A required network is a logical network that must be available to all hosts in a cluster. 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 scheduling policy. This is beneficial if you have virtual machines running mission critical workloads.
An optional network is a logical network that has not been explicitly declared as Required. Optional networks can be implemented on only the hosts that use them. The presence or absence of optional networks does not affect the Operational status of a host. 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. Note that when a logical network is created and added to clusters, the Required box is checked by default.
To change a network's Required designation, from the Administration Portal, select a network, click the Cluster tab, and click the button.
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. Virtual machines that uses an optional virtual machine network will only start on hosts with that network.