Let's say at site A, I have one server with Client Access and HUB roles. Another server is the mailbox store server.
At site B, I have one server with a mailbox store role and the minimal role required for that site to function (is it Client Access or HUB?).
Now if I set up a DAG so that site B has a copy of all the databases from Site A's mailbox server, and Site A goes down, what do I need to do with exchange to be back up and working at site B (besides make it active in exchange, and the external DNS and firewall type stuff)?
Do I simply need to install one more exchange server there? If that's the case, then can I put all 3 roles at Site A and Site B so that I'd have a better DR scenario?