[c-nsp] IGMP differences between c3560 & c4948

[cheddar cheese]

I think if you have a router upstream configured for PIM you won't need to
configure the switch as the querier

On Sun, Apr 11, 2010 at 4:07 PM, <Christopher.Marget at usc-bt.com> wrote:

> Said Vince:
> > On the C3560, with default config, i believe igmp snooping is enabled
> > by default.  A multicast source floods all traffic to all ports in the
> > vlan, until I configure 'ip igmp snooping querier' at which point the
> > flooding stops.  I would have expected igmp snooping to figure out that
> no
> > receiver requested traffic, so no flooding should never occur at all ?
>
> The behavior you describe sounds correct.  IGMP snooping depends on IGMP
> traffic in order to have something to snoop.  Without an IGMP querier, you
> can only count on the initial (unsolicited) IGMP host report.  There might
> be no other traffic, rendering IGMP snooping is totally helpless.  It makes
> sense that traffic would flood to all ports until you enable a querier.
>  This sounds like a feature:  The switch is smart enough to disable
> multicast filtering when the information required for good decision-making
> isn't available.
>
> > If I remove 'ip igmp snooping querier', and configure 'ip pim sparse-
> > mode' on an SVI in that vlan, again I would expect the 3560 to stop the
> > flooding, but the flooding goes on an on, which i dont expect.
>
> Is 'ip multicast routing' enabled?  What does 'sho ip igmp snooping
> querier' have to say about it?
>
> > On the 4948, flooding again does occurs with default config with igmp
> > snooping enabled by default, but different to the c3560 both 'ip igmp
> > snooping querier' and/or 'ip pim sparse-mode' take care of controlling
> > that.
>
> Last time I needed 'ip igmp snooping querier', it wasn't available on the
> 4948.  What version are you running?
>
> > am i getting it wrong, or is it indeed by design that the boxes behave
> > differently ?
>
> It could be different defaults (or manual configuration) around enabling L3
> multicast.  Without 'ip multicast routing', 'ip pim sparse=mode' is a no-op.
>
> Good luck.
>
> /chris
>
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