When you create the Active Directory forest, by default the first Domain Controller will serve as the Global Catalog Server, but we can designate any Domain Controller as the Global Catalog Server.
To configure a Domain Controller as Global Catalog Server, follow these steps.
• Open Active Directory Sites and Services (Start > Programs > Administrative Tools > Active Directory Sites and Services).
• Select the Sites branch.
• Select the site that owns the server, and expand the Servers branch.
• Expand the Domain Controller by double clicking on it.
• Right click the “NTDS Settings” and select properties from the popup menu.
If you want to assign this Domain Controller as the Global Catalog, check the checkbox "Global Catalog" in "NTDS Settings Properties" dialog box.