Red Hat Enterprise Virtualization 3.1 and higher distinguishes between required networks and optional networks.

Required networks must be applied to all hosts in a cluster for the cluster and network to be Operational. Logical networks are added to clusters as Required networks by default.

When a host's required network becomes non-operational, virtual machines running on that host are migrated to another host; the extent of this migration is dependent upon the chosen cluster policy. This is beneficial if you have machines running mission critical workloads.

When a non-required network becomes non-operational, the virtual machines running on the network are not migrated to another host. This prevents unnecessary I/O overload caused by mass migrations.

Optional networks are those logical networks that have not been explicitly declared Required networks. Optional networks can be implemented on only the hosts that use them. The presence or absence of these networks does not affect the Operational status of a host.

Use the Manage Networks button to change a network's Required designation.

Virtual machine networks (called a VM network in the user interface) are logical networks designated to carry only virtual machine network traffic. Virtual machine networks can be required or optional.