Troubleshooting vSphere HA Agent in Unreachable State

Products

• VMware vCenter Server

Issue/Introduction

The vSphere HA agent on a host may enter  "Agent Unreachable" state for a minute or longer. This status occurs when the primary host or vCenter Server is unable to communicate with the HA agent running on the affected host.

When an agent is unreachable, vSphere HA cannot effectively monitor the virtual machines on that host and may fail to restart those VMs automatically after a host failure. This can lead to increased downtime and impact the resiliency of your virtual infrastructure.

 

• Ha agent unresponsive

Environment

• VMware vCenter Server
• ESXi 7.x
• ESXi 8.x

Cause

The Agent Unreachable status often points to communication issues between the vSphere HA agent and the primary host or vCenter Server. The common causes for this state include:

• Networking Problems: Network connectivity issues such as misconfigured VLANs, firewall rules blocking HA traffic, physical network failures, or network partitioning can prevent communication between the HA agent and vCenter Server or the primary host.

• Cluster-Wide Failures: If all hosts in the cluster are experiencing issues, the HA agent may become unreachable due to widespread failure or misconfiguration.

• HA Reactivation While Disconnected: If vSphere HA was deactivated and reactivated on the cluster while the vCenter Server lost communication with the host agent, it can cause the agent to become unreachable.

• ESXi Host Agent Failure: In rare cases, the ESXi host agent managing the HA process may fail, and the internal watchdog process is unable to restart it, causing loss of communication.

It is important to note that when a host enters the Agent Unreachable state, vSphere HA does not trigger a failover event, meaning virtual machines on that host may not be protected during this period.

 

Resolution

To troubleshoot and resolve "Agent Unreachable state", follow these steps:

1. Assess Host Responsiveness via vCenter Server:

   • Check if the affected ESXi host is reported as Not Responding by vCenter Server.

   • If the host is unresponsive, it often indicates an underlying networking issue, an ESXi host agent failure. Investigate network connectivity, validate host health, and check for physical or virtual network misconfigurations.

   • Resolve any identified network or host-level issues. Once connectivity is restored, vSphere HA should resume normal operation.

2. Hosts Reported as Responding but Agent Remains Unreachable:

   • If vCenter Server shows the host as responding but the HA agent is still unreachable, the issue is likely isolated to the HA agent process on that host.

   • In this case, manually reconfigure vSphere HA on the affected host through your vSphere Client. This process will restart the HA agent and re-establish communication with the primary host.

3. Additional Recommendations:

   • Review network firewall and security policies to ensure required HA ports and protocols are allowed between hosts and vCenter Server.

   • Monitor the host logs (hostd, vpxa, and fdm logs) for errors that could point to agent failures or network disruptions.

Additional Information

Refer: vSphere HA Agent Is in the Agent Unreachable State