Learn how to fix the “Cannot Find vSphere HA Master Agent” error in vSphere. Our VMware Support team is here to help you with your questions and concerns.
“Cannot Find vSphere HA Master Agent” in vSphere: Causes and Fixes
The vSphere High Availability (HA) plays a key role in managing a vSphere environment. It helps minimize downtime and ensures VM availability during host failures. However, we may run into the error “Cannot find vSphere HA master agent”. This can leave the infrastructure vulnerable.
Today, we will break down what this error means, its implications, and how to troubleshoot and fix it.
An Overview:
What Does“Cannot Find vSphere HA Master Agent” Error Mean?
This error indicates that vCenter Server cannot communicate with the ESXi hosts in the cluster through the Fault Domain Manager (FDM) agent. As a result, vSphere HA cannot function. In other words, the environment loses its ability to automatically restart virtual machines (VMs) if a host fails.
Here is the error syntax:
Cannot find vSphere HA master agent
This error will have the following impacts:
- VMs won’t restart on other hosts in case of a failure.
- Hosts may show an uninitialized or error state.
- HA agent errors during cluster upgrades.
Common Causes & Fixes
1. Network Connectivity Issues
Kernel management ports may be blocked by firewalls, located on different subnets without gateways, or experiencing poor routing.
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- – Ensure all ESXi hosts are on the same subnet or are properly routed.
- Open ports 443 and 902 between all hosts and the vCenter.
- Use `vmkping` to verify communication.
- Verify physical switch configurations, VLAN settings, and ensure port-channel mode is enabled where necessary.
If you’re also troubleshooting ARP-related networking issues, this guide on VMware ARP cache issues can help you understand another possible layer of network disruption.
2. VSAN Misconfiguration
In VSAN clusters, the Virtual SAN traffic service might not be enabled on vmkernel ports.
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- First, go to Configure > Networking > VMkernel adapters for each host.
- Then, enable the checkbox for Virtual SAN traffic.
- Ensure all nodes in the cluster are configured consistently.
3. Heartbeat Failures
The master host is unable to detect heartbeat signals from other hosts.
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- Enable Host Monitoring in the HA settings.
- Check that all hosts have access to heartbeat datastores.
- Also, check for network congestion or outages.
In some environments, services like SLP can cause unexpected behaviors. Learn how to disable or enable the SLP service on VMware ESXi to eliminate that as a potential factor.
4. Agent Unreachable
vCenter Server can’t reach the HA agent on a host.
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5. HA Configuration Issues
Incorrect HA settings may prevent agents from initializing.
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- Disable and re-enable HA on the cluster by going to Configure > vSphere Availability > Edit.
- Review HA settings and apply again.
Improper setups can originate from the initial deployment. If you’re managing hosted environments, this guide on installing VMware ESXi on OVH offers insights into a clean install process.
6. Incompatible or Faulty Memory
Rarely, new memory hardware or a bad installation can interfere with HA.
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- Check the hardware documentation.
- Ensure memory modules are properly installed and compatible.
- Run diagnostics to check for hardware faults.
In rare cases, hardware vulnerabilities like unmitigated hyperthreading may impact HA and VM behavior. This post on ESXi hyperthreading unmitigated issues discusses that risk in more depth.
Prevention Tips
- Confirm all ESXi hosts have open lines of communication with vCenter and each other.
- Monitor HA heartbeat statuses in vCenter.
- Always apply the latest updates to vSphere, ESXi, and cluster components.
- Ensure VSAN services are properly enabled.
- Simulate failures to verify that HA is working as expected.
- Regularly inspect vCenter and host logs for early signs of HA issues.
[Need assistance with a different issue? Our team is available 24/7.]
Conclusion
The “Cannot find vSphere HA master agent” error can be resolved with the right troubleshooting approach. Keep the environment healthy by monitoring communication paths, ensuring consistent configurations, and testing HA regularly.
In brief, our Support Experts demonstrated how to fix the “Cannot Find vSphere HA Master Agent” error in vSphere
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