Exchange Servers in active/passive and N+1 configurations are supported. Active/active configurations are also possible, but not recommended by Microsoft.

Figure: Active/passive configuration illustrates an example basic active/passive Exchange Server cluster configuration.

There is one Exchange Virtual Server, called EVS1, which can run on either node. As it is currently running on NODEA, this is the active node. If a problem occurs on this node, EVS1 will failover to NODEB, which then becomes the active node. Mailbox and public folder information stores and registered forms are held on the shared disks. The configuration information for the Exchange Virtual Server is held in Active Directory. In a basic active/passive configuration, there is one standby node for each active node.

N+1 clusters are similar to active/passive configurations in that there is a standby (passive) node to which applications on an active node can failover. However, in an N+1 configuration, the passive node is standby for multiple active nodes.

In Figure: N+1 configuration, NODEC is the standby node for NODEA and NODEB.

The Exchange Virtual Server, EVS1, can run on either NODEA or NODEC. The Exchange Virtual Server, EVS2, can run on either NODEB or NODEC.

Figure: Alternative N+1 configuration illustrates an alternative N+1 configuration, in which any of the nodes can act as standby for either of the Exchange Virtual Servers.

Each of the Exchange Virtual Servers, EVS1 and EVS2, can run on NODEA, NODEB or NODEC.

Figure: Active/active configuration illustrates an active/active configuration.

Note that Microsoft does not recommend active/active configurations.

In these configurations there are no passive standby nodes; if the Exchange Virtual Server, EVS1, fails over, then both Exchange Virtual Servers will be running on NODEB, which could cause performance issues.

When configuring Enterprise Vault OWA and RPC Extensions for clustered environments, the extensions must be installed and configured on each node on which the Exchange Virtual Server can run.

Additional information on installing the extensions in active/passive and active/active clustered environments is given in the following sections.