A computer that is running Microsoft Internet Security and Acceleration (ISA) Server 2004 may block User Datagram Protocol (UDP) traffic between two networks.
This problem occurs because the ISA Server 2004 incorrectly processes the UDP requests.
For more information and a solution, check out: http://support.microsoft.com/default.aspx?scid=kb;en-us;915461&sd=rss&spid=2108#
Thomas W Shinder, M.D.
MVP -- ISA Firewalls