A PDC, in its original NT4 implementation, was the domain controller that actively responded to all requests from clients in the domain. For redundancy, BDCs, or Backup Domain Controllers, could also be created that held a replica of the data on the PDC. If a PDC were to fail, then a BDC could be promoted to the PDC role.

This relationship has been deprecated since the release of Windows 2000 Server, which introduced the concept of multi-master peers.

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