Avoiding bridge flood of multicast when L2 switch is used

Avoiding bridge flood of multicast when L2 switch is used

[Murali Karicheri]

netdev experts,

When there is a L2 switch underneath the network interface a flooding of
multicast at the bridge interface results in one copy of this going through each
of the slave resulting in multiple multicast packets reaching the underlying switch.
L2 switch also duplicates the multicast packet on each egress port as done
in software bridge. This is undesirable as it reduces the available bandwidth to
half when multicast packets are involved.

Is there something that can be done to avoid software bridge flooding mulicast
through a ip command or similar or to be done in the network driver? I have a TODO
item to add dsa support in our netcp driver. If this is already solved by dsa, I can
handle it as part of this work. 

Thanks in advance for your response! 
-- 
Murali Karicheri
Linux Kernel, Keystone

Re: Avoiding bridge flood of multicast when L2 switch is used

[Andrew Lunn]

> netdev experts,
> 
> When there is a L2 switch underneath the network interface a flooding of
> multicast at the bridge interface results in one copy of this going through each
> of the slave resulting in multiple multicast packets reaching the underlying switch.

Yes, this is something Florian, Vivien and i have discussed. We don't
yet see a way around it. Neither the Marvell or Broadcast switches
have a mechanism to say: Send this frame out of this list of
interfaces. All we can do is send it out a single interface. Even if
we could direct a single frame out multiple interfaces, we still have
the problem of how to teach the bridge about this, in a clean and
elegant fashion.

Overall, this seems like a hard problem. Since it is a hard problem,
implementing IGMP snooping first would make sense. Don't forward the
packet out a port unless you know there is somebody interested in it.
The swichdev parts of IGMP snooping are in place, we just need to
continue them down into DSA.

> L2 switch also duplicates the multicast packet on each egress port as done
> in software bridge. This is undesirable as it reduces the available bandwidth to
> half when multicast packets are involved.

Once you have a DSA driver, the switch will not do this replication.
So you will gain something there.

     Andrew