Domain Controller on Multi Subnet Always on Availability Group This is will be my first big project. We have 4 Replica Always on availability group  with 3 Replica on site and another replica offsite (different subnet) the File share witness will be placed on site on the primary data center. On various online resources i saw that we need 2 Domain controllers one onsite and the other one on the Disaster Recovery site. My question is what will be the  problems that i will face if all the replicas are joined to one Domain controller .

Suppose if the Primary site goes down the server on the DR will still be able to used cached credentials. 

The distance between Primary and Disaster Recovery site only 20 Miles !

Any advice on this will be highly appreciated.
Suppose if the Primary site goes down the server on the DR will still be able to used cached credentials. 


Assuming the DCs and the rest of your infrastructure will fail over to DR by some other means, your DR SQL Server may not be usable until that happens.  So the AG will give you a  very good Recovery Point Objective (RPO), but the Recovery Time Objective (RTO) won't be any better than a VM failover.