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The “VMware cannot synchronize host” error is a common hurdle for vSphere administrators. When this alert appears in the vSphere Client, it indicates a communication breakdown between the vCenter Server and the ESXi host. While the VMs on the host usually continue to run without interruption, the management plane is compromised, preventing you from performing tasks like vMotion, configuration changes, or power operations.
Fixing this error requires identifying where the communication chain has broken. This guide provides a proven workflow to resolve vCenter cannot synchronize host failures by troubleshooting service hangs, Port 902 connectivity, and time synchronization issues.
Why VMware Cannot Synchronize Host
The “Cannot synchronize host” error appears when vCenter cannot reliably verify an ESXi host’s management state, even if the host is still reachable.
This typically happens due to:
- Management agent communication failure– vCenter depends on ESXi management agents (vpxa and hostd) to collect host status. If these agents stop responding or lose connectivity, synchronization fails.
- Hung or overloaded management services– Storage latency, resource exhaustion, or faulty CIM providers can cause management services to hang, leaving the host reachable but unmanaged.
- Heartbeat or state validation issues– vCenter expects regular heartbeat signals and successful state checks. If heartbeats are inconsistent or configuration verification fails, the host cannot be synchronized.
- Time drift or SSL certificate problems– Incorrect NTP settings or significant clock drift can break SSL trust between vCenter and ESXi, causing synchronization tasks to be rejected.
In essence, vCenter cannot synchronize the host because it no longer trusts the host’s reported state.
Quick Fixes: VMware Cannot Synchronize Host
When a VMware cannot synchronize host error occurs, the priority is to restore management connectivity between vCenter and the ESXi host without impacting running workloads. The following actions are safe in production and resolve most common cases.
Manual Connection Refresh
Temporary network interruptions or stale authentication sessions can prevent vCenter from synchronizing a host.
A. Disconnect and Reconnect
- In the vSphere Client, navigate to Hosts and Clusters.
- Right-click the affected ESXi host → Connection → Disconnect.
- Wait ~30 seconds, then select Connection → Connect.
B. Re-authenticate Host Credentials (if required)
- Right-click the host → Connection → Edit Connection.
- Verify the IP/FQDN and re-enter the root credentials.
- Accept the SSL thumbprint if prompted.
Restart ESXi Management Services
If the host remains out of sync, restart the ESXi management agents (hostd and vpxa).
This operation does not reboot the host and does not affect running virtual machines.
A. Via DCUI(Direct Console User Interface)
- Log in to the host console (F2).
- Go to Troubleshooting Options → Restart Management Agents.
- Confirm and wait for completion.
B. Via SSH
localcli system management agents restart
(Alternatively, restart hostd and vpxa individually.)
Wait 2 – 3 minutes for the services to reinitialize, then verify the host status in vCenter.
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Resolving Time Sync and Credential Issues
If restarting management agents does not fix the VMware cannot synchronize host error, the root cause is often time drift or credential desynchronization. vSphere depends on accurate timestamps and valid credentials to maintain secure host communication.
Host and vCenter Time Synchronization (NTP)
Time drift is a leading cause of an ESXi host out of sync status. If the clock on an ESXi host deviates from the vCenter Server by more than five minutes, SSL handshakes fail. This prevents vCenter from validating the host’s certificate.
How to verify and fix:
- Check the host clock: Connect via SSH and run
esxcli system time get - Set shared NTP sources: Ensure both vCenter and your ESXi hosts point to the same reliable NTP servers.
- Configure the NTP Service: In the vSphere Client, go to Configure > System > Time Configuration and ensure the service is set to “Start and stop with host.”
Addressing VMware Synchronize Guest Time with Host Settings
Host time health directly impacts your workloads. If the VMware synchronize guest time with host option is enabled in a VM’s settings, a drifting host clock will push that incorrect time to the guest OS. This can cause application errors and Kerberos authentication failures within your VMs.
Fixing the vpxuser Password Mismatch
vCenter uses a unique account called vpxuser to manage ESXi hosts. If the password for this account becomes desynchronized, you will see a persistent cannot synchronize host in vCenter error.
To force a credential reset:
- Connect to the ESXi host via SSH.
- Remove the existing account:
esxcli system account remove --id vpxuser - In the vSphere Client, right-click the host and select Connection > Connect.
- vCenter will automatically recreate the account and push fresh management credentials.
Infrastructure-Level Fixes: Network, DNS, and NTP
A vCenter host synchronization issue often points to a breakdown in the communication path between the vCenter Server Appliance (VCSA) and the ESXi host.
Verifying Network Connectivity and Ports
vCenter requires specific ports to be open to maintain a “Synced” status. If firewalls block these, the host heartbeat is lost.
- Port 902 (UDP/TCP): Vital for host heartbeats. If UDP 902 is blocked, vCenter cannot verify if the host is alive.
- Port 443 (TCP): Used for management traffic and configuration synchronization.
Quick Check: From the VCSA command line, test connectivity using nc -zv <ESXi-IP> 443.
DNS Resolution and FQDN Accuracy
vSphere is highly dependent on DNS. Even if you can ping a host by IP, a mismatch in Forward or Reverse (PTR) lookups can cause a VMware cannot synchronize host error.
- Validate Lookups: Run
nslookupfor both the FQDN and the IP of the host from the vCenter shell. - Emergency Fix: If DNS is down, manually add the host’s mapping to the
/etc/hostsfile on the VCSA to restore management immediately.
Host and vCenter Time Synchronization (NTP)
Beyond the software configuration, ensure your network allows NTP traffic.
- Verify UDP 123: Ensure your network firewalls allow NTP traffic to reach your hosts.
- Check NTP Stats: Use
esxcli system ntp stats geton the host. High “jitter” values suggest network congestion is causing the host to fall out of sync.
Fixing these infrastructure pillars ensures that vCenter and ESXi maintain a stable, trusted connection, preventing future synchronization failures.
Advanced Troubleshooting: Certificates and Log Analysis
If standard fixes fail, VMware cannot synchronize host error, which usually points to a deeper security mismatch or a version incompatibility. At this stage, you must verify the vSphere trust certificates and system logs.
Host Certificate Management
vCenter uses SSL certificates to verify the identity of every ESXi host. If a certificate expires or the thumbprint changes unexpectedly, a vCenter host synchronization issue occurs.
How to renew host certificates:
- In the vSphere Client, select the affected host.
- Navigate to Configure > System > Certificates.
- Click Renew orRefresh CA Certificates.
- This forces vCenter to push a new certificate from the VMware Certificate Authority (VMCA), re-establishing the trusted link.
Version and Build Compatibility
A common cause of the “cannot synchronize host with vCenter” error is a version mismatch. According to the VMware interoperability matrix, vCenter must always be at a version equal to or higher than the ESXi hosts it manages.
If you recently patched an ESXi host to a newer build while the vCenter Server remains on an older update, synchronization will fail because vCenter cannot interpret the newer host’s APIs. Always upgrade your vCenter Server Appliance (VCSA) before patching your hosts
Log File Inspection for Synchronization Errors
When the vSphere Client doesn’t provide enough detail, the log files will reveal the specific failure point of the ESXi host out of sync alert.
A. vCenter Server Logs (vpxd):
- Path:
/var/log/vmware/vpxd/vpxd.log - What to look for: Search for the host’s IP and terms like “SSL Error” or “Handshake failed.” This confirms if the issue is at the network transport or security layer.
B. ESXi Host Logs (vpxa):
- Path:
/var/run/log/vpxa.log - What to look for: Search for “Login failed” or “Connection refused.” If you see “Memory limit exceeded,” the management agent is likely crashing and needs a service restart.
By identifying the specific error string in these logs, you can target the root cause of the VMware cannot synchronize host failure rather than relying on trial-and-error.
Conclusion
Resolving a VMware cannot synchronize host error is a process of elimination that moves from the management agents to the network and security layers. By systematically checking the vpxa service, NTP synchronization, and Port 902 connectivity, you can restore management without impacting your running virtual machines.
Consistent synchronization is not just a management convenience—it is essential for operational stability and production-grade backups. If these steps do not clear the alert, your log files (vpxa.logand vpxd.log) will provide the specific handshake or resource error needed to finalize the resolution. For long-term stability, ensure your infrastructure services like DNS and NTP are robust, as they form the foundation of the vSphere management plane.
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