The role of a primary and secondary went away when NT went to 2000/2003.
There are a couple of instances that one of the dc's has more respondsibility
than the others in a few cases but for the most part they are pretty much
equal.
The dc's that reside amongst one another in a common site should share
authentication and dc's outside of their site will handle authentications for
the most part only in their site. Sites are defined by ip subnets.
The issue with role transfer is the difference in the dc's this is called
fsmo roles and there are five of them; 2 which are part of the forest and
three which are common for each domain in the forest.
Schema master - forest role
Domain naming master - forest role
RID master - domain role
PDC emulator - domain role
Infrastructure master - domain role
http://support.microsoft.com/kb/q197132/
Is this wht you were looking for?