vSphere HA not getting configured on an Image Based Cluster hosts with error: "vSphere HA agent cannot be installed or configured" and the HA status displays as "Unreachable"

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VMware vSphere ESXi

Symptoms :

Error in the UI: "vSphere HA agent cannot be installed or configured".
vSphere HA not getting configured on the hosts.
On each host in the cluster, you will find that the vSphere HA state is "Unreachable"
In the Update tab of the Image Based Cluster shows :
- Error : Lifecycle Manager is unable to reach the host.
On re-enabling vSphere-HA in cluster, the fdm vib do not get pushed to the ESXi hosts.
- Verify if vib is pushed to ESXi host > SSH to host > ls /tmp/
ESXi host will not resolve to IP / FQDN.
ICMP communication from vCenter Server to the ESXi host fails, while communication from the ESXi host to vCenter Server is successful.

The management vmkernel interface "vmk0" is disconnected or not reachable from vCenter Server or peer ESXi hosts.
vSphere HA configuration fails due to unidirectional network connectivity or DNS resolution issues between the ESXi hosts and the vCenter Server.
Firewall restrictions are blocking vSphere HA communication — required management network ports (TCP/UDP 8182, 8183, 8042, etc.) are not accessible.
Network segmentation or gateway configuration issues are preventing communication between ESXi hosts located on different subnets.
Internal port 53 (DNS) was blocked between the ESXi hosts and the DNS server

Verify DNS and Network Connectivity:
- Check that the ESXi hosts can resolve the vCenter Server and other ESXi hosts via DNS.
- SSH into each ESXi host and run:
  - nslookup <vCenter_FQDN>
  - ping <vCenter_IP>
- If the ESXi hosts fail to resolve the vCenter Server or other hosts, confirm that the configured DNS servers are reachable.
Check Management Network (vmk0) Connectivity:
- Ensure the management vmkernel interface "vmk0" is up and reachable from vCenter Server and peer ESXi hosts.
- From vCenter Server, ping the ESXi host’s management IP. Connectivity must be bidirectional.
Validate Firewall and Port Accessibility:
- Confirm that the firewall is not blocking vSphere HA traffic. Required TCP/UDP ports include: 8182, 8183, 8042, and 53 for DNS.
Check Network Segmentation:
- Verify that ESXi hosts are on the same subnet or that routing/gateway configuration allows communication across subnets.
Enable vSphere HA:
- After confirming DNS, network connectivity, and firewall rules, re-enable vSphere HA on the cluster.
- SSH into an ESXi host and verify that the FDM VIB has been pushed to /tmp/ and installed.
Verify HA Status:
- In vCenter, check that each host’s HA state changes from “Unreachable” to “Connected/Enabled.”
- Ensure no errors are reported in the cluster Update tab or the vSphere HA events.

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