This blog post will show you how to configure a Windows Server 2012 Domain Controller as a Global Catalog Server.
A domain controller configured as Global Catalog Server Is a DC that contain a copy of all the objects In the directory.
Global Catalog server they uses TCP port 3268.
By default the first DC Installed In the forest Is configured as a Global Catalog.
To configure a Global Catalog Server open dssite.msc
Right click on NTDS Settings and Click on Properties
Tick the Box Global Catalog
How To Verify that the Global Catalog Is ready and working
Type the sever name you need to check
Verify that isGlobalCatalogReady value Is True