A global catalog server is a domain controller it is a master searchable database that contains information about every object in every domain in a forest. The global catalog contains a complete replica of all objects in Active Directory for its host domain, and contains a partial replica of all objects in Active Directory for every other domain in the forest. It have two important functions:
- Provides group membership information during logon and authentication
- Helps users locate resources in Active Directory
Click Start->Administrative Tools-> and then click Active Directory Sites and Services->In the console tree, expand Sites, and then expand the site that contains the domain controller which you want to configure as a GC server->Expand the Servers folder, and locate and then click the domain controller that you want to designate as a GC server-> right-click NTDS Settings and click Properties ->The General tab->.Enable the Global Catalog checkbox->Click OK.