l3 neighbors on bridges are not properly following fdb updates #482
Description
Currently we don't correctly handle fdb updates for l3 neighbors behind them, but we need the fdb entries to lookup the l3 neighbors/nexthops' actual ports.
The following cases are currently unhandled:
- l3 neighbors may live longer than their fdb entries. We should treat a fdb entry timing out as a l3 neighbor becoming unreachable (and once we learn a matching entry, treat it as reachable again).
- fdb entries may move from one port to another. In this case we need to update the l3 interface's port. These updates do not trigger a l3 neigh update from the kernel, so we need to pass that info on to the l3 module.
- In rare cases, we may learn a l3 neighbor before the fdb entry. If this happens fdb lookup fails, and we crash and burn.
For this to work, we need to have a notification system similar to what we have for nexthops, so that the bridge code can notify about them.