In addition to the primary domain controller (PDC), you should have at least one backup domain controller (BDC) in every domain.
When a backup domain controller is promoted to primary domain controller, an up-to-date copy of the domain’s directory database is replicated from the old PDC to the new one, and the old PDC is demoted to a BDC.
If a BDC is promoted to PDC while the existing PDC is unavailable (for example, while it is being repaired), and if the former PDC later returns to service, you must demote the former PDC to BDC. Until the former PDC is demoted to a BDC, it will not run the Net Logon service nor participate in authentication of user logons, and its icon in the Server Manager window will be dimmed.
Note
Usually, when a BDC is promoted to a PDC, the system automatically demotes the former PDC to a BDC. However, if Server Manager cannot locate the PDC, the PDC is not demoted and the user receives a message indicating this condition. The user can choose to proceed without demoting the PDC or wait until the PDC can be demoted.
For information on how to promote and demote domain controllers, see "Promoting a Backup Domain Controller to Primary Domain Controller" and "Demoting a Primary Domain Controller to Backup Domain Controller" in Server Manager Help.
