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

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 29/07/2019 16:52, Allan W. Nielsen wrote:
> The 07/29/2019 15:50, Nikolay Aleksandrov wrote:
>> On 29/07/2019 15:22, Nikolay Aleksandrov wrote:
>>> Hi Allan,
>>> On 29/07/2019 15:14, Allan W. Nielsen wrote:
>>>> 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?
> Yes, it will most likely become a linked list
> 
>> 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 do not think it can be supported with the facilities we have today in tc.
> 
> We can do half of it (copy more fraems to the CPU) with tc, but we can not limit
> the floodmask of a frame with tc (say we want it to flood to 2 out of 4 slave
> ports).
> 

Why not ? You attach an egress filter for the ports and allow that dmac on only
2 of the ports.

>> 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.
> I do not consider it rarely, controling the forwarding of L2 multicast frames is
> quite common in the applications we are doing.
> 
>> If you find a way to achieve this without incurring a performance hit or significant
>> code complexity increase, and without breaking current use-cases (e.g. unexpected default
>> forwarding behaviour changes) then please send a patch and we can discuss it further with
>> all details present. People have provided enough alternatives which avoid all of the
>> problems.
> Will do, thanks for the guidance.
> 
> /Allan
>