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RFC4541, section 2.1.2 says: Packets with a destination IP (DIP) address in the 224.0.0.X range which are not IGMP must be forwarded on all ports. And section 3 says: In IPv6, the data forwarding rules are more straight forward because MLD is mandated for addresses with scope 2 (link-scope) or greater. The only exception is the address FF02::1 which is the all hosts link-scope address for which MLD messages are never sent. Packets with the all hosts link-scope address should be forwarded on all ports. However, currently when a listener on FF12::1 or FF12:🔢0:1 for example joins then not only packets to these addresses but also for FF02::1 won't be flooded to all ports anymore, too. Which violates RFC4541. This happens because A): They all map to the same ethernet multicast address, that is 33:33:00:00:00:01. And B) the VLAN profile L2 unknown MC flood setting will only apply flooding of 33:33:00:00:00:01 if there is no specific listener registered for it. So to fix this, avoid registering an MDB entry in the switch for 33:33:00:00:00:01 at all. The downside of this is that FF12::1, FF12:🔢0:1 etc. will always be flooded, too. However fixing the handling of 224.0.0.X and FF02::1 and adhering to RFC4541 must have priority to avoid undesired packetloss, to avoid breaking IPv4/IPv6. Tested-on: ZyXEL GS1900-24HP v1 Fixes: cde31976e375 ("realtek: Add support for Layer 2 Multicast") Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue> Link: https://github.com/openwrt/openwrt/pull/18769 Signed-off-by: Robert Marko <robimarko@gmail.com>