Now that we have high availability and failover at the namespace level between datacenters, we need to achieve the same for the Mailbox server role. This is accomplished in a similar way to Exchange 2010, by extending a DAG across two or more datacenters.

An organization's SLA covering failure and disaster recovery scenarios is what mostly influences a DAG's design. Every aspect needs to be considered: the number of DAGs to be deployed, the number of members in the DAG(s), the number of database copies, if site resilience is to be used and whether it has to be used for all users or just a subset, if the multiple site solution will be active/active or active/passive, and so on. As to the latter, there are generally three main scenarios when considering a two-datacenter model.