We are in the process of migrating our datacenter to a new location. Soon we will be moving 3 of the 4 domain controllers in our primary AD site to the new datacenter leaving one DC up and running while the other 3 DC's will be down for about 30 minutes. We also have 3 other AD sites that have DC's in them as well. To add to this picture, we have 2 Exchange DAG nodes in the primary site that the DC's will be migrating from. One of those nodes is our active mailbox node. we do have another node in another site as well.
My thinking is since we have one DC still up in the primary site Exchange will not be impacted. But I can failover to the Exchange DAG node in another site just to be safe. Is this really needed though?
How does Exchange choose what DC/GC to communicate with? If it's preferred DC is offline what happens?