Unable to shutdown vSAN due to prechecks failing MTU check

Products

• VMware vSAN

Issue/Introduction

When the vSAN Shutdown Wizard runs its prechecks, it tries to ensure seamless communication on your vSAN network. Part of this involves sending large network packets of MTU size 8972 (Jumbo Frames) between all vSAN hosts.

 

Environment

vSAN 8.x

vSAN 7.x

Cause

The precheck fails because of an MTU mismatch. That is the vSAN VMkernel adapters have an MTU 9000 set, but the physical network switches or even other vSAN hosts are still set to 1500 MTU. When those large packets hit a device that can't handle them, they get dropped.

Resolution

To clear the MTU precheck, ensure consistent MTU settings across the entire vSAN network:

1. Check the MTU on all  vSAN VMkernel and vMotion VMkernel adapters and all the physical network switches in the respective data paths.

2. Align MTU Values:

   • If using Jumbo Frames, make sure every single device involved is configured for large MTU (e.g., 9000 bytes).

   • If not using Jumbo Frames, confirm all components are at the default 1500 bytes.

3. Test It: Use the vmkping command from an ESXi host, specifying a large packet size like so: vmkping -I vmk<x> -d -s 8972 <vSAN VMK adapter IP>

See vSAN Health Service - Network Health - Hosts small ping test (connectivity check) and Hosts large ping test (MTU check) for additional testing details.

Additional Information

If the MTU precheck failure cannot be determined immediately, but the vSAN cluster still needs to be shutdown, an alternative is to Manually Shut Down and Restart the vSAN Cluster.