Exchange 2013 CU6 and later uses out-of-site domain controllers and global catalog servers


Starting with Microsoft Exchange Server 2013 Cumulative Update 6 (CU6), Exchange Server may unexpectedly use out-of-site domain controllers and global catalog servers.

In pre-CU6 installations, when the Exchange process requests domain controllers, as long as there is a single suitable domain controller in the In-Site list, the topology service returns it and does not search the Out-of-Site list any further. This behavior occurs regardless of how many domain controllers are requested by the client. This may cause an unbalanced load issue, especially during site failover. Domain controllers that remain in the failed-out site bear a greater load than domain controllers from outside the site.

To fix this issue, the MinSuitableServer configurable setting has been introduced. The topology service first checks whether there's a sufficient number of suitable servers in the In-Site list. If not, it adds servers from the Out-of-Site list. A similar change has also been made in topology discovery.

For example, if the number of in-site domain controllers is fewer than what's specified by the MinSuitableServer value (which is 3, by default), out-of-site domain controllers will be used. As soon as the number of in-site domain controllers is greater than the MinSuitableServer value, out-of-site domain controllers will not be used any longer.

This default value can be changed in the Microsoft.Exchange.Directory.TopologyService.exe.config file. You can do this by adding the MinSuitableServer value in the Topology MinimumPrefixMatch = "2" section.

More Information

To reduce the minimum number of suitable in-site domain controllers (to match the configuration of the RTM version of Exchange Server 2013), follow these steps:
  1. Open the Microsoft.Exchange.Directory.TopologyService.exe.config file.
  2. Locate the following text in the "Topology" section:

    EnableWholeForestDiscovery = "true"
  3. Add the following line after the line in step 2:

    MinSuitableServer = "1"

    The section should now look like this:

    EnableWholeForestDiscovery = "true"
    MinSuitableServer = "1"
    ForestWideAffinityRequested = "true"/>
  4. Save the changes.
  5. Restart the Microsoft Exchange Active Directory Topology service.

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