netdev - Re: What is the correct way to install an L2 multicast route into a bridge?

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Date:   Wed, 8 Jul 2020 12:42:00 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Cc:     roopa@...ulusnetworks.com, netdev@...r.kernel.org
Subject: Re: What is the correct way to install an L2 multicast route into a
 bridge?

On Wed, Jul 08, 2020 at 12:16:27PM +0300, Nikolay Aleksandrov wrote:
> On 08/07/2020 12:04, Vladimir Oltean wrote:
> > Hi,
> > 
> > I am confused after reading man/man8/bridge.8. I have a bridge br0 with
> > 4 interfaces (eth0 -> eth3), and I would like to install a rule such
> > that the non-IP multicast address of 09:00:70:00:00:00 is only forwarded
> > towards 3 of those ports, instead of being flooded.
> > The manual says that 'bridge mdb' is only for IP multicast, and implies
> > that 'bridge fdb append' (NLM_F_APPEND) is only used by vxlan. So, what
> > is the correct user interface for what I am trying to do?
> > 
> > Thank you,
> > -Vladimir
> > 
> 
> Hi Vladimir,
> The bridge currently doesn't support L2 multicast routes. The MDB interface needs to be extended
> for such support. Soon I'll post patches that move it to a new, entirely netlink attribute-
> based, structure so it can be extended easily for that, too. My change is motivated mainly by SSM
> but it will help with implementing this feature as well.
> In case you need it sooner, patches are always welcome! :)
> 
> Current MDB fixed-size structure can also be used for implementing L2 mcast routes, but it would
> require some workarounds. 
> 
> Cheers,
>  Nik
> 
> 

Thanks, Nikolay.
Isn't mdb_modify() already netlink-based? I think you're talking about
some changes to 'struct br_mdb_entry' which would be necessary. What
changes would be needed, do you know (both in the 'workaround' case as
well as in 'fully netlink')?

-Vladimir