A common question for multi-datacenter deployments is how can we configure a DAG where failure will enable a DAG member locally vs. the other datacenter. This requires DAC mode. DAC or Datacenter Activation Mode is a property of the DAG that can be turned on or off and is disabled by default.
In Exchange 2010 SP1, DAC Mode has been extended to support two-member DAGs that each have a member in a separate datacenter. Bottom-line – you can now use DAC with DAG members in same or different AD Sites allowing the capability of using DAC with two or more members of a DAG.
A common deployment I have is with 4 members of DAG, 2 servers in each datacenter. FSW is in the primary Datacenter and will be quorum. Now Primary Datacenter has power outage. Manually, Exchange admin activates the secondary datacenter with an alternate file share witness.
When the power is restored in the primary site. When the two DAG members and witness server comes online, it has a quorum (majority) and will try to activate the databases. This will cause a “split brain syndrome” where both datacenters think that they are hosting the active databases.
DAC mode is used to avoid this and if enabled when the DAG members come online they will leverage Datacenter Activation Coordination Protocol (DACP) before trying to mount databases.
For more information on how this works check out the following Technet article. http://technet.microsoft.com/en-us/library/dd979790.aspx
Great write-up from EighTwOne (821) here.