[Openstack] MTU 1400? No thanks! - VXLAN users not affected!

Darragh O'Reilly dara2002-openstack at yahoo.com
Mon Oct 28 08:31:57 UTC 2013

Hi Thiago,

you don't say what version of OVS. It must be >=1.10.0 because you are using
VXLAN. From that version the way MTU and tunnels is handled has changed.

See "- Tunneling: " in http://openvswitch.org/releases/NEWS-1.10.0

Here is a tcpdump with with OVS 1.10.2  where all interfaces have an MTU of 

1500. GRE tunnels are used and there are no connectivity problems.

The messages with "IP truncated-ip - 18 bytes missing!" mean IP fragmentation 

is happening.

Re, Darragh.

> Guys,
> I am not detecting any problems related to "MTU = 1500" when using VXLAN!
> It is easy to reproduce the "GRE MTU problem", when using GRE tunnels with
> MTU = 1500, from a Instance, it is impossible to use RubyGems (Ubuntu 12.04
> Instance), for example, "gem install bundler" (neither "bundle install"
> your RoR App) doesn't work with MTU 1500 on top of a GRE tunnel, with MTU
> 1400 is fine.
> But, now with VXLAN, I'm using MTU 1500 again without any problems!
> Honestly, I don't know for sure if RubyGems changed too or, if it is
> related to VXLAN being a bit better than GRE, anyway, I'll keep my Havana
> cloud running with MTU 1500 from now on.
> Cheers!
> Thiago

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