Exchange 2010 Site Resilience with two active Datacenters

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If you want an active/active user distribution you need to determine the failure models and scenarios you are willing to tolerate.

For example, if you have a WAN failure between the two datacenters and you deploy a single DAG, then one of the datacenters is going to have an outage event for the mailboxes because quorum cannot be maintained. If this is unacceptable for you, then you will need to deploy two DAGs to maintain user locality for failure events like that. This will increase the number of mailbox servers as you will need at least a mailbox server from each DAG in the other datacenter. The below diagram clarifies this:

The way to deal with this in the Mailbox Role Calculator:

  1. Launch two versions of the calculator.
  2. Populate the first version for the first DAG in your design. This DAG (DAG01) will utilize Datacenter 1 as its primary location (and thus its user population is based out of Datacenter 1). It has site resiliency by having servers and database copies located in Datacenter 2 that can be activated in the event Datacenter 1 is lost.
  3. Populate the second version for the second DAG in your design. This DAG (DAG02) will utilize Datacenter 2 as its primary location (and thus its user population is based out of Datacenter 2). It has site resiliency by having servers and database copies located in Datacenter 1 that can be activated in the event Datacenter 2 is lost.

By implementing the architecture in this way, you can ensure that for the majority of scenarios except loss of datacenter, the users remain operational in their primary datacenter location.