netdev - Re: [RFC net-next] net: dsa: add support for MC

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Date:   Sun, 23 Jun 2019 10:44:27 +0300
From:   Ido Schimmel <idosch@...sch.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Ido Schimmel <idosch@...lanox.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...nulli.us>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [RFC net-next] net: dsa: add support for MC_DISABLED attribute

On Sun, Jun 23, 2019 at 08:26:05AM +0100, Russell King - ARM Linux admin wrote:
> On Sun, Jun 23, 2019 at 07:09:52AM +0000, Ido Schimmel wrote:
> > When multicast snooping is enabled unregistered multicast traffic should
> > only be flooded to mrouter ports.
> 
> Given that IPv6 relies upon multicast working, and multicast snooping
> is a kernel configuration option, and MLD messages will only be sent
> when whenever the configuration on the target changes, and there may
> not be a multicast querier in the system, who does that ensure that
> IPv6 can work on a bridge where the kernel configured and built with
> multicast snooping enabled?

See commit b00589af3b04 ("bridge: disable snooping if there is no
querier"). I think that's unfortunate behavior that we need because
multicast snooping is enabled by default. If it weren't enabled by
default, then anyone enabling it would also make sure there's a querier
in the network.