Understanding How Failover Works in an Exchange Cluster

As noted earlier, Exchange Virtual Servers are the basic units of failover for your cluster. However, failover occurs differently in active/passive clusters and active/active clusters.

In an active/passive cluster, such as the 3-active/1-passive cluster shown in the following figure, there are three Exchange Virtual Servers: EVS1, EVS2, and EVS3. This configuration can handle a single node failure at a time and still maintain 100 percent availability after a failure occurs. That is, if Node 3 fails, Node 1 still owns EVS1, Node 2 still owns EVS2, and Node 4 takes ownership of EVS3 with all the storage groups mounted after the failure. However, if a second node fails while Node 3 is still down, the Exchange Virtual Server associated with the second failed node remains in a failed state because there is no stand-by node available for failover.

Effect of failures on an active/passive cluster

In an active/active cluster as shown in the following figure, there are only two Exchange Virtual Servers: EVS1 and EVS2. This configuration can handle a single node failure at a time and still maintain 100 percent availability after the failure occurs. That is, if Node 2 fails, Node 1 still owns EVS1, and Node 1 also takes ownership of EVS2 with all the storage groups mounted after the failover. However, if Node 1 fails while Node 2 is still down, the whole cluster is in a failed state, because no nodes are available for failover.

Effect of failures on an active/active cluster