B

backup domain controller

In a Windows NT® server 4.0 or earlier domain, the backup domain controller is the server host computer that receives a copy of the domain's directory database, with all account and security policy information for the domain. The copy is synchronized periodically and automatically with the master copy on the primary domain controller (PDC). BDCs also authenticate user logons and can be promoted to function as PDCs as needed. Multiple BDCs can exist on a domain.

In a Windows® 2000 domain, backup domain controllers are not required; all domain controllers are peers, and all can perform maintenance on the directory. Windows NT 4.0 and 3.51 Backup domain controllers can participate in a Windows 2000 domain when it is running in mixed mode.