[ovs-dev] Odd gre_sys MTU setting (2.6.x vs 2.8.x)

Eric Garver e at erig.me
Fri Jan 12 15:51:11 UTC 2018


On Fri, Jan 12, 2018 at 12:33:23PM +0000, James Page wrote:
> Hi
> 
> We're currently trying to debug an issue we're seeing when using OpenStack
> with OVS 2.8.1 (see [0]).
> 
> When using GRE tunnels, we're seeing broken/slow performance due to what
> appears to be an incorrect MTU setting on the gre_sys device; for previous
> OVS releases (2.6.1 specifically but I think this applies back to 2.5.x as
> well) this device was created with an MTU of 65490; however under 2.8.1 it
> gets created with an MTU of 1472, which when used with GRE networks with
> larger MTU settings causes problems with fragmentation and broken path MTU
> discovery.
> 
> I've tested under Linux 4.4 and 4.13 and see the problem with both kernels.
> 
> Its possible to workaround the issue by setting the MTU on the gre_sys
> device directly, but this change will get reset back to 1472 on a restart
> of openvswitch.
> 
> I know the mtu_request feature was introduced around 2.7.0 - groking the
> code I can't see where the MTU of this device is actually configured...
> 
> Any help much appreciated
> 
> James
> 
> [0] https://bugs.launchpad.net/ubuntu/+source/openvswitch/+bug/1742505

Most likely you are seeing this bug:

  https://bugzilla.redhat.com/show_bug.cgi?id=1488484

It's actually an issue with the kernel GRE driver ignoring IFLA_MTU when
the device is created.


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