[c-nsp] IGMP differences between c3560 & c4948

vince anton mvanton at gmail.com
Tue Apr 13 15:10:40 EDT 2010

> 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.
ok - i follow - makes sense, thanks

> > 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?

must have been an isolated issue, perhaps due to "multicast routing", as it
worked fine on another 3560 !

> > 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?
yes its there on 12.2(50)SG6

now to troubleshoot why the CPU is at 99% due to high "IP Input" and "PIM
Process" when i connected multiple sources and recievers..... somehow TTL=1
is a suspicion.




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