GRE tunneling is broken if hosts are on multiple subnets (multi-homed)

[Adin Scannell]

Basic setup:
* Bunch of hosts on subnet X
* Host on subnet X and subnet Y (controller)
* Bunch of hosts on subnet Y

If local_ip for controller is from subnet X, then GRE tunnels are broken from controller to subnet Y.

-- more detail --

Because you can only specify a single local_ip when using GRE tunneling in openvswitch and this information is propagated to all hosts regardless of their subnet -- allowing GRE to choose the local_ip for tunnels results in one-directional flows because the IP won't be recognized (as one or more hosts may be sending traffic on an IP that is not their recognized local_ip).

There is a pretty straight-forward fix -- the local_ip should be specified for all GRE tunnels, that way all traffic will originate from the IP that the hosts in the cluster are aware of. The local_ip needs to be routable from all hosts, but this is no different than before. There are more complex ways of dealing with this problem, but I think that this is the right fix and keeps it simple.

I will be submitting a fix shortly via Gerrit.

[Adin Scannell (amscanne)]

Patch submitted:
https://review.openstack.org/#/c/30637/

[Adin Scannell (amscanne)]

I've updated the commit message and title to indicate that this specifically addresses the problem of having some multi-homed hosts.

[Kyle Mestery (mestery)]

This bug was addressed by this review [1] and was merged in Havana. Marking this as "Fix Committed" since for some reason this didn't happen automatically.

[1] https://review.openstack.org/#/c/30637/