…Overview…
  

The Global Catalog contains a partial replica of every object in the Active Directory. It’s what is accessed (unwittingly) by the end user and applications when searching for names, logon details, printers etc. It contains details of every object in the Active Directory but with only a small number of their attributes. In this article I describe the Global Catlogue in more detail and best practices for deploying and configuring.

 
Planning Global Catalog Deployment

There are several things to consider when planning how many Global Catalog servers you are going to deploy. If you only have one domain then there is no need to have more than one global catalog server. If you have a forest you need to think about the positioning of the global catalog servers as these are the Domain Controllers that are aware of other domains and most importantly recognise Universal Groups. Having a Universal Group in a single domain is pointless but as non Global Catalog Domain Controllers can’t recognise Universal groups from other domains then you need to try and install Global Catalogs locally to speed up logons etc.

Exchange server 2003 uses the Global Catalog for resolving email addresses so if you are running Exchange in a large organisation it may be worthwhile installing more Global Catalog servers. Microsofts recommendation is typically four Exchange Servers to every one GC. This is what is accessed when people are looking up the
You need to be aware that more Global Catalog servers mean more replication and therefore more network traffic. If it’s of no benefit having another Global Catalog server (EG only one domain) then don’t have one. 

Another thing to note is that Microsoft warn of potential conflicts between the global catalog and the Infrastructure Master. If you have only one domain controller then of course they will have to remain on the same server but where possible the Global Catlog and Infrastructure Master should be hosted on different domain controllers to prevent conflicts, replication issues and problems such as certain information not being seen at all. If your environment hosts the Global Catalogue on every domain controller then this is an exception to the rule. 

Installing a Global Catalog Server

The first domain controller installed in your forest will automatically be a Global Catalog server. If you wish to add more it’s a simple matter of a tick box. It’s not a simple matter of finding it unless you know where to look.

1.) Open up Active Directory Sites and Services.
2.) Expand Sites, Site Name, Servers.
3.) Select your server and then the NTDS Settings, right click and choose Properties.
4.) Tick the box marked Global Catalog.

Moving a Global Catalog Server

1.) If you are moving a global catalog server then follow the instructions above for first installing a new one.
2.) Wait for the information to replicate to the new GC. This is very important and may take some time in Multi Domain Forests.
3.) Navigate to the NTDS settings of the original server and remove the tick from Global Catalog.

If you are running Windows 2000 server you will be prompted to reboot. If you are running 2003 there is no prompt and no need to although I always feel better when making a change like this to reboot anyway.

Afterthought

Please note that in the UK we spell catalog with a “ue” on the end and I have spelled it both the American and English way in this article. We also use the words lift instead of elevator, pavement instead of sidewalk and boot instead of trunk but these are all pretty irrelevant in the world of Windows servers so probably won’t be mentioned anywhere else on the website!