MTU Error on vSAN

[pedretti diego]

Hello,
I'm receiving the following error in vSAN Skyline Health: "vSAN: MTU check (ping with large packet size)", stating that every host cannot ping others with 8972 bytes via vmk3.

However, if I manually run vmkping from each host via SSH using the same packet size, it works correctly. So I can't really understand where the problem might be.

I'm using the latest vCenter 8 version, and I have set the MTU to 9000 bytes everywhere the documentation suggests.

Does anyone have any ideas on what might be causing this issue?

Thank you!

[mketech]

Ensure that you have all the hosts setup using vmk3 for vSAN traffic. Try this from all a host to see if it is a single host or all by using `vmkping -I vmk3 -d 10.1.52.5` where the IP is the address of vmk3 of the other hosts. Link with other troubleshooting / setup

[a_p_]

Can you confirm that your manual test has the "don't fragment" (-d) option set?

vmkping -I vmk3 -s 8972 -d IP-Address

André

[pedretti diego]

I'll write my solution here in case anyone else encounters this problem:

After André's and mketech's answers, I tried using the -d parameter that I had forgotten, and I found that I could ping other hosts with 8000 bytes, but not with 8972 bytes.

After some testing, I discovered that increasing the MTU on the switch side solved the problem. I have 9000 MTU set in VMware and 9216 bytes on the switch side.

I still don't fully understand the root cause. Traffic is not tagged, and the only feature enabled is LACP.

[a_p_]

Glad to read that you've solved the issue.

Please take a look at https://rickardnobel.se/troubleshoot-jumbo-frames-with-vmkping/ which may answer you questions about the MTU/packet sizes.

André