[openstack-dev] [neutron] MTU native ovs firewall driver

Ihar Hrachyshka ihrachys at redhat.com
Wed Sep 20 16:58:30 UTC 2017

On Wed, Sep 20, 2017 at 9:33 AM, Ajay Kalambur (akalambu)
<akalambu at cisco.com> wrote:
> So I was forced to explicitly set the MTU on br-int
> ovs-vsctl set int br-int mtu_request=9000
> Without this the tap device added to br-int would get MTU 1500
> Would this be something the ovs l2 agent can handle since it creates the bridge?

Yes, I guess we could do that if it fixes your problem. The issue
stems from the fact that we use a single bridge for different networks
with different MTUs, and it does break some assumptions kernel folks
make about a switch (that all attached ports steer traffic in the same
l2 domain, which is not the case because of flows we set). You may
want to report a bug against Neutron and we can then see how to handle
that. I will probably not be as simple as setting the value to 9000
because different networks have different MTUs, and plugging those
mixed ports in the same bridge may trigger MTU updates on unrelated
tap devices. We will need to test how kernel behaves then.

Also, you may be interested in reviewing an old openvswitch-dev@
thread that I once started here:
Sadly, I never followed up with a test scenario that wouldn't involve
OpenStack, for OVS folks to follow up on, so it never moved anywhere.


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