AutoFailover to Additional Domain Controller

[laserted]

Greets - I'm looking to set up an additional domain controller for our network and am curious about how an auto-failover setup can be achieved in the event the PDC goes down [temporarily].
Currently, DNS and AD is hosted inside the Zentyal PDC, while routing and address provisioning (DHCP) is in a traditional Cisco router setup.

As I'm adding the ADC to an established domain with clients already joined, is it just a matter of adding the ADC's address as the second entry in my router's DNS server table, then refreshing the local DNS caches of each client (basic reboots, etc) or do I have to un-join/rejoin those clients so they have a fresh instance of the catalog?

I have a couple custom applications deployed that query LDAP resources on-demand for simple password auth (versus say, just Windows machines that will cache domain profiles and continue to function off-network), so just relying on local-machine caches isn't necessarily an option.

Adding the ADC to the domain itself seems rather trivial as there's lots of tutorials out there, I'm really interested in the technique of the handoff from the PDC to the alternative when the PDC isn't reachable.

Thanks,
Ted.