What is a PDC, BDC?

A. A PDC is a Primary Domain Controller, and a BDC is a Backup Domain Controller. You must install a PDC before any other domain servers. The Primary Domain Controller maintains the master copy of the directory database and validates users. A Backup Domain Controller contains a copy of the directory database and can validate users. If the PDC fails then a BDC can be promoted to a PDC. Possible data loss is user changes that have not yet been replicated from the PDC to the BDC. A PDC can be demoted to a BDC if one of the BDC's is promoted to the PDC.