Configuring DAG witness in a non-Exchange Server
Exchange Server 2010 is just around the corner and today we are going over the witness server/directory creation in a regular member server. The first question is why? Well, we already know that in this new release we can have only 2 servers and provide high availability for all roles the essential roles with those two servers. Remember that the same thing was accomplished with 4 servers in Exchange Server 2007 (2 CAS/HUB and 2 MBX - CCR).
However DAG witness has some requirements, such as: it cannot be a member of the DAG; must be in the same forest; it can run on top of Windows Server 2003/2008/2008 R2.
So, if you have only two Exchange Servers and wants to create a DAG a member server must be used as DAG witness server. In order to do that make sure that the group Exchange Trusted Subsystem is added to the local administrators of the server that will be the witness server, as shown in the figure below. After that you will be able to create a DAG using Exchange Management Console.