When you get to larger orgs, the DC doesnt always have to be the logon server. In fact, you're liable to see a slew of logon kerb redirectors talking to a few ldap backends running as pri and secondary AD.
I have to admit I’m not familiar with anything in your comment. Is this different terminology for a RODCs? I was also under the impression that AD didn’t have a primary DC, the only vestige of that is the PDC Emulator FSMO role.
Explorer address bar (or start>run) > %logonserver%