To set an additional Domain Controller as a Global Catalog Server, follow the below steps:

1. Open Active Directory Sites and Services, click Start > Administrative Tools > then click Active Directory Sites and Services.
   
   
   
   
2. From the left side pane, expand Sites > Default-First-Site-Name ( If you have renamed the Site name, then this name will reflect your Site name) > Servers > Domain Controllers .
   
   
   
   
3. Now expand the Windows Server 2008 that you want to set as a Global Catalog and right Click its NTDS Settings then click on Properties
   
   
   
   
   
4. Under the General Tab, there is a check box beside Global Catalog, if it is unselected then this means that this Domain Controller is not a Global Catalog, if you do want to set it as a Global Catalog server, then select this checkbox and click on Apply then OK. The process it takes the Global Catalog to replicate itself throughout the forest takes time depending on your Active Directory infrastructure.
   
   
   
   
   
   
5. Close Active Directory Sites and Services and then restart the Domain Controller.

Summary

A global catalog server is a domain controller that, in addition to its full, writable domain directory partition replica, also stores a partial, read-only replica of all other domain directory partitions in the forest. In this article, I have shown you how easy it is to set an additional Domain Controller to be configured as a Global Catalog.