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                                                                     Exhibit 2.1

                                AMENDMENT NO. 3
                                       TO
                              AMENDED AND RESTATED
                          AGREEMENT AND PLAN OF MERGER

     This Amendment No. 3 (this "Amendment") to the Amended and Restated
                                 ---------
Agreement and Plan of Merger, dated as of August 4, 1999, as amended (the
"Merger Agreement"), by and among CAIS Internet, Inc., a Delaware corporation
-----------------
("CAIS"), CIAM Corp., a California corporation (the "Company"), and Atcom, Inc.,
------                                               -------
a California corporation ("Atcom"), is made effective as of March 21, 2000.
                           -----
Unless otherwise defined herein, capitalized terms used herein shall have the
same meanings as those set forth in the Merger Agreement.

     WHEREAS the parties hereto desire to amend the Merger Agreement as provided
below.

     NOW, THEREFORE, in consideration of the mutual promises herein contained,
the parties, intending to be legally bound, agree as follows:

     1.  Amendment to Section 2.12 of the Merger Agreement.  Section 2.12 of the
         -------------------------------------------------
Merger Agreement is hereby amended to read in full as follows:

     (a)(1) Within twenty-five (25) days after the date hereof, CAIS shall
distribute to Qualified Shareholders (as defined below) for each share of CAIS
Common Stock held or underlying warrants held by the Qualifying Shareholders
("Qualified Shares"), 500,000 shares of CAIS Common Stock divided by the total
number of shares of CAIS Common Stock held by, or underlying warrants owned by,
all Qualifying Shareholders in the aggregate as of the Effective Time ("Per
Share Contingent Consideration").  For purposes of this Section 2.12, "Qualified
Shareholders" shall mean the holders of Atcom Common Stock or warrants to
acquire Atcom Common Stock, as of the Effective Time.

     (2) CAIS shall, subject to the Redemption Notice set forth below, redeem up
to an aggregate of 600,000 Qualified Shares at a redemption price of $30 per
share.  If Qualified Shareholders in the aggregate tender for redemption in
excess of 600,000 Qualified Shares, CAIS shall redeem an aggregate of 600,000
Qualified Shares on a pro rata basis, based on the ratio of each tendering
Qualified Shareholder's tendered Qualified Shares to the total of all tendered
Qualified Shares in the aggregate.  CAIS shall provide notice to all Qualified
Shareholders, pursuant to Section 13.6 of the Merger Agreement, of their limited
redemption right set forth in this Section 2.12(a)(2).  The Qualified Shares
redeemed pursuant to this Section 2.12(a)(2) shall cease to be Qualified Shares
for purposes of Section 2.12(a)(1) hereof.  Within five (5) days after the date
hereof, CAIS shall offer in writing to repurchase the 600,000 Qualified Shares
in accordance with the terms of this Section 2.12 (the "Redemption Notice").
The Holders shall notify CAIS in writing within fourteen (14) days after the
date of the Redemption Notice (the "Offer Period") as to the number of Qualified
Shares that they wish CAIS to redeem pursuant to this Section 2.12.   Closing on
the redemption of such Qualified
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Shares pursuant to this Section 2.12(a)(2) shall occur within five (5) business
days after the expiration of the Offer Period.

     2.  Governing Law.  This Amendment shall be construed in accordance with
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and governed by the laws of the State of California without regard to choice of
law principals.

     3.  Counterparts.  This Agreement may be executed in any number of
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counterparts, each of which shall be deemed an original but all of which shall
constitute one and the same instrument.

     4.  Effect of Amendment.  Except as expressly provided in this Amendment,
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the Merger Agreement shall remain unmodified and in full force and effect.
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     IN WITNESS WHEREOF, the undersigned have executed this Amendment, all as of
the day and year first above written.

CAIS INTERNET, INC.

    /s/ Ulysses G. Auger, II
By:________________________________

       CEO
Title:_____________________________




CAIS SOFTWARE SOLUTIONS, INC.
(successor in interest to CIAM Corp. and
formerly known as Atcom, Inc.)


By:________________________________


Title:_____________________________




     /s/ Neil Senturia
     ____________________________________
     Neil Senturia, In his individual capacity



Funds Managed by Patricof & Co.
Ventures, Inc.

      /s/ Thomas P. Hirschfeld
By: ___________________________________
      Thomas P. Hirschfeld
      Managing Director