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Date:   Fri, 11 Jan 2019 16:43:31 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Ido Schimmel <idosch@...lanox.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "vivien.didelot@...il.com" <vivien.didelot@...il.com>,
        "cphealy@...il.com" <cphealy@...il.com>,
        Jiri Pirko <jiri@...lanox.com>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        "nikolay@...ulusnetworks.com" <nikolay@...ulusnetworks.com>,
        "roopa@...ulusnetworks.com" <roopa@...ulusnetworks.com>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        "ivan.khoronzhuk@...aro.org" <ivan.khoronzhuk@...aro.org>
Subject: Re: [PATCH net-next v4] Documentation: networking: Clarify switchdev
 devices behavior

> > +IGMP snooping
> > +~~~~~~~~~~~~~
> > +
> > +The Linux bridge allows the configuration of IGMP snooping (compile and run
> > +time) which must be observed by the underlying switchdev network device/hardware
> > +in the following way:
> > +
> > +- when IGMP snooping is turned off, multicast traffic must be flooded to all
> > +  switch ports within the same broadcast domain. The CPU/management port
> > +  should ideally not be flooded and continue to learn multicast traffic through
> > +  the network stack notifications. If the hardware is not capable of doing that
> > +  then the CPU/management port must also be flooded and multicast filtering
> > +  happens in software.
> > +
> > +- when IGMP snooping is turned on, multicast traffic must selectively flow
> > +  to the appropriate network ports (including CPU/management port) and not flood
> > +  the switch.
> > +
> > +Note: reserved multicast addresses (e.g.: BPDUs) as well as Local Network
> > +Control block (224.0.0.0 - 224.0.0.255) do not require IGMP and should always
> > +be flooded.
> 
> I'm not sure that these paragraphs are actually needed. You're basically
> describing RFC 4541 on which the IGMP snooping functionality in the
> Linux bridge is based on.

Hi Ido

My experience talking with people is that IGMP snooping is a bit
mystical and not well understood. I would not be surprised if
community driver writers, as opposed to vendor driver writers, don't
actually know how snooping works. So i find having some hints is good.

	 Andrew

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