Single global catalog is present in topology

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at]  

Topic Last Modified: 2010-05-05

The Microsoft® Exchange Server Analyzer Tool queries the Exchange_DSAccess Microsoft Windows® Management Instrumentation (WMI) class in the root\MicrosoftExchangeV2 namespace to determine the number of global catalog servers present in the topology. The number of global catalog servers is determined by counting the number of entries for the DSAccessNameGC WMI key. If the Exchange Server Analyzer finds that only one global catalog server exists in the topology, the Exchange Server Analyzer displays a best practice recommendation.

For fault tolerance, it is a best practice to have at least two global catalog servers available to service Exchange Servers and users. A single global catalog server represents a single point of failure. Exchange 2000 Server, Exchange Server 2003, and Exchange Server 2007 depend on global catalog servers for a variety of functions. If a global catalog server is not available, mail delivery can become slow or stop completely. Also, if a global catalog server is not available, Exchange Server services may not start. If Exchange Server services do not start, users cannot access their mailboxes.

For scalability and for fault tolerance, it is recommended that you configure at least two global catalog servers in each Active Directory® directory service site. If a site spans multiple domains, it is recommended that you configure a global catalog for each domain where Exchange 2000 Server computers, Exchange Server 2003 computers, or Exchange Server 2007 computers and clients reside. Also, it is recommended that you deploy one global catalog server for every four Exchange Server computer processors.

For details about how to create additional global catalog servers, see Microsoft Knowledge Base article 313994, "How to create or move a global catalog in Windows Server 2003, Windows 2000, or Small Business Server 2000" (

For more information about global catalog server placement and availability as it relates to Exchange Server, see the following Knowledge Base articles: