IGMP snooping when there is no querier

[CSCO10631105]

Hi,

i seem to have experienced a 2960 switch with IGMP Snooping enabled, flooding multicast out of all trunked ports. When I enable a querier IGMP Snooping kicks in and starts to do its job.

(Portfast enabled so shouldn't be flooding on tcn)

I can't find any documentation to support this finding  and I'd really like to see more diagnostic info on this. I've done the usual show IP IGMP snooping etc but all it ever says is that snooping is enabled. It feels like there should be a enabled but inactive status somewhere.  When the querier is shut down, 300 seconds elapses before flooding restarts. Debug shows flooding being activated at this point.  

Any pointers appreciated. 

Thanks!

[Jon Marshall]

This is exactly what you would expect to see.

IGMP snooping listens to responses to IGMP requests to work out which ports to send the multicast stream to.

But something has to make the IGMP queries for IGMP snooping to work.

If you have PIM enabled on the L3 interface for that vlan then IGMP queries will be sent. But if you don't need PIM ie. you don't need to route between subnets then you need something else to make the queries.

This is what the "igmp snooping querier" function does on the switch.

Jon

[desweiler]

Ok. But what he is saying is:

With IGMP snooping enabled and no querier (no querier, no PIM) the switch should drop all multicast packets because of igmp snooping and no known groups. It should not flood them. This is what the switch should do with IGMP snooping disabled.

Some switches magically and silently disable igmp snooping in the absence of a querier. This behaviour is what he said. And this is not documented and happens on some switches and on others it does not.

[Jon Marshall]

Ahh okay, I understand what you (he) is saying now.

From my experience I always thought without a querier it would always simply treat the packets as broadcasts but you are saying that is dependant on the switch.

To be honest I thought that was the default behaviour.

Shows how observant I am :-)

Thanks for clarifying, learn something new every day.

Jon

[CSCO10631105]

Thanks, yes this is exactly it. It's working in my favour in this instance however shame it's not well documented.

i think ive seen a 6500 output on a website somewhere saying IGMP admin status active and IGMP operational status active. I wonder whether that's where it would be displayed. Anyway on my 2960 is just says active or inactive.

glad I'm not going mad!

cheers for the responses.