[openstack-dev] Neutron and MTU advertisements -- post newton

Ian Wells ijw.ubuntu at cack.org.uk
Mon Jul 11 19:04:33 UTC 2016


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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