This article discusses the ratio of global catalog servers to Microsoft Exchange Server 2003 computers or to Microsoft Exchange 2000 Server computers in your Exchange organization. Additionally, this article describes some of the issues that you should consider when you run Exchange Server on a domain controller.
Global catalog server ratio and global catalog server placement
The ratio of servers to global catalog servers in your Exchange organization depends on all the following:
The performance capabilities of the servers in your organization.
The number of users in your organization.
The message volume that you experience in your organization.
The available network bandwidth in your organization.
The other factors that affect computer processor load.
For scalability and for fault tolerance, we recommend that you configure at least two global catalog servers in each Active Directory site. If a site spans multiple domains, we recommend that you configure a global catalog for each domain where Exchange 2000 Server computers or Exchange Server 2003 computers and clients reside.
A general guideline is to deploy one global catalog server for every four Exchange computer processors. Therefore, by using this general guideline, you might deploy your global catalog servers as follows.
Note This general guideline assumes that all the computer processors are of the same type and of the same speed.
One single-processor global catalog server to support one four-processor Exchange 2000 Server computer or Exchange Server 2003 computer.
Two single-processor global catalog servers to support one eight-processor Exchange 2000 Server computer or Exchange Server 2003 computer.
Four quadruple-processor global catalog servers to support eight eight-processor Exchange 2000 Server computers or Exchange Server 2003 computers.
You can adjust these guidelines to meet the specific requirements of your Exchange organization.
Exchange Server computer placement
For performance and for scalability, we recommend that you install Exchange 2000 Server or Exchange Server 2003 on a member server instead of on a domain controller. However, sometimes you may have to install Exchange on a domain controller. Consider all the following if you install Exchange on a domain controller:
The domain controller must not be a node in a server cluster. We do not support Exchange 2000 Server clusters or Exchange Server 2003 clusters that are configured on Active Directory domain controllers.
The installation of Exchange on a domain controller may affect the performance of that domain controller. In some scenarios, this performance effect prevents the Exchange-related services from starting successfully when you start the computer. In these scenarios, you must manually start the Exchange-related services.
The domain controller that is running Exchange must also be a global catalog server.
All the Exchange-related services run under the Local System security account. Therefore, you may be at more risk if you run Exchange on a domain controller, and if a problem is discovered in Exchange.
If you permit Exchange administrators to log on locally to the domain controller that is running Exchange, a malicious user could try to elevate their permissions in Active Directory directory service.
You may experience slow shutdown times on a domain controller that is running Exchange. In some scenarios, the domain controller may take 10 minutes or more to shut down. This behavior occurs because the Lsass.exe service shuts down before the Exchange-related services shut down. (Lsass.exe is an Active Directory service.) Therefore, the DSAccess component times out many times before it shuts down. (The DSAccess component is an Exchange component.) To work around this issue, manually stop the Exchange-related services before you shut down or restart the domain controller. Specifically, shut down the information store service before you restart the domain controller that is running Exchange.
For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base:
322801 Exchange System Attendant does not start, and you receive a "Global catalog servers not responding" error message
829361 Exchange Server 2003 computer takes longer than you expect to shut down
313994 How to create or move a global catalog in Windows 2000
304403 Exchange considerations for promoting a domain controller to a global catalog server
305065 Exchange Server-related considerations for demoting a global catalog server to a domain controller
813033 Availability of the "Understanding and Troubleshooting Directory Access" book
For additional information about how to plan and configure an Exchange messaging system, visit the following Microsoft Web sites: