vMotion Compatibility Warning - MTU Configuration Mismatch

Products

• VMware vSphere ESXi

Issue/Introduction

When attempting to migrate a Virtual Machine (VM) between ESXi hosts using vMotion, the following compatibility warning is displayed in the vSphere Client:

"The source and destination hosts have different vMotion vNIC MTU configurations."

While the migration may sometimes proceed, it often fails or experiences significant performance degradation and network timeouts.

Environment

VMware vSphere ESXi

Cause

This issue occurs when the Maximum Transmission Unit (MTU) settings are inconsistent across the vMotion network path. Specifically:

• The newly added host is using the default MTU 1500 while existing hosts are configured for Jumbo Frames  9000

• The TCP/IP stack selection differs between the source and destination (e.g., using the "vMotion Stack" on one host and the "Default Stack" on another).

For vMotion to function reliably, the MTU must be uniform across the entire data path, including the VMkernel ports, virtual switches, and physical switch fabric.

Resolution

To resolve the mismatch, the VMkernel port on the inconsistent host must be reconfigured to match the cluster standard.

1. Identify the Mismatch

• Navigate to Host > Configure > Networking > VMkernel adapters.

• Compare the MTU and TCP/IP Stack columns for the vMotion adapter (usually vmk2) across all hosts in the cluster.

2. Reconfigure the VMkernel Port

If a mismatch is found on a specific host:

1. Place the host in Maintenance Mode.

2. Select the affected VMkernel adapter and click Delete.

3. Click Add Networking > VMkernel Network Adapter.

4. Select the existing Port Group used for vMotion.

5. In the Port properties settings:

   • Ensure the TCP/IP stack matches the rest of the cluster.

   • Enable the vMotion service.

6. In the IPv4 settings:

   • Enter the correct IP address and Subnet Mask.

   • Set the MTU to match the cluster (e.g., 9000 for Jumbo Frames).

7. Click Finish.

3. Validation

• Perform a test vMotion of a non-critical VM to the reconfigured host.

• Perform a return vMotion to the original host.

• Verify that the compatibility warning no longer appears.

Additional Information

 

• Uniformity: Always ensure MTU settings are consistent end-to-end. If using MTU 9000, ensure the physical switches are configured to support frames slightly larger (e.g., 9100 or 9216) to account for overhead.

• Provisioning: Dedicate at least one 1 GbE (minimum) or 10 GbE (recommended) physical NIC to vMotion traffic.

• Security: Keep vMotion traffic on a dedicated, isolated VLAN.

• Firewalls: Ensure Port 8000 is open bidirectionally between all ESXi hosts.
  
  

Reference KBs : 

vMotion compatibility warning: "The source and destination hosts have different vMotion vNlC MTU configurations. This could result in a lower network copy throughput