Redundancy is a key idea behind a Windows domain. The domain controller that is currently active on a domain is called the
primary domain controller (PDC). There can be one or more
backup domain controllers (BDCs) in the domain as well, which will take over in the event that the primary domain controller fails or becomes inaccessible. BDCs frequently synchronize their SAM data with the primary domain controller so that, if the need arises, any one of them can perform DC services transparently without impacting its clients. Note that BDCs, however, have only read-only copies of the SAM; they can update their data only by synchronizing with a PDC. A server in a Windows domain can use the SAM of any primary or backup domain controller to authenticate a user who attempts to access its resources and logon to the domain.
Note that in many aspects, the behaviors of a Windows workgroup and a Windows domain overlap. This is not accidental since the concept of Windows domains did not evolve until Windows NT 3.5 was introduced, and Windows domains were forced to remain backwards compatible with the workgroups present in Windows for Workgroups 3.1. The key thing to remember here is that a Windows domain is simply a Windows workgroup with one or more domain controllers added.
Samba can function as a primary domain controller for Windows 95/98 machines without any problems. However, Samba 2.0 can act as a primary domain controller only for authentication purposes; it currently cannot assume any other PDC responsibilities. (By the time you read this, Samba 2.1 may be available so you can use Samba as a PDC for NT clients.) Also, because of the closed protocol used by Microsoft to synchronize SAM data, Samba currently cannot serve as a backup domain controller.
