Earlier today I had to prepare for taking one of my servers offline to be shipped to another site. I did a poor man's failover by updating the RPCClientAccess values for my Exchange Databases for both sites, and pointed DNS to the CAS server that would remian up. Everything seemed to work okay for the most part. When it came time to power off my server and shit it, all hell broke loose. Users would lose connectivity to Exchange. Mailbox Databases were shown as completely dismounted. People were unable to send/rec mail; and were getting password prompts all over the place.

My sr. admin told me it has to do with Database Activation Coordination, basically what's responsible for telling Exchange which member in a DAG is allowed to amount it's databases. This is where my understanding of this issue stops. From running Get-DatabaseAvailabilityGroup, the output I get is that Database Activation mode is disabled; which is by default.

Here's where I will need help: what needs to be done when that servers gets back on site? If that CAS server & mailbox server are powered up, do they automagically try to mount each respective copy of the DAG? My understanding is this shouldn't necessarily matter since until they were dismounted, I had them still up on my current server. Is there a way I can prevent split-brain from occuring in this situation, or do I have to ensure that DAC is setup first on the still-standing server? I'm slightly confused. :(