| Principal Shareholders PRINCIPAL STOCKHOLDERS
 
 The following table sets forth certain information regarding beneficial
 ownership of our common stock as of January 25, 2000, and as adjusted to reflect
 the sale of the shares of common stock in this offering, by:
 
 - each person known by us to be the beneficial owner of more than 5% of our
 common stock;
 - each of our executive officers listed in the Summary Compensation table;
 - each of our directors; and
 - all of our executive officers and directors as a group.
 
 To our knowledge, each person has sole voting and investment power over the
 shares shown as beneficially owned except to the extent authority is shared by
 spouses under applicable law and except as described in the footnotes to the
 table. The number of shares of common stock owned by each person listed includes
 shares of common stock underlying options held by that person that are
 exercisable within 60 days after January 25, 2000. The number of outstanding
 shares of common stock used in calculating the percentage ownership for each
 person listed includes the shares of common stock underlying options held by
 that person that are exercisable within 60 days after January 25, 2000, but
 excludes shares of common stock underlying options held by any other person.
 Percentage ownership calculations are based on 29,210,864 shares of common stock
 outstanding as of January 25, 2000, and the additional                shares of
 common stock to be sold in this offering.
 
 PERCENTAGE OF
 COMMON STOCK
 OUTSTANDING
 --------------------
 NUMBER OF SHARES      BEFORE      AFTER
 NAME OF BENEFICIAL OWNER              BENEFICIALLY OWNED    OFFERING    OFFERING
 ------------------------              ------------------    --------    --------
 North Bridge Venture Partners(1)...        5,585,190         19.12%
 Matrix Partners(2).................        7,684,806         26.31
 Accel Partners(3)..................        2,428,684          8.31
 Chin-Cheng Wu(4)...................        2,581,600          8.84
 Louis J. Volpe.....................        1,300,000          4.45
 Cynthia M. Deysher(5)..............          371,600          1.27
 Christopher P. Lynch(6)............          400,000          1.37
 Peter M. Piscia(7).................          166,066             *
 Paul J. Ferri(8)...................        7,684,806         26.31
 Edward T. Anderson(9)..............        5,585,190         19.12
 James A. Dolce, Jr.(10)............           68,920             *
 All executive officers
 and directors as a group
 (eight persons)..................       18,158,182         62.11
 
 ---------------
 * Less than 1%.
 
 (1) Composed of 5,303,206 shares held by North Bridge Venture Partners II,
 L.P., 191,156 shares held by North Bridge Venture Partners IV-A, L.P. and
 90,828 shares held by North Bridge Venture Partners IV-B, L.P. The address
 of North Bridge Venture Partners, L.P. is 950 Winter Street, Suite 4600,
 Waltham, MA 02451.
 (2) Composed of 6,931,976 shares held by Matrix Partners IV, L.P., 364,840
 shares held by Matrix IV Entrepreneurs Fund, L.P., 333,672 shares held by
 Matrix Partners VI, L.P. and 54,318 shares held by Weston & Co., as nominee
 for certain persons. The address of Matrix Partners is Bay Colony Corporate
 Center, 1000 Winter Street, Suite 4500, Waltham, MA 02154.
 (3) Composed of 1,906,518 shares held by Accel V L.P., 252,582 shares held by
 Accel Internet/Strategic Technology Fund L.P., 99,576 shares held by Accel
 Keiretsu V L.P., 116,578 shares held by Accel
 Investors '97 L.P. and 53,430 shares held by Ellmore C. Patterson Partners.
 The address of Accel Partners is One Palmer Square, Princeton, New Jersey
 08542.
 (4) Includes 1,000,000 shares held by the Chin-Cheng Wu 1996 Irrevocable
 Children's Trust dated December 9, 1996. The address of Chin-Cheng Wu is
 c/o ArrowPoint Communications, Inc., 50 Nagog Park, Acton, Massachusetts
 01720.
 (5) Includes 16,000 shares held for the benefit of Ms. Deysher's minor children
 under the Massachusetts Uniform Transfer to Minors Act.
 (6) Includes 8,000 shares held for the benefit of Mr. Lynch's minor children
 under the Massachusetts Uniform Transfer to Minors Act.
 (7) Includes 26,066 shares issuable upon the exercise of options exercisable
 within 60 days after January 25, 2000.
 (8) Composed of 6,931,976 shares held by Matrix Partners IV, L.P., 364,840
 shares held by Matrix IV Entrepreneurs Fund, L.P., 333,672 shares held by
 Matrix Partners VI, L.P. and 54,318 shares held by Weston & Co. Mr. Ferri
 is a general partner of Matrix IV Management Co., L.P., which is a general
 partner of each of Matrix Partners IV, L.P. and Matrix IV Entrepreneurs
 Fund, L.P. Mr. Ferri is a member of Matrix VI Management Co., L.L.C., which
 is the general partner of Matrix Partners VI, L.P. Mr. Ferri is also an
 authorized signatory for Weston & Co.
 (9) Composed of 5,303,206 shares held by North Bridge Venture Partners II,
 L.P., 191,156 held by North Bridge Venture Partners IV-A, L.P. and 90,828
 shares held by North Bridge Venture Partners IV-B, L.P. Mr. Anderson is a
 general partner of North Bridge Venture Management II, L.P., which is the
 general partner of North Bridge Venture Partners II, L.P. Mr. Anderson is
 also a general partner of North Bridge Venture Management IV, L.P., which
 is the general partner of each of North Bridge Venture Partners IV-A, L.P.
 and North Bridge Venture Partners IV-B, L.P.
 (10) Composed of 50,000 shares held by the Alexer Family Limited Partnership and
 18,920 held by Mr. Dolce. Mr. Dolce is a general partner of the Alexer
 Family Limited Partnership.
 
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