I believe we need to emit ICMP / need to frag messages to have proper support on different MTUs (on router sides), I wonder how does it work the other way around (when external net is 1500, and internal net is 1500-geneve overhead).
Is there any way to match packet_size > X on a flow? How could we implement this? On Wed, Jul 11, 2018 at 1:01 PM Daniel Alvarez Sanchez <dalva...@redhat.com> wrote: > On Wed, Jul 11, 2018 at 12:55 PM Daniel Alvarez Sanchez < > dalva...@redhat.com> wrote: > >> 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. >> > > s/1900/1500 > >> >> 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: >> > > Just to clarify, in the reference implementation (ML2/OVS) the routing > takes place with iptables rules so we assume that it's the kernel > processing those ICMP packets. > >> >> 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 >> >> > > _______________________________________________ > discuss mailing list > disc...@openvswitch.org > https://mail.openvswitch.org/mailman/listinfo/ovs-discuss >
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