[OVN] External subscribers to feed generated in Openstack fails if no internal VM is subscribed to that feed first

[Diko Parvanov]

Commit d02fb560aa35c4931d40000e8ca4c0e4105299b7 (1)(2) to Neutron disabled 'mcast_flood' on localnet ports, in order to avoid multicast packet duplication. This introduces a regression because outbound multicast traffic from VMs is no longer sent to the provider network unless it is part of a multicast group that OVN has snooped. This makes it impossible for something external to OpenStack to subscribe to a multicast group running on a VM (in OpenStack) unless another VM (in OpenStack) subscribes to it first (because the multicast traffic never leaves the OVS).

In my opinion https://review.opendev.org/c/openstack/neutron/+/797418 has to be reverted and a change implemented in ovn itself.

1 https://review.opendev.org/c/openstack/neutron/+/797418
2 https://bugs.launchpad.net/neutron/+bug/1933207

[Diko Parvanov (dparv)]

Opened issue on ovn to track progress: https://github.com/ovn-org/ovn/issues/125

[Diko Parvanov (dparv)]

A quick workaround for this is to subscribe the VM generating/sending the traffic from within the cloud to the IGMP group(s) it's streaming to.

[Lucas Alvares Gomes (lucasagomes)]

This is a bug in core OVN, it has been discussed at: https://github.com/ovn-org/ovn/issues/126#issuecomment-1092776871. Keeping this LP open in case we need to change some of the options in ML2/OVN