DAC(Datacenter Activation Coordination) Mode

DAC stands for Datacentre Activation Coordination and it works using a new protocol named Datacentre Activation Coordination Protocol (DACP). 


DAC Mode makes sure a check is made before automatically mounting the database from a DAG member that went down.
This avoids SPLIT BRAIN SYNDROME i.e. a situation where both members of a cluster dont have a coordination between them and are operating at the same time leading to Database issues.

By default, this feature is disbaled. To enable DAC Mode open Exchange Management Shell:



Set-DatabaseAvailibilityGroup-Identity-DatacenterActivationMode DagOnly


A great article with a very clear description and diagram is available- 
http://www.howexchangeworks.com/2011/10/how-dac-mode-works-in-exchange-2010-sp1.html