Editing Domain controller (Windows) (section)

==Backup domain controller==
In Windows NT 4 domains, the backup domain controller (BDC) is a computer that has a copy of the user accounts database.  Unlike the accounts database on the PDC, the BDC database is a read-only copy. When changes are made to the master accounts database on the PDC, the PDC pushes the updates down to the BDCs. These additional domain controllers exist to provide fault tolerance. If the PDC fails, then it can be replaced by a BDC.  In such circumstances, an administrator promotes a BDC to be the new PDC. BDCs can also authenticate user logon requests and take some of the authentication load from the PDC.

When [[Windows 2000]] was released, the NT domain as found in NT 4 and prior versions was replaced by [[Active Directory]].  In Active Directory domains running in native mode, the concept of the PDC and BDC do not exist. In these domains, all domain controllers are considered equals. A side effect of this change is the loss of ability to create a "read-only" domain controller. [[Windows Server 2008]] reintroduced this capability.