Every Forest requires at least one Global Catalog Server; You can promote more DC to GCS if you want


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Every forest requires at least one Global Catalog server, that’s why by default When you create a Windows Server Active Directory Forest, the first domain controller in the forest is automatically configured to be a Global Catalog server. The global catalog is a distributed data repository that contains a searchable, partial representation of every object in every domain in a multi-domain Active Directory forest. 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.

http://www.elmajdal.net/win2k8/Setting_a_Windows_Server_2008_Domain_Controller_as_a_Global_Catalog.aspx

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

Open Active Directory Sites and Services, click Start > Administrative Tools > then click Active Directory Sites and Services
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 .

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

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.

Close Active Directory Sites and Services and then restart the Domain Controller

Thank you for the contribution Tarek Majdalani ( ElMajdal )
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http://technet.microsoft.com/en-us/library/ee198775.aspx
Identifying Global Catalog Servers

Microsoft® Windows® 2000 Scripting Guide

Knowing which computers in your organization are global catalog servers is very helpful for administering Active Directory efficiently.

Because global catalog servers are required for user authentication, it is important to have an adequate number of global catalog servers at a site to handle the volume of authentication requests and a quick connection to handle each request efficiently. If a remote office does not have its own global catalog server and is connected over a slow network link, authentication of the site’s users will be slow.

However, having too many global catalog servers can slow down your network by creating unnecessary replication traffic. Global catalog servers contain information about every item in the forest and can generate a considerable amount of replication data. A remote office that has its own global catalog servers and is connected over a slow network link can provide efficient authentication to its local users, but it might have trouble communicating with other sites because of the quantity of replication data moving across the network link.

Thank you for the contribution Tarek Majdalani ( ElMajdal )

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