Exchange 2010 Database Accessability Group Witness Server Requirements

In the Exchange Server 2010 Database Accessibility Group (DAG) documentation (http://technet.microsoft.com/en-us/library/dd298065.aspx), it states the following:

“The witness server and its directory are used only for quorum purposes where there’s an even number of members in the DAG.”

This is semi-confusing during the planning stages of an Exchange 2010 infrastructure, as you must have a witness server regardless of how many mailbox servers are members of a DAG.  It’s just that the witness only comes in to play when their are an even number of active members in a DAG.

Even a little more confusing is that a witness server is typically just an Exchange Hub Transport server that does not have the mailbox role installed (i.e. a Witness can never host a copy of the database it’s witnessing).  So, if your environment consists of dedicated mailbox servers separated from the Client Access and Hub Transport roles, you really don’t have to install any additional servers.

In our environment, we’re taking Microsoft’s recommendation of using only mutli-role servers, so we had to set up a dedicated Hub Transport server to act as the Witness for our DAG.

While you can set up a Windows Server as a witness without Exchange installed on it (see the above article), it’s really not a good idea as that prevents your Exchange administrators from properly managing the entire environment from within Exchange (and we get cranky when we can’t control everything).

Additionally, while your mailbox-role servers must be running the Enterprise Edition of Windows Server to be a part of the DAG, the Hub Transport (Witness) server does not need to be running Enterprise Edition, even though it is a proper part of the DAG.

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