Dcdiag.exe: Domain Controller Diagnostic Tool
The Dcdiag command-line tool is included when you install Windows Server 2003 Support Tools from the product CD or from the Microsoft Download Center
This command-line tool analyzes the state of domain controllers in a forest or enterprise and reports any problems to assist in troubleshooting. As an end-user reporting program, Dcdiag encapsulates detailed knowledge of how to identify abnormal behavior in the system. Dcdiag displays command output at the command line.
Dcdiag consists of a framework for executing tests and a series of tests to verify different functional areas of the system. This framework selects which domain controllers are tested according to scope directives from the user, such as enterprise, site, or single server.
All domain controllers in the same domain are peers of one another and any domain controller can make directory updates.
However, given the way in which directory updates are replicated from one domain controller to another, it is possible that difficulties can arise. For example, if the necessary domain controllers are not connected by a replication topology, the appropriate domain controllers do not receive directory updates when replication occurs.
Also, in order for the (Domain Controller) Locator to find a domain controller, it must have accurate information so that it can properly locate the resource. If a domain controller is incorrectly advertised, the Locator is unable to find it.