Ihar's e-mail summarizes things quite nicely. We have an ongoing topic of conversation[1] about MTU configuration in Neutron. It is my personal viewpoint that the current Neutron configuration for MTUs is not intuitive and can be improved. There are a number of options that you had to enable in the past, and we are attempting to change Neutron, in order for things to Just Work™ [1]: http://lists.openstack.org/pipermail/openstack-dev/2016-January/084241.html -- Sean M. Collins