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, vSphere HA applies the power-off or shutdown host isolation response to virtual machines running on the host. vSphere HA continues to monitor the virtual machines that are left powered on. While a host is in this state, vSphere HA's ability to restart virtual machines after a failure is affected. vSphere HA only powers off or shuts down a virtual machine if the agent on the host determines that a master host is responsible for the virtual machine.
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.