Whether you are using a Microsoft Cluster Server (MSCS)-based clustering solution or a third-party clustering solution, we strongly recommend that you install Microsoft Exchange Server software only on cluster node computers that are configured as domain member servers. Microsoft does not support the use of Exchange on cluster node computers that are also configured as domain controllers.
During typical operation, Exchange is intended to run without problems on identically-configured cluster nodes where each node is configured as a domain controller or as a global catalog server. However, if a problem does occur, this configuration may greatly limit the ability to troubleshoot or to correct problems without potentially disrupting critical domain communications.
Additionally, you may experience performance issues and security issues when you run Exchange in an environment where each node is configured as a domain controller or as a global catalog server. The additional load on a server that is caused by servicing Active Directory directory service requests may affect the performance of your installed Exchange software. You should also consider the fact that administrators for Exchange-based servers that also serve as domain controllers will necessarily have extensive administrative rights in the Active Directory forest.