The master host of a vSphere HA cluster is responsible for detecting the failure of slave hosts. Depending on the type of failure detected, the virtual machines running on the hosts might need to be failed over.
In a vSphere HA cluster, three types of host failure are detected:
A host stops functioning (that is, fails).
A host becomes network isolated.
A host loses network connectivity with the master host.
The master host monitors the liveness of the slave hosts in the cluster. This communication is done through the exchange of network heartbeats every second. When the master host stops receiving these heartbeats from a slave host, it checks for host liveness before declaring the host to have failed. The liveness check that the master host performs is to determine whether the slave host is exchanging heartbeats with one of the datastores. See Datastore Heartbeating. Also, the master host checks whether the host responds to ICMP pings sent to its management IP addresses.
If a master host is unable to communicate directly with the agent on a slave host, the slave host does not respond to ICMP pings, and the agent is not issuing heartbeats it is considered to have failed. The host's virtual machines are restarted on alternate hosts. If such a slave host is exchanging heartbeats with a datastore, the master host assumes that it is in a network partition or network isolated and so continues to monitor the host and its virtual machines. See Network Partitions.
Host network isolation occurs when a host is still running, but it can no longer observe traffic from vSphere HA agents on the management network. If a host stops observing this traffic, it attempts to ping the cluster isolation addresses. If this also fails, the host declares itself as isolated from the network.
The master host monitors the virtual machines that are running on an isolated host and if it observes that they power off, and the master host is responsible for the virtual machines, it restarts them.
If you ensure that the network infrastructure is sufficiently redundant and that at least one network path is available at all times, host network isolation should be a rare occurrence.