Least Cost Routing is Enabled in a Mixed-Mode Organization

[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 http://go.microsoft.com/fwlink/?linkid=34707.]  

Topic Last Modified: 2005-11-18

The Microsoft® Exchange Server Analyzer Tool queries the Active Directory® directory service to determine whether least cost routing is enabled on the server running Exchange Server version 5.5 in a mixed-mode environment. The Exchange Server Analyzer displays a warning if the following conditions exist:

  • Least cost routing is enabled.

  • Your organization is running Exchange Server 2003 in a mixed-mode environment with Exchange Server 5.5.

When least cost routing is enabled in a mixed-mode environment, message loops may be created. Exchange Server 2003 uses link state routing. Therefore, Exchange Server 2003 reroutes messages to another server when it detects that a link is down. When an Exchange Server 5.5 server is using the Only use least cost routes option in the Message Transfer Agent properties page, the best route frequently is determined to be back to the Exchange Server 2003 server from which the message was just sent. When the message is sent back to the Exchange Server 2003 server, the route becomes a loop.

In any mixed-mode organization that is running both Exchange Server 5.5 and Exchange Server 2003, it is recommended that the least cost routing option be disabled on all servers running Exchange Server 5.5.

To disable least cost routing in Exchange Server 5.5
  1. In the Microsoft Exchange Server Administrator program, expand the Organization container, expand the Site container, expand the Configuration container, expand the Servers container, and then click your server object.

  2. In the right pane, click the Message Transfer Agent object.

  3. Click the File menu, and then click Properties.

  4. In the Message Transfer Agent Properties dialog box, on the General tab, clear Only use least cost routing.

  5. Click OK.


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