It is a 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

Site is a collection of well-connected TCP/IP subnets. Sites are used for defining the topology of Active Directory replication.

Schedule enables you to list weekdays or hours when the site link is available for replication to happen in the give interval. Interval is the re occurrence of the inter site replication in given minutes. It ranges from 15 - 10,080 mins. The default interval is 180 mins.

The KCC(Knowledge Consistency Checker) is a built-in process that runs on all domain controllers and generates replication topology for the Active Directory forest. The KCC creates separate replication topologies depending on whether replication is occurring within a site (intrasite) or between sites (intersite). The KCC also dynamically adjusts the topology to accommodate new domain controllers, domain controllers moved to and from sites, changing costs and schedules, and domain controllers that are temporarily unavailable.

Intersite Topology Generator (ISTG), which is responsible for the connections among the sites. By default Windows 2003 Forest level functionality has this role.
By Default the first Server has this role. If that server can no longer preform this role then the next server with the highest GUID then takes over the role of ISTG.