Process %1 (PID=%2). The configuration domain controller specified in the registry (%3) is unreachable. Exchange Active Directory Provider will select the configuration domain controller from the list of available domain controllers.

This Warning event indicates that the computer specified in the Windows registry that must be used as a Configuration Domain Controller (CDC) cannot be reached. This situation can occur if one or more of the following conditions are true:

• A registry key or server name is configured incorrectly. For example, you may have incorrectly spelled the name of the domain controller in the registry.

• The computer specified in the event description has a network connectivity issue.

• The domain controller specified in the event description is not running.

If the correct server name was specified, the specified server may be down or the network path may be down. Usually, if a configured domain controller is not reachable for any reason, Exchange will select an appropriate configuration domain controller from the list of available domain controllers. Therefore, in most cases, this event can be ignored. However, if there is an accompanying MSExchangeDSAccess Event ID 2102, the Exchange Services might not start. In this case, you must determine the basis of the problem and make sure that the domain controllers are reachable.

If the Exchange Services start without any problems, you can safely ignore this warning. However, if the Exchange Services do not start, do one or more of the following

• Make sure that the registry key and the server name are configured to use the correct values.

• Use the Ping or PathPing command-line tools to test basic connectivity. Use Ping to isolate network hardware problems and incompatible configurations. Use PathPing to detect packet loss over multiple-hop trips. For more information, see Microsoft Knowledge Base article 325487, How to troubleshoot network connectivity problems.

• Run the Dcdiag command line tool to test domain controller health. To do this, run dcdiag /s:<Domain Controller Name> at a command prompt on the Exchange Server. Use the output of Dcdiag to discover the root cause of any failures or warnings that it reports. For more information, see Dcdiag Overview at the Microsoft Windows Server TechCenter.

Note  If you aren’t already doing so, consider running the tools that Exchange offers to help administrators analyze and troubleshoot their Exchange environment. These tools can help you make sure that your configuration is in line with Microsoft best practices. They can also help you identify and resolve performance issues, improve mailflow, and better manage disaster-recovery scenarios. Go to the Toolbox node of Exchange Management Console to run these tools now. For more information about these tools, see Toolbox in the Exchange Server 2007 Help.