I had covered the topic of what to look for before creating a DAG in one of my previous article. In this one, I am going to highlight the facts you need to consider while designing a Database Availability Group. Without going over the DAG feature again, let me list the points to note.
  • Each member of the DAG must be running the same operating system.
  • It is not supported to add an Exchange 2010 mailbox server that is also a directory server to a DAG.
  • A DAG can contain a mix of servers running Exchange 2010 Standard and Enterprise editions.
  • Each DAG must have no more than one MAPI network.
  • DAG members with a single NIC (for both MAPI & Replication) is supported.
  • Each DAG member must have the same number of networks. For example, if you use a single NIC in one DAG member, then all members of the DAG must also use single NIC.
  • Regardless of whether you use static or DHCP addresses, any IP address assigned to the DAG must be on the MAPI network.
  • DAG networks support IPv4 and IPv6. IPv6 is supported only when IPv4 is also used. A pure IPv6 environment is not supported.
  • Configure the network connection order so that the MAPI network is at the top of the connection order.
  • It is not a requirement that the version of operating system of the witness server should match the operating system used by the DAG members.
  • MAPI networks should be isolated from Replication networks.
  • Use static routes to configure connectivity across Replication networks.
  • When a DAG is extended across multiple datacenters, it should be designed so that either the majority of the DAG members are located in the primary datacenter or when each datacenter has the same number of members, the primary datacenter hosts the witness server.
  • Each time the DAG's MAPI network is extended across an additional subnet, an additional IP address for that subnet must be configured for the DAG.
  • Each IP address that is configured for the DAG is used by the failover cluster. The name of the DAG is also used as the name for the underlying failover cluster.
  • If the replication network fails and the MAPI network is unaffected, log shipping & seeding will revert to use the MAPI network. When the failed replication network is restored, log shipping & seeding will revert back to the replication network.
  • Each server in the DAG can be on a different subnet, but the MAPI and Replication networks must be routable and provide connectivity.

http://www.howexchangeworks.com/2010/02/points-to-note-while-designing-database.html