Server clustering is a method of tying two or more servers together by using a high-speed network connection so that the group of servers functions as a single, logical server. If one of the servers fails, the other servers in the cluster continue operating, picking up the operations that the failed server performs.

VMware tests Microsoft Cluster Service in conjunction with ESX/ESXi systems, but other cluster solutions might also work. Different configuration options are available for achieving failover with clustering:

Cluster in a box

Two virtual machines on one host act as failover servers for each other. When one virtual machine fails, the other takes over. This configuration does not protect against host failures and is most commonly used during testing of the clustered application.

Cluster across boxes

A virtual machine on an ESX/ESXi host has a matching virtual machine on another ESX/ESXi host.

Physical to virtual clustering (N+1 clustering)

A virtual machine on an ESX/ESXi host acts as a failover server for a physical server. Because virtual machines that run on a single host can act as failover servers for numerous physical servers, this clustering method is a cost-effective N+1 solution.