The server discovery flow is this:
The Client looks into the Authenticore Servers group.
Then the Client selects a random server from that group which belongs to the same AD site as the client PC.
The Client tries to establish RPC connection with the Authenticore Server on given Server.
If the Server is operable, Client caches it and continues to work with it.
If it is down, Client gets another Authenticore server from the list which belongs to the AD site and continues from step #3.
If there are no servers belonging to the site, the client will try to connect to the servers from another site.
For more information on how to setup Active Directory Sites check the following technet article.