Agents on the different hosts contact and monitor each other through the exchange of heartbeats, by default every second. If a 15-second period elapses without the receipt of heartbeats from a host, and the host cannot be pinged, the host is declared as failed. The virtual machines running on the failed host are restarted on the alternate hosts with the most available unreserved capacity (CPU and memory).
Host network isolation occurs when a host is still running, but it can no longer communicate with other hosts in the cluster. With default settings, if a host stops receiving heartbeats from all other hosts in the cluster for more than 12 seconds, it attempts to ping its isolation addresses. If this also fails, the host declares itself isolated from the network.
When the isolated host's network connection is not restored for 15 seconds or longer, the other hosts in the cluster treat that host as failed and try to fail over its virtual machines. However, when an isolated host retains access to the shared storage it also retains the disk lock on virtual machine files. To avoid potential data corruption, VMFS disk locking prevents simultaneous write operations to the virtual machine disk files. Therefore attempts to fail over the isolated host's virtual machines do not succeed. By default, the isolated host leaves its virtual machines powered on, but you can change HA host isolation response as described in the vSphere Availability guide.
