vSAN network alarm after cluster upgrade. - vSAN Healthcheck -- 'vSAN: MTU check (ping with large packet size) Fails

Products

• VMware vSAN

Issue/Introduction

After Cluster Upgrade vSAN network alarm 'vSAN: MTU check (ping with large packet size) is failing on the vSAN witness network. 

 

Environment

vSAN (all versions)

Cause

This Alert will be present when you have over 30 percent packet loss when pinging with large packets. 

For example, you can use the MTU of 1500 on the VMkernel adapter and the MTU 9000 on the physical switch. 
If the source VMkernel adapter has an MTU of 1500, it will fragment the 9000 Byte packet. 
The fragments then travel along the network to the other ESXi host where they are reassembled. 
As long as all network devices along the path use a higher or equal MTU, the test passes.

 

Resolution

Check the physical network to determine what is causing the fragmentation of large sized packets being dropped on the network. 

See KB vSAN Healthcheck -- vMotion: MTU check (ping with large packet size) for more details.