[openstack-dev] [neutron] What semantics are expected when booting a VM on an external network?

Neil Jerram Neil.Jerram at metaswitch.com
Thu Sep 17 13:58:29 UTC 2015

Thanks to the interesting 'default network model' thread, I now know
that Neutron allows booting a VM on an external network. :-)  I didn't
realize that before!

So, I'm now wondering what connectivity semantics are expected (or even
specified!) for such VMs, and whether they're the same as - or very
similar to - the 'routed' networking semantics I've described at [1].


Specifically I wonder if VM's attached to an external network expect any
particular L2 characteristics, such as being able to L2 broadcast to
each other?

By way of context - i.e. why am I asking this?...   The
networking-calico project [2] provides an implementation of the 'routed'
semantics at [1], but only if one suspends belief in some of the Neutron
semantics associated with non-external networks, such as needing a
virtual router to provide connectivity to the outside world.  (Because
networking-calico provides that external connectivity without any
virtual router.)  Therefore we believe that we need to propose some
enhancement of the Neutron API and data model, so that Neutron can
describe 'routed' semantics as well as all the traditional ones.  But,
if what we are doing is semantically equivalent to 'attaching to an
external network', perhaps no such enhancement is needed...

[2] https://git.openstack.org/cgit/openstack/networking-calico

Many thanks for any input!


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