A global catalog server (GC) contains information about objects from all domains in an Active Directory forest. When you promote Windows Server 2012 to be a domain controller (DC), the option to make the new DC a global catalog server is selected by default. If you choose not to do this at the time of promoting the server to a DC, you can make the DC a global catalog server at a later stage.
In an Active Directory forest where there is only one domain, every DC can be a global catalog server without affecting performance. GCs are used when a client requests information about AD objects in different domains to avoid directing the queries to DCs in different domains until the right domain is located.
While DCs in a single-domain forest contain information about all AD objects, GC searches occur on a different port to standard AD queries, so you should configure every DC to be a global catalog server for load-balancing. If you make use of Universal Groups in AD, a global catalog is required for domain logons.
In general, you should consider having at least one global catalog server per AD site; but this will depend on whether clients and applications in a site need to look up information from different domains. If no GC is available, AD operations could be significantly slower if search queries have to be directed over a wide area network. GC placement in a multi-domain forest should take into consideration the increased replication overhead.
Log in to Windows Server 2012 with an account that is a member of the Domain Administrators group and follow the instructions below.
A DC won’t be marked as a GC in Active Directory until replication has completed, so you may need to be patient, depending on the speed of your network connections and number of objects in AD.