The global catalog is a repository of information that contains a subset of attributes for all objects in Active Directory. By default, the attributes that are stored in the global catalog are those that are most frequently used in queries (such as a user's first name, last name, and logon name). The global catalog contains the information that is necessary to determine the location of any object in the directory.

A global catalog server is a domain controller that stores a copy of and processes queries to the global catalog. Global catalog servers improve the performance of forest-wide searches in Active Directory. For example, if you search for all of the printers in a forest, a global catalog server processes the query against the global catalog and then returns the results. Without a global catalog server, this query would require a search of every domain in the forest.

The global catalog performs two important directory roles:

When a user logs on to the network, the global catalog server provides universal group membership information for the account to the domain controller that processes the user logon information. If a global catalog server is not available when a user initiates a network logon process, then the user is only able to log on to the local computer. However, if a user is a member of the Domain Admins group, then the user can log on to the network even when the global catalog server is not available.

The first domain controller you create in Active Directory is a global catalog server. You can configure additional domain controllers to be global catalog servers in order to balance the logon authentication traffic and query traffic.

The global catalog server is designed to respond to queries about objects anywhere in the domain tree or forest with maximum speed and minimum network traffic. Because a single global catalog server contains information about objects in all domains in the forest, a query about an object can be resolved by a global catalog server in the domain in which the query is initiated. Thus, finding information in the directory does not produce unnecessary traffic across domains.