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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<mailto: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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