vSphere HA agent on host cannot reach isolation address in vSAN clusters

Products

• VMware vCenter Server
• VMware vSphere ESXi

Issue/Introduction

In a VMware vSAN environment, a host may report that the vSphere HA agent cannot reach the configured isolation address. This occurs even when network connectivity between the host and the gateway appears functional.

• Error message: vSphere HA agent on this host could not reach isolation address.

• HA state for the host shows as Unreachable or Configuration Pending.

• Manual HA reconfiguration tasks may initially fail or hang.

Environment

VMware  vCenter

Cause

This behavior can occur if there is a mismatch or timeout during the HA agent election process, or if the isolation address is incorrectly resolving to an IPv6 address in an IPv4-only vSAN environment. When the HA configuration task is in a "Pending" state, the agent may report as unreachable until the task successfully completes and the election process is finalized.

Resolution

To resolve this issue, perform a manual reconfiguration of the HA agent and verify the network settings:

1. Verify Connectivity: Log in to the affected ESXi host via CLI and verify you can ping the isolation address (typically the vSAN gateway): vmkping -I vmkX <Gateway_IP>

2. Check Isolation Address: Ensure that the das.isolationaddress advanced setting is set to a valid, pingable IPv4 address if your cluster is utilizing IPv4.

3. Reconfigure HA:

   • Right-click the affected host in the vSphere Client.

   • Select VMware HA > Reconfigure for vSphere HA.

4. Monitor Task Status: Wait for the "Pending" task in the vCenter task console to complete. In vSAN environments, this may take several minutes as the FDM agent synchronizes with the vSAN object metadata.

Additional Information

VMware High Availability cluster reports the error: "vSphere HA agent on this host could not reach isolation address <IP Address>"