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On 11 July 2016 at 12:52, 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
>
> 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
>

To be fair, what was said on there doesn't apply to this fix (which I
hadn't seen - I think it was in the channel before I joined).

You were suggesting that you should still be able to disable MTU
advertisement - which, as I pointed out, wouldn't do what you wanted, as it
would leave the ports used for DHCP, routing and metatdata servers all with
different IPs to your instances in some cases, and likely this would break
your system in exciting and unpredictable ways.

The fix proposed does *not* disable MTU advertisement, however.  It will
make Neutron calculate a 1500 MTU for VLAN and VXLAN networks, both - which
it will advertise, but that's effectively a no-op and I imagine you don't
really care.  The more important thing, though, is that Neutron now
understands that the MTUs are 1500 and it will apply that properly to its
service ports.  The solution above makes good sense and is much better than
disabling advertisement - go with it.
-- 
Ian.
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