A Microsoft Windows NT domain controller containing a read-only copy of the master domain directory database located on the primary domain controller (PDC).
A Windows NT domain can have zero or more backup domain controllers (BDCs) for load balancing and redundancy. The BDCs periodically undergo directory synchronization in a Windows NT domain by retrieving a copy of the directory database from the PDC. A BDC can perform logon validation and authentication like a PDC, but it cannot manage accounts—for example, it cannot change user passwords.
Backup domain controller two choices for BDC
The placement of BDCs in wide area networks (WANs) that are based on Windows NT is an important issue. In a master domain model scenario, user accounts are centralized in a master domain located at company headquarters, while users and shared network resources are distributed in resource domains located at branch offices in different locations. The users in this scenario must log on to the master domain in order to access resources in the enterprise. There are two ways of facilitating this:
Every Windows NT network should have at least one BDC for fault tolerance. If the PDC fails, the BDC can be promoted to take its place. One BDC can support approximately 2000 users on a network, but many factors can affect this figure.