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

Rick Jones rick.jones2 at hp.com
Mon Oct 28 15:33:43 UTC 2013

On 10/27/2013 11:59 AM, Martinx - ジェームズ wrote:
> 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.

I'm not certain, but if you dig a little - either a tcpdump trace in one 
of your instances, or a netperf -t TCP_STREAM -H <somewhere> -- -o 
transport_mss, that in your VXLAN case you may be automagically getting 
a TCP MSS that does not result in fragmentation/etc.


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