Recent versions of neutron have made significant changes to how MTUs
are calculated. This work appears to have been completed and it's had
knock-on effects for some installers [1] which have changed their
default MTUs accordingly. In general, it is not recommended that one
change the MTU of a network segment in-flight as it can result in
dropped packets (depending on the change). However, the combination of
a live upgrade and these recent neutron changes means this exact
thing is now happening in some deployments.
Make life a little easier for the users who see these issues by
configuring as much of the network "plumbing" as possible to use the
the latest MTU whenever we plug interfaces. This won't necessarily
resolve all packet losses immediately - the guest will have to wait for
the new MTU to be propogated with a new DHCP lease or have it set
manually by a user - but it will make the issue eventually solvable.
[1] https://bugs.launchpad.net/tripleo/+bug/1590101
Change-Id: If09eda334cddc74910dda7a4fb498b7987714be3
Closes-bug: #1649845
01da454fc8