Global catalog servers are responsible for keeping a fully writable copy of objects in its own domain and a partial copy of all other domains objects in the forest. It facilitates querying about objects in the entire forest. Global catalog service is part of the domain controller services, and it cannot be separated.

In a single-forest-single-domain environment, all the domain controllers can be global catalog servers as it will not be different from domain replication. But in a multi-domain environment, global catalog server placement involves planning as it increases the amount of data to be replicated and the bandwidth.

There are certain things that you need to consider when placing a global catalog server:

• Number of users: It is recommended that you place a global catalog server on any site that is over 100 users. This will help maintain site availability in the event of WAN link failure.
• WAN link reliability: If you are struggling with link availability between sites, it's recommended that you place global catalog servers in a remote site, and enable universal membership caching.
• Roaming users: Roaming users are required to connect to the global catalog server when they log in for the first time from any location in the infrastructure. Therefore, if users are using roaming profiles in remote sites, it's important that you place the global catalog server.
• Application requirements: Applications such as Microsoft Exchange heavily depend on global catalog servers. If similar applications are hosted on remote sites, they will be required to have a global catalog server.

Once the global catalog server placement is determined, the Active Directory design phase is completed. Now we have the logical and physical design for the Active Directory infrastructure ready.