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> On 11 Jul 2016, at 15:52, Sam Yaple <samuel at yaple.net> wrote:
> 
> After lots of fun on IRC I have given up this battle. I am giving up quickly because frickler has purposed a workaround (or better solution depending on who you ask). So for all of you keeping track at home, if you want your vxlan and your vlan networks to have the same MTU, here are the relevant options to set as of Mitaka.
> 
> [DEFAULT]
> global_physnet_mtu = 1550
> [ml2]
> path_mtu = 1550
> physical_network_mtus = physnet1:1500

I believe your request is corner case, and as long as we have a way to solve the issue for you, we should be ok.

Note that if you use IPv6 endpoints, you need to use a higher value for path_mtu:

https://review.openstack.org/#/c/320121/

> 
> This should go without saying, but i'll say it anyway: Your underlying network interface must be at least 1550 MTU for the above config to result in all instances receiving 1500 mtu regardless of network type. If you want some extra IRC reading, there was a more extensive conversation about this [1]. Good luck, you'll need it.
> 
> [1] http://eavesdrop.openstack.org/irclogs/%23openstack-neutron/%23openstack-neutron.2016-07-11.log.html#t2016-07-11T13:39:45
> 
> Sam Yaple
> 
> On Mon, Jul 11, 2016 at 7:22 PM, Fox, Kevin M <Kevin.Fox at pnnl.gov> wrote:
> I fought for two weeks to figure out why one of my clouds didn't seem to want to work properly. It was in fact one of those helpful souls you mention below filtering out PMTU's. I had to play with some rather gnarly iptables rules to workaround the issue. -j TCPMSS --clamp-mss-to-pmtu....
> 
> So it does happen.
> 
> Thanks,
> Kevin
> 
> From: Ian Wells [ijw.ubuntu at cack.org.uk]
> Sent: Monday, July 11, 2016 12:04 PM
> To: OpenStack Development Mailing List (not for usage questions)
> Subject: Re: [openstack-dev] Neutron and MTU advertisements -- post newton
> 
> On 11 July 2016 at 11:49, Sean M. Collins <sean at coreitpro.com> wrote:
> Sam Yaple wrote:
> > In this situation, since you are mapping real-ips and the real world runs
> > on 1500 mtu
> 
> Don't be so certain about that assumption. The Internet is a very big
> and diverse place....
> 
> OK, I'll contradict myself now - the original question wasn't L2 transit.  Never mind.
> 
> That 'inter' bit is actually rather important.  MTU applies to a layer 2 domain, and routing is designed such that the MTUs on the two ports of a router are irrelevant to each other.  What the world does has no bearing on the MTU I want on my L2 domain, and so Sam's point - 'I must choose the MTU other people use' - is simply invalid.  You might reasonably want your Neutron router to have an external MTU of 1500, though, to do what he asks in the face of some thoughtful soul filtering out PMTU exceeded messages.  I still think it comes back to the same thing as I suggested in my other mail.
> 
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