Global catalogs

The global catalog is a partial read-only copy of all objects in the forest. The purpose of the global catalog is to speed up searching for objects stored on different domains in the forest. Within a single domain, each query for objects is sent directly to the domain controllers in that domain, but if you want to include results from other domains in the forest, the query needs to be sent to the global catalog server. The global catalog server is the domain controller that hosts the global catalog, which, by default, is the first deployed domain controller in the forest root domain. The global catalog maintains the subset of attributes that are useful in cross-domain searches, such as givenName, displayName, and the mail.