[ovs-discuss] OVN - MTU path discovery

[Daniel Alvarez Sanchez]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openvswitch.org/pipermail/ovs-discuss/attachments/20180711/84ed2990/attachment-0001.html>