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