> From: [email protected]
> Date: Thu, 15 May 2014 14:43:12 -0700
> Subject: Re: [ovs-discuss] MTU/fragmentation issue in 2.0.1/openstack
> icehouse using GRE
> To: [email protected]
> CC: [email protected]
>
> On Wed, May 14, 2014 at 8:18 PM, Tom Christensen <[email protected]> wrote:
> > I'm seeing an issue in openstack icehouse running on ubuntu 14.04, ovs
> > version 2.0.1, package version 2.0.1+git20140120-0ubuntu2 where gre packets
> > cannot pass between 2 ovs bridges (br-int and br-tun) when the original
> > source packet is larger than 1438 bytes (mtu set to 1500 everywhere). I've
> > confirmed that openstack havana running on ubuntu 12.04 (ovs 1.10.2) does
> > not have this same issue, so it feels like a regression. I don't know where
> > this should be reported, or if its been fixed in subsequent versions of
> > openvswitch.
> >
> > I also am pretty new to openvswitch so, any help in really nailing down
> > exactly what is going on inside these bridges would be appreciated greatly.
>
> Tunnel fragmentation isn't really supported well in OVS and even in
> cases where it does work it performs poorly. It is best if you either
> increase the MTU on the physical network or decrease it in the sending
> VMs.
To be clear, with the MTU set at 1400 in the VM, the size of packet that causes
the problem remains the same. If you ping -s 1430 <vm on different host> it
will work, the packet will be fragmented on the tap interface, and will reach
the other host and vm. If you ping -s 1431 <vm on different host> it will be
fragmented on the tap interface, but will not leave the source host, arrive at
the destination host, or arrive at the destination vm.
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