The global catalog is a distributed data repository that contains a searchable, partial representation of every object in every domain in a multidomain Active Directory Domain Services (AD DS) forest. The global catalog is stored on domain controllers that have been designated as global catalog servers and is distributed through multimaster replication. Searches that are directed to the global catalog are faster because they do not involve referrals to different domain controllers.
The global catalog provides the ability to locate objects from any domain without having to know the domain name. A global catalog server is a domain controller that, in addition to its full, writable domain directory partition replica, also stores a partial, read-only replica of all other domain directory partitions in the forest . The additional domain directory partitions are partial because only a limited set of attributes is included for each object. By including only the attributes that are most used for searching, every object in every domain in even the largest forest can be represented in the database of a single global catalog server.
Common Global Catalog Scenarios
The following events require a global catalog server:
- Forest-wide searches. The global catalog provides a resource for searching an AD DS forest. Forest-wide searches are identified by the LDAP port that they use. If the search query uses port 3268, the query is sent to a global catalog server.
- User logon. In a forest that has more than one domain, two conditions require the global catalog during user authentication:
- In a domain that operates at the Windows 2000 native domain functional level or higher, domain controllers must request universal group membership enumeration from a global catalog server.
- When a user principal name (UPN) is used at logon and the forest has more than one domain, a global catalog server is required to resolve the name.
- Universal Group Membership Caching : In a forest that has more than one domain, in sites that have domain users but no global catalog server, Universal Group Membership Caching can be used to enable caching of logon credentials so that the global catalog does not have to be contacted for subsequent user logons. This feature eliminates the need to retrieve universal group memberships across a WAN link from a global catalog server in a different site.
- Exchange Address Book lookups . Servers running Microsoft Exchange Server rely on access to the global catalog for address information. Users use global catalog servers to access the global address list (GAL).
