Primary and backup domain controllers

You've already read about master and backup browsers. Domain controllers are similar in that a domain has a primary domain controller (PDC) and can have one or more backup domain controllers (BDCs) as well. If the PDC fails or becomes inaccessible, its duties are automatically taken over by one of the BDCs. BDCs frequently synchronize their SAM data with the PDC so if the need arises, any one of them can immediately begin performing domain-controller services without impacting the clients. However, note that BDCs have read-only copies of the SAM database; they can update their data only by synchronizing with a PDC. A server in a Windows domain can use the

SAM of any PDC or BDC to authenticate a user who attempts to access its resources and log on to the domain.

All recent versions of Windows can log on to a domain as clients to access the resources of the domain servers. The systems that are considered members of the domain are a more exclusive class, composed of the PDC and BDCs, as well as domain member servers, which are systems that have joined a domain as members, and are known to the domain controllers by having a computer account in the SAM database.

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