Is it necessary DAC enable

is it necessary DAC enable one of them data-center location? Why it necessary?
Muhammad AliAsked:
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Andrej PirmanCommented:
DAC should be:
- ENABLED on all Exchange Database Availability Groups when you use at least 2 nodes with Exchange NATIVE Continous Replication topology
- DISABLED if you use SINGLE Exchange node, or 3rd party replication mechanism (unless 3rd party instructs you to enable DAC)

DAC mechanism prevents split-brain problems, meaning in scenario with 2 or more Exchange servers serving same content/mailboxes with real-time replication, they all must act as one, one server must be avare of actions taken on all other servers. When DAC mode is enabled, Datacenter Availability Group  members won't automatically mount databases even if they have quorum. Instead DACP (DAC Protocol) is used to determine the current state of the Datacenter Availability Group and whether Active Manager should attempt to mount the databases.
Muhammad AliAuthor Commented:
For example: If  I have 3 exchange members are in primary location and 3 are DR location, then only one Exchange member on primary location have DAC enable?


Note: I am new in DAG environment.
Jeff GloverSr. Systems AdministratorCommented:
You set the DAC on the DAG, not on a specific Server.
Set-DatabaseAvailabilityGroup -Identity "DAgname" -DataCenterActivationMode DAGOnly

You only do this on a DAG that has members in different AD Sites. Enabling this does 2 things. First, it sets the Database startup bit to 0 in the Active Manager instead of a 1. This means if you lose a datacenter and mount the databases in the DR site and the primary comes back online, they will not mount until they hear from all DAG members, not just a quorum n/2+1.  This means no Split-Brain scenario where your DAG now behaves like 2 separate DAGs, one in each datacenter.
 It also enables 3 Powershell cmdlets that are specifically for DR.
Stop-DatabaseAvailabilityGroup - Lets you evict a server from the DAG
Restore-DatabaseAvailabilityGroup - Restarts the DAG with fewer members
Start-DatabaseAvaialbilityGroup - Adds servers back into DAG once the servers are backup and communicating.

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