[ovs-discuss] MTU/fragmentation issue in 2.0.1/openstack icehouse using GRE

[Tom Christensen]

> From: jesse at nicira.com
> 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: pavera at live.com
> CC: discuss at openvswitch.org
> 
> On Wed, May 14, 2014 at 8:18 PM, Tom Christensen <pavera at live.com> 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.

 		 	   		  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://openvswitch.org/pipermail/ovs-discuss/attachments/20140515/adb62afc/attachment-0002.html>