Comment 6 for bug 1504265

OK, in Kilo, we have path_mtu option in ml2 plugin that allows to calculate network MTU based on tenant network type. Please try to set it to your physical infrastructure MTU (1500), recreate networks (or update network table mtu fields with proper values based on tunneling overhead, f.e. 1450 for vxlan networks; sadly, there is no way to reset those MTU values thru API) , and retry.

When retrying, please also configure your dhcp agent to have advertise_mtu = True. In that case, the service will attach DHCP option with proper MTU to all allocations, and your instances will then use the value for their network interfaces.

Note that DHCP agent feature does not work for networks with no DHCP enabled (specifically, stateless IPv6 networks are still affected).

You can obviously use some other mechanism to make sure your instances use the proper MTU (f.e. modifying running images to run a script for that).

You may also want to look at setting network_device_mtu for L3 and DHCP agents to 1450.

Note that in Mitaka, we try to fix this MTU mess in several ways. F.e. we already enabled advertise_mtu = True by default, we set path_mtu = 1500 too; we added a feature to propagate MTU values using RA packets sent for IPv6 networks.

We also work on other fixes to allow for more smooth MTU setup.