Dynamically change MTU

SUSE Product Topics SLES Networking
1

Hi,
For testing purposes I need to change mtu of interface multiple times on the fly using either ip or ifconfig. I am experiencing a delay in the used value, meaning:

  1. Set mtu to 3000 on interface
  2. Run ‘ping <client_ip> -s 8700 -M do’ which will fail (expected) with:

From 3.0.0.14 icmp_seq=1 Frag needed and DF set (mtu = 3000) From 3.0.0.14 icmp_seq=1 Frag needed and DF set (mtu = 3000) From 3.0.0.14 icmp_seq=1 Frag needed and DF set (mtu = 3000)
3. Set mtu to 9000 on same interface
4. Run ‘ping <client_ip> -s 8700 -M do’ which will fail again (unexpected) with same output as above:

From 3.0.0.14 icmp_seq=1 Frag needed and DF set (mtu = [COLOR="#FF0000"]3000[/COLOR]) From 3.0.0.14 icmp_seq=1 Frag needed and DF set (mtu = [COLOR="#FF0000"]3000[/COLOR]) From 3.0.0.14 icmp_seq=1 Frag needed and DF set (mtu = [COLOR="#FF0000"]3000[/COLOR])
After some time (~5 or more minutes) the ping will fix itself and start passing. My question: is there some sort of cache for mtu value in the system? Can it be cleared?

Thanks

2

mac84,

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3

Hi mac84,

I just wanted to let you know that I was able to re-create the symptoms you’re facing, so you’re not chasing ghosts. Unfortunately, neither do I yet fully understand the root cause nor do I have a work-around, that’s why I didn’t respond so far…

Regards,
Jens