Just curious, what is the nic model ? I was already doing mtu 9000 15years ago with gigabit intel nic, I have never see nics with mtu limitations under 9000.It's an older network card. It can't do more.
>>but how can I hide this warning for the link 2 on node 2?
you need to set mtu in /etc/network/interfaces to lowest value on all nodes