[Bridge] [PATCH] net: bridge: Allow bridge to joing multicast groups

Allan W. Nielsen allan.nielsen at microchip.com
Mon Jul 29 13:14:21 UTC 2019

The 07/29/2019 15:22, Nikolay Aleksandrov wrote:
> Yes, all of the multicast code is handled differently, it doesn't go through the fdb
> lookup or code at all. I don't see how you'll do a lookup in the fdb table with a
> multicast mac address, take a look at br_handle_frame_finish() and you'll notice
> that when a multicast dmac is detected then we use the bridge mcast code for lookups
> and forwarding.

Here is my thinking (needs much more elaboration, which will come if we do a
patch to test it out):

In br_pkt_type

Add a new type called BR_PKT_MULTICAST_L2

In br_handle_frame_finish

	if (is_multicast_ether_addr(dest)) {
		/* by definition the broadcast is also a multicast address */
		if (is_broadcast_ether_addr(dest)) {
			pkt_type = BR_PKT_BROADCAST;
			local_rcv = true;
		} else {
			pkt_type = BR_PKT_MULTICAST;
			if (br_multicast_rcv(br, p, skb, vid))
				goto drop;

Change the code above to detect if it is a BR_PKT_MULTICAST_IP or a

In this section:

switch (pkt_type) {

if (dst) {
} else {

Add awareness to the BR_PKT_MULTICAST_L2 type, and allow it do forwarding
according to the static entry if it is there.

> If you're trying to achieve Rx only on the bridge of these then
> why not just use Ido's tc suggestion or even the ip maddr add offload for each port ?
> If you add a multicast mac in the fdb (currently allowed, but has no effect) and you
> use dev_mc_add() as suggested that'd just be a hack to pass it down and it is already
> possible to achieve via other methods, no need to go through the bridge.

Well, I wanted the SW bridge implementation to behave the same with an without
HW offload.

And also, I believe that is conceptually belongs to the MAC tables.


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