Document:

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                                                                    EXHIBIT 10.1

                  THIRD AMENDMENT TO FIRST AMENDED AND RESTATED
                                CREDIT AGREEMENT

         This Third Amendment to First Amended and Restated Credit Agreement is
made and effective as of March 31, 2000, by and between CARBO CERAMICS INC., a
Delaware corporation (the "Borrower"), and BROWN BROTHERS HARRIMAN & CO., a New
York limited partnership (the "Lender").

                                   WITNESSETH:

         WHEREAS, the Lender and the Borrower entered into a First Amended and
Restated Credit Agreement dated as of February 12, 1998 by and between the
Lender and the Borrower (as heretofore amended by that certain First Amendment
dated as of March 31, 1999 and that certain Second Amendment dated as of
December 31, 1999, the "Credit Agreement"), pursuant to which the Lender
committed, among other things, to make loans, advances, and other credit
facilities available to the Borrower;

         WHEREAS, the parties desire to amend the Credit Agreement in certain
respects, as hereinafter set forth;

         NOW, THEREFORE, in consideration of the foregoing, for other valuable
consideration hereby acknowledged, and subject to the other terms and conditions
hereof, the Lender and the Borrower agree that Subsection (c) of Section 5.01 of
the Credit Agreement is hereby amended and restated, effective the date hereof,
as follows:

         "(c)     Earnings Ratio. Maintain a ratio of its EBITDA to its Fixed
                  Charges of not less than 1.75 to 1.0 at March 31, 2000,
                  calculated quarterly on a rolling four (4) quarter basis for
                  the four (4) quarters immediately preceding the date of such
                  calculation.

         1. By its execution hereof, the Borrower hereby certifies to the Lender
that as of the date hereof (i) all representations and warranties heretofore
made by it under the Credit Agreement and the other Loan Documents are true in
all material respects, (ii) the Borrower does not have or claim to have any
offset, counterclaim, or defense to any of its obligations under the Credit
Agreement or any of the Loan Documents, and (iii) in consideration of and to
induce the Lender's agreement to the terms of this Third Amendment, the Borrower
for itself and its successors and assigns, does hereby RELEASE, ACQUIT, and
FOREVER DISCHARGE the Lender, its partners, officers, employees, agents,
attorneys, other representatives, and all other persons (collectively, the
"Indemnitees") who might be liable, from any and all claims, demands,

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liabilities, and causes of action of whatsoever nature, whether in contract or
in tort, or arising under or by virtue of any statute, rule, regulation, or
other laws, for all losses and damages, including but not limited to exemplary
and punitive damages, that have accrued or may ever accrue to the Borrower and
its successors and assigns, and which arise out of, result from, or are caused
by any act or omission of any one or more of the Indemnitees on or prior to the
date hereof in connection with the Loan, the Loan Documents, this Third
Amendment, or any matter or thing in connection therewith or related thereto.

         2. Except as expressly set forth herein, all terms and provisions of
the Credit Agreement are hereby confirmed as in full force and effect. The
Borrower expressly agrees by its execution hereof that all benefits created and
conferred by the Credit Agreement cover and extend to all obligations evidenced
and created by the Note, the Credit Agreement as amended hereby, and all
renewals, extensions, and modifications thereof, in whole or in part. All terms
with their initial letters capitalized herein and not otherwise defined herein
have the same meanings set forth in the Credit Agreement.

         3. Subject to satisfaction of the requirements for the effectiveness of
this Third Amendment, as set forth in paragraph 4 hereof, and the agreement of
the Borrower to the terms and provisions in this Third Amendment, the Lender
hereby agrees to the foregoing amendment to the Credit Agreement.

         4. This Third Amendment shall not be effective until the Lender shall
have received the following, each dated such day and in form and substance
satisfactory to the Lender:

         a. This Third Amendment, duly executed by all parties hereto; and

         b. Payment by the Borrower of all costs, expenses, and fees, including
            but not limited to reasonable attorneys' fees and costs, incurred by
            the Lender in connection with this Third Amendment.

         5. This Third Amendment may be executed in any number of counterparts,
all of which taken together shall constitute one and the same instrument. In
making proof hereof, it shall not be necessary to produce or account for any
counterpart other than one signed by the party against which enforcement is
sought.

               REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK.

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         IN WITNESS WHEREOF, the parties hereto have caused this Third Amendment
to be executed by their respective officers thereunto duly authorized as of the
date first above written.

                                         CARBO CERAMICS INC.

                                         By:
                                             --------------------------------
                                         Print Name:
                                                     ------------------------
                                         Title:
                                                 ----------------------------

                                         per pro BROWN BROTHERS HARRIMAN & CO.

                                         By:
                                             --------------------------------
                                         Print Name:
                                                     ------------------------
                                         Title:
                                                 ----------------------------

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                                                                     EXHIBIT 4.1

                          REGISTRATION RIGHTS AGREEMENT

         This Agreement dated as of October 29, 1999 is entered into by and
among Concord Communications, Inc., a Massachusetts corporation (the "Company"),
Cheryl Krupczak and Robert Krupczak (the "Purchasers").

         Recitals

         WHEREAS, the Company, E Acquisition Corp., Empire Technologies, Inc.
and the Purchasers have entered into an Agreement and Plan of Reorganization of
even date herewith (the "Merger Agreement"); and

         WHEREAS, the Company and the Purchasers desire to provide for certain
arrangements with respect to the registration of shares of capital stock of the
Company under the Securities Act of 1933;

         NOW, THEREFORE, in consideration of the mutual promises and covenants
contained in this Agreement, the parties hereto agree as follows:

         1.       Certain Definitions.

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

                  "Commission" means the Securities and Exchange Commission, or
any other federal agency at the time administering the Securities Act.

                  "Common Stock" means the common stock, $.01 par value per
share, of the Company.

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

                  "Initiating Holders" means the Stockholders initiating a
request for registration pursuant to Section 2.1.

                  "Prospectus" means the prospectus included in any Registration
Statement, as amended or supplemented by an amendment or prospectus supplement,
including post-effective amendments, and all material incorporated by reference
or deemed to be incorporated by reference in such Prospectus.

                  "Registration Expenses" means the expenses described in
Section 2.4.
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                  "Registration Statement" means a registration statement filed
by the Company with the Commission for a public offering and sale of securities
of the Company, including, without limitation, any public offering on behalf of
any other person granted registration rights by the Company and any registration
statement covering securities proposed to be issued in exchange for securities
or assets of another corporation (other than a registration statement on Form
S-8 or Form S-4, or their successors, or any other form for a similar limited
purpose, ).

                  "Registrable Shares" means the shares of Common Stock issued
to either Purchaser in connection with the Merger; provided, however, that
shares of Common Stock which are Registrable Shares shall cease to be
Registrable Shares upon any sale pursuant to a Registration Statement.

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

                  "Selling Stockholder" means any Stockholder owning Registrable
Shares included in a Registration Statement.

                  "Stockholders" means the Purchasers and any permitted assigns
pursuant to Section 3 hereof.

         Capitalized terms not otherwise defined herein shall have the meanings
provided in the Merger Agreement.

         2.       Registration Rights

                  2.1      Incidental Registration.

                           (a) If the Company proposes to file a Registration
Statement it will, prior to such filing, give written notice to all Stockholders
of its intention to do so; provided that no such notice need be given if no
Registrable Shares are to be included therein as a result of a determination of
the managing underwriter; provided that the Company will exercise its best
efforts to cause the managing underwriter to include at least 200,000
Registrable Shares in such Registration Statement. Upon the written request of a
Stockholder or Stockholders given within 7 days after the Company provides such
notice (which request shall state the intended method of disposition of such
Registrable Shares), the Company shall use its commercially reasonable efforts
to cause all Registrable Shares which the Company has been requested by such
Stockholder or Stockholders to register to be registered under the Securities
Act to the extent necessary to permit their sale or other disposition in
accordance with the intended methods of distribution specified in the request of
such Stockholder or Stockholders; provided that the Company shall have the right
to postpone or withdraw any registration effected pursuant to this Section 2.1
without obligation to any Stockholder.

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                           (b) If the registration for which the Company gives
notice pursuant to Section 2.1(a) is a registered public offering involving an
underwriting, the Company shall so advise the Stockholders as a part of the
written notice given pursuant to Section 2.1(a). In such event, the right of any
Stockholder to include its Registrable Shares in such registration pursuant to
Section 2.1 shall be conditioned upon such Stockholder's participation in such
underwriting on the terms set forth herein. All Stockholders proposing to
distribute their securities through such underwriting shall enter into an
underwriting agreement in customary form with the underwriter or underwriters
selected for the underwriting by the Company. Notwithstanding any other
provision of this Section 2.1, if the managing underwriter determines that the
inclusion of all shares requested to be registered would adversely affect the
offering, the Company may limit the number of Registrable Shares to be included
in the registration and underwriting. The Company shall so advise all holders of
Registrable Shares requesting registration. The securities of the Company held
by the Stockholders shall be excluded from such registration and underwriting to
the extent deemed advisable by the managing underwriter, and underwriting shall
be allocated among all Stockholders requesting registration in proportion, as
nearly as practicable, to the respective number of shares of Common Stock which
they held at the time the Company gives the notice specified in Section 2.1(a).
If any Stockholder would thus be entitled to include more securities than such
holder requested to be registered, the excess shall be allocated among other
requesting Stockholders pro rata in the manner described in the preceding
sentence. If any holder of Registrable Shares or any officer or director
disapproves of the terms of any such underwriting, such person may elect to
withdraw therefrom by written notice to the Company, and any Registrable Shares
or other securities excluded or withdrawn from such underwriting shall be
withdrawn from such registration.

                  2.2.     Registrations on Form S-3.

         If at any time after October 29, 2000, (i) a Stockholder requests 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 Registrable Shares held by
such Stockholder, the reasonably anticipated aggregate price to the public of
which would exceed $5,000,000, 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 the number of Registrable Shares;
provided, however, that (A) the number of registrations on Form S-3 which may be
requested and obtained under this Section 2.2 shall be limited to three, and (B)
the Company shall not be obligated to file a registration statement on Form S-3
if at the time of any such request the Registrable Shares are eligible for sale
pursuant to Rule 144 and the volume limitations, if any, imposed on the
Stockholders at the time of such request would prevent the Stockholders from
selling up to an aggregate of 200,000 of such shares during any ninety (90) day
period. The registration rights in this section 2.2 are in addition to the
registration rights afforded under section 2.1. Notwithstanding anything to the
contrary contained herein, the Holders agree not to sell under any registration
statement or otherwise more than 25% of their Registrable Shares in any one
quarter unless the Company otherwise agrees in writing.

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                  2.3      Registration Procedures.

                           (a) If and whenever the Company is required by the
provisions of this Agreement to effect the registration of any Registrable
Shares under the Securities Act, the Company shall:

                                    (i) file with the Commission a Registration
Statement on Form S-3 with respect to such Registrable Shares and use its
commercially reasonable efforts to cause that Registration Statement to become
effective as soon as possible;

                                    (ii) as expeditiously as possible prepare
and file with the Commission any amendments and supplements to the Registration
Statement on Form S-3 and the prospectus included in the Registration Statement
on Form S-3 as may be necessary to comply with the provisions of the Securities
Act (including the anti-fraud provisions thereof) and to keep the Registration
Statement on Form S-3 effective until all such Registrable Shares are sold;

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

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

                                    (v) as expeditiously as possible, cause all
such Registrable Shares to be listed on each securities exchange or automated
quotation system on which similar securities issued by the Company are then
listed;

                                    (vi) promptly provide a transfer agent and
registrar for all such Registrable Shares not later than the effective date of
such registration statement;

                                    (vii) promptly make available for inspection
by the Selling Stockholders, any managing underwriter participating in any
disposition pursuant to such Registration Statement, and any attorney or
accountant or other agent retained by any such underwriter or selected by the
Selling Stockholders, all financial and other records, pertinent corporate
documents and properties of the Company and cause the Company's officers,
directors, employees and independent accountants to supply all information
reasonably requested by any

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<PAGE>   5
such seller, underwriter, attorney, accountant or agent in connection with such
Registration Statement;

                                    (viii) as expeditiously as possible, notify
each Selling Stockholder, promptly after
it shall receive notice thereof, of the time when such Registration Statement
has become effective or a supplement to any Prospectus forming a part of such
Registration Statement has been filed; and

                                    (ix) as expeditiously as possible following
the effectiveness of such Registration
Statement, notify each seller of such Registrable Shares of any request by the
Commission for the amending or supplementing of such Registration Statement or
Prospectus.

                           (b) If the Company has delivered a Prospectus to the
Selling Stockholders and after having done so the Prospectus is amended to
comply with the requirements of the Securities Act, the Company shall promptly
notify the Selling Stockholders and, if requested, the Selling Stockholders
shall immediately cease making offers of Registrable Shares and return all
Prospectuses to the Company. The Company shall promptly provide the Selling
Stockholders with revised Prospectuses and, following receipt of the revised
Prospectuses, the Selling Stockholders shall be free to resume making offers of
the Registrable Shares.

                           (c) In the event that, in the judgment of the
Company, it is advisable to suspend use of a Prospectus included in a
Registration Statement due to pending material developments or other events that
have not yet been publicly disclosed and as to which the Company believes public
disclosure would be detrimental to the Company, the Company shall notify all
Selling Stockholders to such effect, and, upon receipt of such notice, each such
Selling Stockholder shall immediately discontinue any sales of Registrable
Shares pursuant to such Registration Statement until such Selling Stockholder
has received copies of a supplemented or amended Prospectus or until such
Selling Stockholder is advised in writing by the Company that the then current
Prospectus may be used and has received copies of any additional or supplemental
filings that are incorporated or deemed incorporated by reference in such
Prospectus. Notwithstanding anything to the contrary herein, the Company shall
not exercise its rights under this Section 2.3(c) to suspend sales of
Registrable Shares for a period in excess of 180 days in any 365-day period.

                  2.4 Allocation of Expenses. The Company will pay all
Registration Expenses for all registrations under this Agreement; provided,
however, that if a registration under Section 2.1 is withdrawn at the request of
the Initiating Holders (other than as a result of information concerning the
business or financial condition of the Company which is made known to the
Stockholders after the date on which such registration was requested) and if the
Initiating Holders elect not to have such registration counted as a registration
requested under Section 2.1, the requesting Stockholders shall pay the
Registration Expenses of such registration pro rata in accordance with the
number of their Registrable Shares included in such registration. For purposes
of this Section, the term "Registration Expenses" shall mean all expenses
incurred by the Company in complying with this Agreement, including, without
limitation, all registration

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and filing fees, exchange listing fees, printing expenses, fees and expenses of
counsel for the Company and the reasonable fees and expenses of one counsel
selected by the Selling Stockholders to represent the Selling Stockholders,
state Blue Sky fees and expenses, and the expense of any special audits incident
to or required by any such registration, but excluding underwriting discounts,
selling commissions and the fees and expenses of Selling Stockholders' own
counsel (other than the counsel selected to represent all Selling Stockholders).

                  2.5      Indemnification and Contribution.

                           (a) In the event of any registration of any of the
Registrable Shares under the Securities Act pursuant to this Agreement, the
Company will indemnify and hold harmless each Selling Stockholder, each
underwriter of such Registrable Shares, and each other person, if any, who
controls such Selling Stockholder or underwriter within the meaning of the
Securities Act or the Exchange Act against any losses, claims, damages or
liabilities, joint or several, to which such Selling Stockholder, underwriter or
controlling person may become subject under the Securities Act, the Exchange
Act, state securities or Blue Sky laws or otherwise, insofar as such losses,
claims, damages or liabilities (or actions in respect thereof) arise out of or
are based upon any untrue statement or alleged untrue statement of any material
fact contained in any Registration Statement under which such Registrable Shares
were registered under the Securities Act, any preliminary prospectus or final
prospectus contained in the Registration Statement, or any amendment or
supplement to such Registration Statement, or arise out of or are based upon the
omission or alleged omission to state a material fact required to be stated
therein or necessary to make the statements therein not misleading; and the
Company will reimburse such Selling Stockholder, underwriter and each such
controlling person for any legal or any other expenses reasonably incurred by
such Selling Stockholder, underwriter or controlling person in connection with
investigating or defending any such loss, claim, damage, liability or action;
provided, however, that the Company will not be liable in any such case to the
extent that any such loss, claim, damage or liability arises out of or is based
upon any untrue statement or omission made in such Registration Statement,
preliminary prospectus or prospectus, or any such amendment or supplement, in
reliance upon and in conformity with information furnished to the Company, in
writing, by or on behalf of such Selling Stockholder, underwriter or controlling
person specifically for use in the preparation thereof.

                           (b) In the event of any registration of any of the
Registrable Shares under the Securities Act pursuant to this Agreement, each
Selling Stockholder, severally and not jointly, will indemnify and hold harmless
the Company, each of its directors and officers and each underwriter (if any)
and each person, if any, who controls the Company or any such underwriter within
the meaning of the Securities Act or the Exchange Act, against any losses,
claims, damages or liabilities, joint or several, to which the Company, such
directors and officers, underwriter or controlling person may become subject
under the Securities Act, Exchange Act, state securities or Blue Sky laws or
otherwise, insofar as such losses, claims, damages or liabilities (or actions in
respect thereof) arise out of or are based upon any untrue statement or alleged
untrue statement of a material fact contained in any Registration Statement
under which such Registrable Shares were registered under the Securities Act,
any preliminary prospectus or final prospectus contained in the Registration
Statement, or any amendment or supplement to the

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Registration Statement, or arise out of or are based upon any omission or
alleged omission to state a material fact required to be stated therein or
necessary to make the statements therein not misleading, if the statement or
omission was made in reliance upon and in conformity with information relating
to such Selling Stockholder furnished in writing to the Company by or on behalf
of such Selling Stockholder specifically for use in connection with the
preparation of such Registration Statement, prospectus, amendment or supplement;
provided, however, that the obligations of a Selling Stockholder hereunder shall
be limited to an amount equal to the net proceeds to such Selling Stockholder of
Registrable Shares sold in connection with such registration.

                           (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, 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 Section except to the extent
that the Indemnifying Party is adversely affected by such failure. The
Indemnified Party may participate in such defense at such party's expense;
provided, however, that the Indemnifying Party shall pay such expense if
representation of such Indemnified Party by the counsel retained by the
Indemnifying Party would be inappropriate due to actual or potential differing
interests between the Indemnified Party and any other party represented by such
counsel in such proceeding; provided further that in no event shall the
Indemnifying Party be required to pay the expenses of more than one law firm per
jurisdiction as counsel for the Indemnified Party. The Indemnifying Party also
shall be responsible for the expenses of such defense if the Indemnifying Party
does not elect to assume such defense. No Indemnifying Party, in the defense of
any such claim or litigation shall, except with the consent of each Indemnified
Party, consent to entry of any judgment or enter into any settlement which does
not include as an unconditional term thereof the giving by the claimant or
plaintiff to such Indemnified Party of a release from all liability in respect
of such claim or litigation, and no Indemnified Party shall consent to entry of
any judgment or settle such claim or litigation without the prior written
consent of the Indemnifying Party, which consent shall not be unreasonably
withheld.

                           (d) In order to provide for just and equitable
contribution in circumstances in which the indemnification provided for in this
Section 2.5 is due in accordance with its terms but for any reason is held to be
unavailable to an Indemnified Party in respect to any losses, claims, damages
and liabilities referred to herein, then the Indemnifying Party shall, in lieu
of indemnifying such Indemnified Party, contribute to the amount paid or payable
by such Indemnified Party as a result of such losses, claims, damages or
liabilities to which such party may be subject in such proportion as is
appropriate to reflect the relative fault of the Company on the one hand and the
Selling Stockholders on the other in connection with the statements or omissions
which resulted in such losses, claims, damages or liabilities, as well as any
other relevant equitable considerations. The relative fault of the Company and
the Selling

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<PAGE>   8
Stockholders shall be determined by reference to, among other things, whether
the untrue or alleged untrue statement of material fact related to information
supplied by the Company or the Selling Stockholders and the parties' relative
intent, knowledge, access to information and opportunity to correct or prevent
such statement or omission. The Company and the Selling Stockholders agree that
it would not be just and equitable if contribution pursuant to this Section 2.5
were determined by pro rata allocation or by any other method of allocation
which does not take account of the equitable considerations referred to above.
Notwithstanding the provisions of this paragraph of Section 2.5, (a) in no case
shall any one Selling Stockholder be liable or responsible for any amount in
excess of the net proceeds received by such Selling Stockholder from the
offering of Registrable Shares and (b) the Company shall be liable and
responsible for any amount in excess of such proceeds; provided, however, that
no person guilty of fraudulent misrepresentation (within the meaning of Section
11(f) of the Securities Act) shall be entitled to contribution from any person
who was not guilty of such fraudulent misrepresentation. Any party entitled to
contribution will, promptly after receipt of notice of commencement of any
action, suit or proceeding against such party in respect of which a claim for
contribution may be made against another party or parties under this Section,
notify such party or parties from whom contribution may be sought, but the
omission so to notify such party or parties from whom contribution may be sought
shall not relieve such party from any other obligation it or they may have
thereunder or otherwise under this Section. No party shall be liable for
contribution with respect to any action, suit, proceeding or claim settled
without its prior written consent, which consent shall not be unreasonably
withheld.

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

                  2.7 "Stand-Off" Agreement; Confidentiality of Notices. Each
Stockholder, if requested by the Company and the managing underwriter of an
underwritten public offering by the Company of Common Stock, shall not sell or
otherwise transfer or dispose of any Registrable Shares or other securities of
the Company held by such Stockholder for a period of 180 days following the
effective date of a Registration Statement. The Company may impose stop-transfer
instructions with respect to the Registrable Shares or other securities subject
to the foregoing restriction until the end of such 180-day period. Any
Stockholder receiving any written notice from the Company regarding the
Company's plans to file a Registration Statement shall treat such notice
confidentially and shall not disclose such information to any person other than
as necessary to exercise its rights under this Agreement.

                  2.8      Rule 144 Requirements.  The Company agrees to:

                           (a) make and keep current public information about
the Company available, as those terms are understood and defined in Rule 144;

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<PAGE>   9
                           (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 (at any time after it has become subject
to such reporting requirements);

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

                           (d) instruct its transfer agent to remove the
restrictive legend on any Registrable Shares which are eligible for resale under
Rule 144(k) provided that the Stockholders execute the customary representation
letter required by the transfer agent.

                  2.9 Termination. All of the Company's obligations to register
Registrable Shares under Section 2.1 and Section 2.2 of this Agreement shall
terminate upon the four year anniversary date of this Agreement.

         3. Transfers of Rights. This Agreement, and the rights and obligations
of each Purchaser hereunder, may be assigned by such Purchaser to (a) the
spouse, children or descendants of any Purchaser; or (b) any trust for the
exclusive benefit of any Purchaser or one or more of the individuals listed in
clause (a); provided that the transferee provides written notice of such
assignment to the Company and agrees in writing to be bound hereby. 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

         4. Representations, Warranties and Covenants of the Company. The
Company represents and warrants to Stockholders 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.

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         5. General.

                           (a) Severability. The invalidity or unenforceability
of any provision of this Agreement shall not affect the validity or
enforceability of any other provision of this Agreement.

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

                           (c) Governing Law. This Agreement shall be governed
by and construed in accordance with the internal laws of the Commonwealth of
Massachusetts (without reference to the conflicts of law provisions thereof).

                           (d) Notices. All notices, requests, consents, and
other communications under this Agreement shall be in writing and shall be
deemed delivered (i) two business days after being sent by registered or
certified mail, return receipt requested, postage prepaid or (ii) one business
day after being sent via a reputable nationwide overnight courier service
guaranteeing next business day delivery, in each case to the intended recipient
as set forth at the address indicated in the Merger Agreement.

         Any party may give any notice, request, consent or other communication
under this Agreement using any other means (including, without limitation,
personal delivery, messenger service, telecopy, first class mail or electronic
mail), but no such notice, request, consent or other communication shall be
deemed to have been duly given unless and until it is actually received by the
party for whom it is intended. Any party may change the address to which
notices, requests, consents or other communications hereunder are to be
delivered by giving the other parties notice in the manner set forth in this
Section.

                           (e) Complete Agreement. This Agreement constitutes
the entire agreement and understanding of the parties hereto with respect to the
subject matter hereof and supersedes all prior agreements and understandings
relating to such subject matter.

                           (f) Amendments and Waivers. Any term of this
Agreement may be amended or terminated and the observance of any term of this
Agreement may be waived (either generally or in a particular instance and either
retroactively or prospectively), with the written consent of the Company and the
holders of at least 66 2/3 % of the Registrable Shares held by all of the
Stockholders; provided, that this Agreement may be amended with the consent of
the holders of less than all Registrable Shares only in a manner which applies
to all such holders in the same fashion. Any such amendment, termination or
waiver effected in accordance with this Section 6(f) shall be binding on all
parties hereto, even if they do not execute such consent. No waivers of or
exceptions to any term, condition or provision of this Agreement, in any one or

                                      -10-
<PAGE>   11
more instances, shall be deemed to be, or construed as, a further or continuing
waiver of any such term, condition or provision.

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

                           (h) Counterparts; Facsimile Signatures. This
Agreement may be executed in any number of counterparts, each of which shall be
deemed to be an original, and all of which together shall constitute one and the
same document. This Agreement may be executed by facsimile signatures.

                           (i) Section Headings. The section headings are for
the convenience of the parties and in no way alter, modify, amend, limit or
restrict the contractual obligations of the parties.

                  [REMAINDER OF PAGE LEFT INTENTIONALLY BLANK.]

                                      -11-
<PAGE>   12
         Executed as of the date first written above.

                                 COMPANY:

                                 CONCORD COMMUNICATIONS, INC.

                                 By:      /s/ Gary E. Haroian
                                    --------------------------------------------
                                          Gary E. Haroian
                                          Sr. Vice President and CFO

                                 PURCHASERS:

                                 /s/ Cheryl Krupczak
                                 -----------------------------------------------
                                 Cheryl Krupczak

                                 /s/ Robert Krupczak
                                 -----------------------------------------------
                                 Robert Krupczak

                                      -12-

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