Description:

Datacenter Activation Coordination (DAC) mode is a property setting for a database availability group (DAG).

DAC mode is disabled by default and should be enabled for all DAGs with 2 or more members that use continuous replication.

That means the majority of Exchange DAG deployments need the DAC mode.

The DAC is most useful in a multi-datacenter configuration to prevent split brain syndrome, a condition that occurs when all networks fail, and DAG members can’t receive heartbeat signals from each other.

However, I suggest you always enable the DAC.

If you enable the DAC and you need to recover, the recovery takes less commands on the command line. Only the following commandlets will be necessary:

Stop-DatabaseAvailabilityGroup

Restore-DatabaseAvailabilityGroup

Start-DatabaseAvailabilityGroup

Also, if you did NOT enable the DAC, you cannot do so if you have a failure. The DAC must be enabled ahead of time.

Instructions:

Here is how to enable the DAC mode:

1. Go to Exchange Management Shell

2. Type the following, where DAG2 is your DAG name:

Set-DatabaseAvailabilityGroup -Identity DAG2 -DatacenterActivationMode DagOnly

More Information:

For more information, read this article:

http://technet.microsoft.com/en-us/library/dd979790.aspx