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

Nikolay Aleksandrov nikolay at cumulusnetworks.com
Mon Jul 29 12:50:18 UTC 2019

On 29/07/2019 15:22, Nikolay Aleksandrov wrote:
> Hi Allan,
> On 29/07/2019 15:14, Allan W. Nielsen wrote:
>> Hi Nikolay,
>> First of all, as mentioned further down in this thread, I realized that our
>> implementation of the multicast floodmasks does not align with the existing SW
>> implementation. We will change this, such that all multicast packets goes to the
>> SW bridge.
>> This changes things a bit, not that much.
>> I actually think you summarized the issue we have (after changing to multicast
>> flood-masks) right here:
>> The 07/26/2019 12:26, Nikolay Aleksandrov wrote:
>>>>> Actually you mentioned non-IP traffic, so the querier stuff is not a problem. This
>>>>> traffic will always be flooded by the bridge (and also a copy will be locally sent up).
>>>>> Thus only the flooding may need to be controlled.
>> This seems to be exactly what we need.
>> Assuming we have a SW bridge (br0) with 4 slave interfaces (eth0-3). We use this
>> on a network where we want to limit the flooding of frames with dmac
>> 01:21:6C:00:00:01 (which is non IP traffic) to eth0 and eth1.
>> One way of doing this could potentially be to support the following command:
>> bridge fdb add    01:21:6C:00:00:01 port eth0
>> bridge fdb append 01:21:6C:00:00:01 port eth1

And the fdbs become linked lists ? So we'll increase the complexity for something
that is already supported by ACLs (e.g. tc) and also bridge per-port multicast
flood flag ?

I'm sorry but that doesn't sound good to me for a case which is very rare and
there are existing ways to solve without incurring performance hits or increasing
code complexity.

>> On 25/07/2019 16:06, Nikolay Aleksandrov wrote:
>>>>>>>>>  In general NLM_F_APPEND is only used in vxlan, the bridge does not
>>>>>>>>>  handle that flag at all.  FDB is only for *unicast*, nothing is joined
>>>>>>>>>  and no multicast should be used with fdbs. MDB is used for multicast
>>>>>>>>>  handling, but both of these are used for forwarding.
>> This is true, and this should have been addressed in the patch, we were too
>> focused on setting up the offload patch in the driver, and forgot to do the SW
>> implementation.
>> Do you see any issues in supporting this flag, and updating the SW
>> forwarding in br_handle_frame_finish such that it can support/allow a FDB entry
>> to be a multicast?
> 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. 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.
>> /Allan

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