Additional information concerning
Backup Domain Controllers:
The BDC can have up to a 5-minute lapse in SAM database entries, since the PDC pushes the SAM at that frequency.
There can be up to 20 BDCs in one domain. There should be at least one BDC at every remote WAN site and link point in a network. Should the communications link go down, the people on the local network would still be able to log on and use their machines. A PDC must be running to allow you to join a new domain.