A. Exchange 2010 is supported on the 64-bit editions of both Server 2008 and Server 2008 R2, but you can't mix the OSs within a single DAG. This limitation is because DAGs use Windows failover clustering behind the scenes, and failover clustering doesn't support a mix of Server 2008 and Server 2008 R2 nodes in the same cluster (because of differences in cluster implementation).

If you want a mix of Server 2008 and Server 2008 R2, you'll need to have multiple DAGs, each containing one version of Windows.

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