The vSphere HA agent on a host is in the Network Isolated state. User intervention is required to resolve this situation.
When a host is in the Network Isolated state, there are two things to consider -- the isolated host and the vSphere HA agent that holds the master role.
On the isolated host, the vSphere HA agent applies the configured isolation response to the running VMs, determining if they should be shut down or powered off. It does this after checking whether a master agent is able to take responsibility for each VM (by locking the VM's home datastore.) If not, the agent defers applying the isolation response for the VM and rechecks the datastore state after a short delay.
If the vSphere HA master agent can access one or more of the datastores, it monitors the VMs that were running on the host when it became isolated and attempts to restart any that were powered off or shut down.
A host is network isolated if both of the following conditions are met:
Isolation addresses have been configured and the host is unable to ping them.
The vSphere HA agent on the host is unable to access any of the agents running on the other cluster hosts.
If your vSphere HA cluster has Virtual SAN enabled, a host is determined to be isolated if it cannot communicate with the other vSphere HA agents in the cluster and cannot reach the configured isolation addresses. Although the vSphere HA agents use the Virtual SAN network for inter-agent communication, the default isolation address is still the gateway of the host. Hence, in the default configuration, both networks must fail for a host be declared isolated.
Resolve the networking problem that is preventing the host from pinging its isolation addresses and communicating with other hosts.