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Since OVS is doing L2 forwarding, you should be fine setting the MTU to as
high as you choose, which would probably be the segment_mtu in the config,
since that's what it defines - the largest MTU that (from the Neutron API
perspective) is usable and (from the OVS perspective) will be used in the
system.  A 1500MTU Neutron network will work fine over a 9000MTU OVS switch.

What won't work is sending a 1500MTU network to a 9000MTU router port.  So
if you're doing any L3 (where the packet arrives at an interface, rather
than travels a segment) you need to consider those MTUs in light of the
Neutron network they're attached to.
-- 
Ian.

On 20 September 2017 at 09:58, Ihar Hrachyshka <ihrachys at redhat.com> wrote:

> 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:
> https://mail.openvswitch.org/pipermail/ovs-dev/2016-June/316733.html
> 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.
>
> Cheers,
> Ihar
>
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