In Windows NT 4 server domains, the Backup Domain Controller (BDC) is a computer that has a copy of the user accounts database.
Unlike the accounts database on the Primary Domain Controller (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.
Most domains will have at least one BDC, often there are several BDCs in a domain. These domains 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.
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