How many Global Catalog Servers to have on your network? Where to place those servers? About this things Administrator should have thing at least once :). You could cause severe performance problems or possibly a single point of failure, If there are jast few Global Catalog Servers. At the same time, having too many Global Catalog Servers can also cause performance problems. In this article, you will see some techniques for determining how many global catalog servers you need.

Why Are Global Catalog Servers So Important? (BTW, about this question you could be asked on every Interview :))

Global Catalog Servers, they are simply domain controllers that have been designated to perform the Global Catalog server role. The first domain controller in the forest is automatically assigned the Global Catalog server roll, because every forest requires at least one Global Catalog server. The Global Catalog server performs a variety of tasks, both for Windows and for Exchange. If a Global Catalog server is not available, then nobody will be able to log into the domain except for the Administrator.

Global Catalog server performs some critical functions at the Active Directory level and in relation to Exchange Server it's also very critical.
In order for clients to be able to send and receive mail, both the Outlook client and the Exchange Server must be able to query a global
catalog server. Without access to a Global Catalog server, Outlook clients will not be able to open the Global Address List or resolve the e-mail addresses of message recipients within the forest.

Now let’s talk about placement. Any domain controller that’s running Windows 2000 or Windows Server 2003 can be designated to be a Global Catalog server. I saw networks, where every domain controller act as a Global Catalog server. Equally, this is a bad idea. Global Catalog servers produce quite a bit of traffic applied to the replication process.

Some Network Consultans or System Administrators says, that you should place a Global Catalog Server into any site that contains a server that’s running an application that makes use of port 3268 (Global Catalog lookup port). Exchange Server is such an application, so if you were to follow this rule, then you would want to place a Global Catalog server into any site that contains an Exchange Server.

You can find a Microsoft document that suggests placing a Global Catalog server into each site. I believe that this is the best idea. The fact that clients make use of the global catalog during the logon process. But, Microsoft also says, that a site doesn’t need a Global Catalog server if the site does not contain any servers running Global Catalog dependant applications, such as Exchange or contains fewer than 100 users.

In large organizations with lots of Exchange mailboxes, Global Catalog server could start working VERY hard. Microsoft recommends having one Global Catalog Server for every four mailbox servers.

If you have a single site, single domain network, then it is safe to designate all of your domain controllers to act as Global Catalog servers. Becouse all of the domain controllers contain full copies of the Active Directory you will not get any performance problems by having multiple Global Catalog servers will be minimal.

It is important to make sure that our system uses Global Catalog servers in an optimal manner. Global Catalog servers are so critically important to the Active Directory and to Exchange.

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