On our network the PDC is a older machine that I don't entirely trust anymore.
( the lights one drive in the raid are out, but seemingly running anyway, one power supply is red lighting as well. And I don't think it's worth putting more $ into it)
There is another machine that has active directory installed, and it is apparently also a domain controller. However, as a test, I powered down the primary to see if the other one would function as the domain controller and authenticate users logging in. It did not. In fact, I could not log onto it via terminal services as it reported that the domain controller could not be found.
I want it to know how to configure these machines so that if (when) the primary one fails, the other one will step up and function as the PDC without any interruption for our users.