Access to a global catalog is required for successful logon attempts. A global catalog is necessary to determine group memberships during the logon process. If your network has any slow or unreliable links, enable at least one global catalog on each side of the link for maximum availability and fault tolerance.
Most of your Active Directory network traffic will be query-related: users, administrators, and programs requesting information about directory objects. Updates to the directory, causing directory replication traffic, should occur much less frequently. To get the best network performance, place at least one domain controller in each site. With a domain controller in each site, all users can access a server that can service query requests over the fastest links available.
To help optimize network traffic, at sites connected by slow links, you can also configure domain controllers to receive directory replication updates only during off-peak hours.
The best network performance comes when the domain controller at a site is also a global catalog. That way, that server can fulfill queries about objects in the entire forest. However, enabling many domain controllers as global catalogs can increase the replication traffic on your network.
In domains with more than one domain controller, do not enable the domain controller holding the infrastructure master role as a global catalog. For more information, see Global catalog and infrastructure master