Here is a scenario (based on a 2 node DFS and 2 node DAG)
All users based at HQ connect to a File Server and the Primary Exchange Server (all Databases mounted here)
They have a passive DFS server and and passive Exchange Server with copies of the databases, and the 2 sites are connected via a VPN.
DFS is configured with over-ride referral on each namespace which dictates that all users will connect to the HQ machine regardless of which site they are at, until such time as the Primary DFS Server goes down.
My main concern is if they have a site down scenario and the primary servers are OFF for an extended period:
1. Will DFS get itself in a muddle as users will immediately connect back to the primary server which has not yet received the replicated changed files
2. How does DAG deal with the same situation? If the main server comes back up, will the database copies automatically revert the the lowest preference server (primary) and if so, then how does it handle missing files/emails while it updates?