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 > >
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