- Backup Domain Controller. In a Windows NT domain, one domain controller is the
primary (the PDC) and any other domain controllers
are backup ones. Each BDC synchronises regularly with the PDC, and can
provide the same services as the PDC. Should the PDC fail then any of the
BDCs can be promoted to replace the PDC but this promotion is a manual
activity that must be done by an administrator.
From Windows 2000 onwards the concept of PDC and BDC no longer exist (although system administrators still use the terms), and all domain controllers have equal status.