Hi all, Miguel Angel Ajo and I have been trying to setup Jumbo frames in OpenStack using OVN as a backend.
The external network has an MTU of 1900 while we have created two tenant networks (Logical Switches) with an MTU of 8942. When pinging from one instance in one of the networks to the other instance on the other network, the routing takes place locally and everything is fine. We can ping with -s 3000 and with tcpdump we verify that the packets are not fragmented at all. However, when trying to reach the external network, we see that the packets are not tried to be fragmented and the traffic doesn't go through. In the ML2/OVS case (reference implementation for OpenStack networking), this works as we're seeing the following when attempting to reach a network with a lower MTU: 10:38:03.807695 IP 192.168.20.14 > dell-virt-lab-01.mgmt.com: ICMP echo request, id 30977, seq 0, length 3008 10:38:03.807723 IP overcloud-controller-0 > 192.168.20.14: ICMP dell-virt-lab-01.mgmt.com unreachable - need to frag (mtu 1500), length 556 As you can see, the router (overcloud-controller-0) is responding to the instance with an ICMP need to frag and after this, subsequent packets are going fragmented (while replies are not): 0:38:34.630437 IP 192.168.20.14 > dell-virt-lab-01.mgmt.com: ICMP echo request, id 31233, seq 0, length 1480 10:38:34.630458 IP 192.168.20.14 > dell-virt-lab-01.mgmt.com: icmp 10:38:34.630462 IP 192.168.20.14 > dell-virt-lab-01.mgmt.com: icmp 10:38:34.631334 IP dell-virt-lab-01.mgmt.com > 192.168.20.14: ICMP echo reply, id 31233, seq 0, length 3008 Are we missing some configuration or we lack support for this in OVN? Any pointers are highly appreciated :) Thanks a lot. Daniel Alvarez
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