"vSphere HA detected that this host is in a different network partition than the master to which vCenter Server is connected" error on vCenter Server

Products

• VMware vSphere ESXi

Issue/Introduction

• ESXi host reports following alert in the vCenter
  • Alert: [Critical] Alarm alarm.HAhostStatus on Host <host-fqdn>
because vSphere HA detected that host <host-fqdn> is in a different network partition than the master <cluster-name> in <Datacenter/vCenter>.
    
    
• The /var/run/log/fdm.log from the primary host reports the following entries
         warning  fdm Sendto[ipv4] ###.###.###.###: Host is down
    verbose  fdm Waited 5 seconds for icmp ping reply for host-##
    verbose  fdm Checking for Partition
    error    fdm  [60 times] sendto ###.###.###.### failed: Host is down
    info     fdm  Host host-## changed state: Partitioned

Environment

VMware vSphere ESXi 7.x
VMware vSphere ESXi 8.x

Cause

If the primary host is unable to communicate directly with the agent on a secondary host—for instance, if the secondary host does not respond to ICMP pings—but the secondary host is still sending heartbeats via the datastore, the primary host considers the secondary host to be either network partitioned or isolated. In this scenario, no VM failover occurs. However, the primary host continues to monitor the secondary host and its virtual machines.

Reference - Network Partitions

Resolution

The alert is triggered due to connectivity issues between the primary and secondary hosts, which should be investigated at the physical network layer. Please refer to the following KB article to test connectivity between the hosts - Testing network connectivity with the ping command

Additional Information

Reference - Host Failure Types