or 6, the Company will indemnify and hold harmless each seller of such
Restricted Stock thereunder, each underwriter of such Restricted Stock
thereunder and each other person, if any, who controls such seller or
underwriter within the meaning of the Securities Act, against any losses,
claims, damages or liabilities, joint or several, to which such seller,
underwriter or controlling person may become subject under the Securities Act or
otherwise, insofar as such losses, claims, damages or liabilities (or actions in
respect thereof) arise out of or are based upon any untrue statement or alleged
untrue statement of any material fact contained in any registration statement
under which such Restricted Stock was registered under the Securities Act
pursuant to Sections 4, 5 or 6, any preliminary prospectus or final prospectus
contained therein, or any amendment or supplement

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thereof, or arise out of or are based upon 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, and will reimburse each such seller, each
such underwriter and each such controlling person for any legal or other
expenses reasonably incurred by them in connection with investigating or
defending any such loss, claim, damage, liability or action, PROVIDED, HOWEVER,
that the Company will not be liable in any such case if and to the extent that
any such loss, claim, damage or liability arises out of or is based upon an
untrue statement or alleged untrue statement or omission or alleged omission so
made in conformity with information furnished by any such seller, any such
underwriter or any such controlling person in writing specifically for use in
such registration statement or prospectus.

            (b) In the event of a registration of any of the Restricted Stock
under the Securities Act pursuant to Sections 4, 5 or 6, each seller of such
Restricted Stock thereunder, severally and not jointly, will indemnify and hold
harmless the Company, each person, if any, who controls the Company within the
meaning of the Securities Act, each officer of the Company who signs the
registration statement, each director of the Company, each underwriter and each
person who controls any underwriter within the meaning of the Securities Act,
against all losses, claims, damages or liabilities, joint or several, to which
the Company or such officer, director, underwriter or controlling person may
become subject under the Securities Act or otherwise, insofar as such losses,
claims, damages or liabilities (or actions in respect thereof) arise out of or
are based upon any untrue statement or alleged untrue statement of any material
fact contained in the registration statement under which such Restricted Stock
was registered under the Securities Act pursuant to Sections 4, 5 or 6, any
preliminary prospectus or final prospectus contained therein, or any amendment
or supplement thereof, or arise out of or are based upon 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, and will reimburse the
Company and each such officer, director, underwriter and controlling person for
any legal or other expenses reasonably incurred by them in connection with
investigating or defending any such loss, claim, damage, liability or action,
PROVIDED, HOWEVER, that such seller will be liable hereunder in any such case if
and only to the extent that any such loss, claim, damage or liability arises out
of or is based upon an untrue statement or alleged untrue statement or omission
or alleged omission made in reliance upon and in conformity with information
pertaining to such seller, as such, furnished in writing to the Company by such
seller specifically for use in such registration statement or prospectus, and
PROVIDED, FURTHER, HOWEVER, that the liability of each seller hereunder shall be
limited to the proportion of any such loss, claim, damage, liability or expense
which is equal to the proportion that the public offering price of the shares
sold by such seller under such registration statement bears to the total public
offering price of all securities sold thereunder, but not in any event to exceed
the proceeds received by such seller from the sale of Restricted Stock covered
by such registration statement.

            (c) Promptly after receipt by an indemnified party hereunder of
notice of the commencement of any action, such indemnified party shall, if a
claim in respect thereof is to be made against the indemnifying party hereunder,
notify the indemnifying party in writing thereof, but the omission so to notify
the indemnifying party shall not relieve it from any liability which it may have
to such indemnified party other than under this Section 9 and shall only relieve
it

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from any liability which it may have to such indemnified party under this
Section 9 if and to the extent the indemnifying party is prejudiced by such
omission. In case any such action shall be brought against any indemnified party
and it shall notify the indemnifying party of the commencement thereof, the
indemnifying party shall be entitled to participate in and, to the extent it
shall wish, to assume and undertake the defense thereof with counsel
satisfactory to such indemnified party, and, after notice from the indemnifying
party to such indemnified party of its election so to assume and undertake the
defense thereof, the indemnifying party shall not be liable to such indemnified
party under this Section 9 for any legal expenses subsequently incurred by such
indemnified party in connection with the defense thereof other than reasonable
costs of investigation and of liaison with counsel so selected, PROVIDED,
HOWEVER, that, if the defendants in any such action include both the indemnified
party and the indemnifying party and the indemnified party shall have reasonably
concluded that there may be reasonable defenses available to it which are
different from or additional to those available to the indemnifying party or if
the interests of the indemnified party reasonably may be deemed to conflict with
the interests of the indemnifying party, the indemnified party shall have the
right to select a separate counsel and to assume such legal defenses and
otherwise to participate in the defense of such action, with the expenses and
fees of such separate counsel and other expenses related to such participation
to be reimbursed by the indemnifying party as incurred.

            (d) In order to provide for just and equitable contribution to joint
liability under the Securities Act in any case in which either (i) any holder of
Restricted Stock exercising rights under this Agreement, or any controlling
person of any such holder, makes a claim for indemnification pursuant to this
Section 9 but it is judicially determined (by the entry of a final judgment or
decree by a court of competent jurisdiction and the expiration of time to appeal
or the denial of the last right of appeal) that such indemnification may not be
enforced in such case notwithstanding the fact that this Section 9 provides for
indemnification in such case, or (ii) contribution under the Securities Act may
be required on the part of any such selling holder or any such controlling
person in circumstances for which indemnification is provided under this Section
9; then, and in each such case, the Company and such holder will contribute to
the aggregate losses, claims, damages or liabilities to which they may be
subject (after contribution from others) in such proportion so that such holder
is responsible for the portion represented by the percentage that the public
offering price of its Restricted Stock offered by the registration statement
bears to the public offering price of all securities offered by such
registration statement, and the Company is responsible for the remaining
portion; PROVIDED, HOWEVER, that, in any such case, (A) no such holder will be
required to contribute any amount in excess of the public offering price of all
such Restricted Stock offered by it pursuant to such registration statement; and
(B) no person or entity guilty of fraudulent misrepresentation (within the
meaning of Section 11(f) of the Securities Act) will be entitled to contribution
from any person or entity who was not guilty of such fraudulent
misrepresentation.

      10. CHANGES IN COMMON STOCK OR PREFERRED STOCK. If, and as often as, there
is any change in the Common Stock or the Preferred Stock by way of a stock
split, stock dividend, combination or reclassification, or through a merger,
consolidation, reorganization or recapitalization, or by any other means,
appropriate adjustment shall be made in the provisions

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hereof so that the rights and privileges granted hereby shall continue with
respect to the Common Stock or the Preferred Stock as so changed.

      11. 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 Stock to the public without registration, at all times
after 90 days after any registration statement covering a public offering of
securities of the Company under the Securities Act shall have become effective,
the Company agrees to:

            (a) make and keep public information available, as those terms
are understood and defined in Rule 144 under the Securities Act;

            (b) use its best efforts to file with the Commission in a timely
manner all reports and other documents required of the Company under the
Securities Act and the Exchange Act; and

            (c) furnish to each holder of Restricted Stock forthwith upon
request a written statement by the Company as to its compliance with the
reporting requirements of such Rule 144 and of the Securities Act and the
Exchange Act, a copy of the most recent annual or quarterly report of the
Company, and such other reports and documents so filed by the Company as such
holder may reasonably request in availing itself of any rule or regulation of
the Commission allowing such holder to sell any Restricted Stock without
registration.

      12. REPRESENTATIONS AND WARRANTIES OF THE COMPANY.  The Company
represents and warrants to you as follows:

            (a) The execution, delivery and performance of this Agreement by the
Company have been duly authorized by all requisite corporate action and will not
violate any provision of law, any order of any court or other agency of
government, the Charter or By-laws of the Company or any provision of any
indenture, agreement or other instrument to which it or any or its properties or
assets is bound, conflict with, result in a breach of or constitute (with due
notice or lapse of time or both) a default under any such indenture, agreement
or other instrument or result in the creation or imposition of any lien, charge
or encumbrance of any nature whatsoever upon any of the properties or assets of
the Company.

            (b) This Agreement has been duly executed and delivered by the
Company and constitutes the legal, valid and binding obligation of the Company,
enforceable in accordance with its terms.

      13. MISCELLANEOUS.

            (a) All covenants and agreements contained in this Agreement by or
on behalf of any of the parties hereto shall bind and inure to the benefit of
the respective successors and assigns of the parties hereto (including without
limitation transferees of any Preferred Shares or Restricted Stock), whether so
expressed or not, PROVIDED, HOWEVER, that registration rights


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conferred herein on the holders of Preferred Shares or Restricted Stock shall
only inure to the benefit of a transferee of Preferred Shares or Restricted
Stock if (i) there is transferred to such transferee at least 20% of the total
shares of Restricted Stock originally issued pursuant to the Purchase Agreement
or the Series A Convertible Preferred Stock Purchase Agreement dated November 8,
1993, as amended ("1993 Purchase Agreement"), as applicable, to the direct or
indirect transferor of such transferee or (ii) such transferee is a partner,
shareholder or affiliate of a party hereto.

            (b) All notices, requests, consents and other communications
hereunder shall be in writing and shall be delivered in person, mailed by
certified or registered mail, return receipt requested, or sent by telecopier or
telex, addressed as follows:

            if to the Company or any other party hereto, at the address of such
      party set forth in the Purchase Agreement or in the 1993 Purchase
      Agreement with respect to any party who is not a Purchaser;

            if to any subsequent holder of Preferred Shares or Restricted Stock,
      to it at such address as may have been furnished to the Company in writing
      by such holder;

or, in any case, at such other address or addresses as shall have been furnished
in writing to the Company (in the case of a holder of Preferred Shares or
Restricted Stock) or to the holders of Preferred Shares or Restricted Stock (in
the case of the Company) in accordance with the provisions of this paragraph.

            (c) This Agreement shall be governed by and construed in accordance
with the laws of the Commonwealth of Massachusetts.

            (d) This Agreement may not be amended or modified, and no provision
hereof may be waived, without the written consent of the Company and the holders
of at least two-thirds of the outstanding shares of Restricted Stock.

            (e) 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.

            (f) The obligations of the Company to register shares of Restricted
Stock under Sections 4, 5 or 6 shall terminate on the fifteenth anniversary of
the date of this Agreement.

            (g) If requested in writing by the underwriters for the initial
underwritten public offering of securities of the Company, each holder of
Restricted Stock who is a party to this Agreement (including any party to the
1993 Agreement who is not a Purchaser) shall agree not to sell publicly any
shares of Restricted Stock or any other shares of Common Stock (other than
shares of Restricted Stock or other shares of Common Stock being registered in
such offering), without the consent of such underwriters, for a period of not
more than 120 days

   12


following the effective date of the registration statement relating to such
offering; PROVIDED, HOWEVER, that all persons entitled to registration rights
with respect to shares of Common Stock who are not parties to this Agreement,
all other persons selling shares of Common Stock in such offering, all persons
holding in excess of 1% of the capital stock of the Company on a fully diluted
basis and all executive officers and directors of the Company shall also have
agreed not to sell publicly their Common Stock under the circumstances and
pursuant to the terms set forth in this Section 13(g).

            (h) Notwithstanding the provisions of Section 7(a), the Company's
obligation to file a registration statement, or cause such registration
statement to become and remain effective, shall be suspended for a period not to
exceed 90 days in any 24-month period if there exists at the time material
non-public information relating to the Company which, in the reasonable opinion
of the Company, should not be disclosed.

            (i) The Company shall not grant to any third party any registration
rights more favorable than any of those contained herein, so long as any of the
registration rights under this Agreement remains in effect.

            (j) If any provision of this Agreement shall be held to be illegal,
invalid or unenforceable, such illegality, invalidity or unenforceability shall
attach only to such provision and shall not in any manner affect or render
illegal, invalid or unenforceable any other provision of this Agreement, and
this Agreement shall be carried out as if any such illegal, invalid or
unenforceable provision were not contained herein.



   13


                                      -13-



               [SIGNATURE PAGES TO REGISTRATION RIGHTS AGREEMENT]

            Please indicate your acceptance of the foregoing by signing and
returning the enclosed counterpart of this letter, whereupon this Agreement
shall be a binding agreement between the Company and you.

                                    Very truly yours,


                                    CONCORD COMMUNICATIONS, INC.


                                    By:/s/ JOHN A. BLAESER
                                       -------------------
                                    Title: President


   14


                                      -14-

AGREED TO AND ACCEPTED as of the date first
above written.


                              Purchasers named in Schedule I to the
                              Purchase Agreement:

                              CHARLES RIVER PARTNERSHIP VII,
                              LIMITED PARTNERSHIP

                              By:  CHARLES RIVER VII GP LIMITED PARTNERSHIP
                                   ----------------------------------------
                              Its: General Partner

                              By:  /s/ R.M. BURNES
                                   ----------------------------------------
                                   General Partner


                              APEX INVESTMENT FUND II, L.P.

                              By:  Apex Management Partnership, L.P.

                              Its: General Partner

                              By:  /s/
                                   ----------------------------------------
                                   General Partner


                              THE PRODUCTIVITY FUND II, L.P.

                              By:  First Analysis Management Co. II

                              Its: General Partner

                              By:  First Analysis Corporation,
                                   General Partner

                              By:  /s/
                                   ----------------------------------------

   15

                                      -15-


                              CANAAN VENTURES II LIMITED PARTNERSHIP

                              By:   Canaan Venture Partners II L.P.

                              Its:  General Partner

                              By:  /s/
                                   ----------------------------------------
                                   General Partner


                              CANAAN VENTURES II OFFSHORE C.V.

                              By:  Canaan Venture Partners II L.P.

                              Its: General Partner

                              By:  /s/
                                   ----------------------------------------
                                   General Partner


                              PIONEER VENTURES LIMITED PARTNERSHIP

                              By:  Pioneer SBIC Corp.

                              Its: General Partner

                              By:  /s/ L.W. DIEK
                                   ----------------------------------------
                                   L.W. Diek
                                   Vice President


                              MASSACHUSETTS TECHNOLOGY
                              DEVELOPMENT CORPORATION

                              By:  /s/ ROBERT J. CREEDON
                                   ----------------------------------------
                                   Vice President
   16


                                      -16-


                              JOHN HANCOCK VENTURE CAPITAL
                              FUND LIMITED PARTNERSHIP II

                              By:  its General Partner:
                                   Back Bay Partners L.P. II

                              By:  One of Its General Partners:
                                   Hancock Venture Partners, Inc.

                              By:  /s/ ROBERT M. WADSWORTH
                                   ----------------------------------------
                                   Vice President


                              BATTERY VENTURES L.P.

                              By:  /s/ RICH FRISBEE
                                   ----------------------------------------

                              EVERGREEN I LIMITED PARTNERSHIP

                              By:  its General Partner:
                                   Back Bay Partners L.P.

                              By:  One of Its General Partners:
                                   Hancock Venture Partners, Inc.

                              By:  /s/ ROBERT M. WADSWORTH
                                   ----------------------------------------
                                   Vice President


                              B.U.N.P

                              By:  /s/ JOHN E. BAGALY
                                   ----------------------------------------

                              BAY PARTNERS IV

                              By:  Bay Management Company IV, L.P.
                                   General Partner

                              By:  /s/ JOHN GORCH
                                   ----------------------------------------
                                   General Partner

   17

                                      -17-



                              CALIFORNIA BPIV, L.P.

                              By:  /s/ JOHN GORCH
                                   ----------------------------------------
                                   General Partner



                              BAY PARTNERS SBIC, L.P.
                              By:  Bay Management Company 1995,
                                   General Partner

                              By:  /s/ JAY FREEDMAN
                                   ----------------------------------------
                                   General Partner


                              /s/ ANTHONY M. HELIES
                              Anthony M. Helies


                              GRACE VENTURES CORP.

                              By:  /s/ ROBERT E. PERDIGO
                                   ----------------------------------------
                                   Robert E. Perdigo
                                   Executive Vice President