With the capabilities introduced by storage area networks (SANs), you cannot only create larger clusters, you can also connect multiple servers to the same storage.
Figure: N+1 configuration shows an N+1 cluster failover configuration.
A dedicated, redundant server is no longer required in the configuration. Instead of N-to-1 configurations, you can use an N+1 configuration. In advanced N+1 configurations, an extra server in the cluster is spare capacity only.
When a server fails, the application service group restarts on the spare. After the server is repaired, it becomes the spare. This configuration eliminates the need for a second application failure to fail back the service group to the primary system. Any server can provide redundancy to any other server.
Figure: N+1 cluster failover configuration requiring failback shows an N+1 cluster failover configuration requiring failback.