Multicast IGMP Snooping confusion

Unanswered Question
Apr 17th, 2008


I have been doing some testing with IGMP snooping. I do not have any multicast routing setup, and am using default IGMP Snooping on a 2960 12.2(44)SE IOS.

When I start a multicast stream using VLC on an IP ( for example) - the switch floods this traffic to all ports in the VLAN. Why?

I thought IGMP snooping was meant to only allow the traffic to ports that the switch notices an IGMP group join request from.

Am I missing something?



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Kevin Dorrell Thu, 04/17/2008 - 04:57

Is the stream really IP? I know you say it is to an IP address, and I presume it is to a 01:00:5e MAC address, but is the type field 0x0800? I found out the hard way that IGMP snooping will only affect genuine IP traffic.


Luxembourg Thu, 04/17/2008 - 05:13

Hi Kevin,

Yes the packets are 0x0800 MAC 01:00:5e:7f:0a:0a. Content is encapsulated in RTP, but I have also tried straight UDP streams with the same outcome.

I have also tried using a C2950 12.1(22)EA11 with the same results.



Jon Marshall Thu, 04/17/2008 - 05:16


IGMP snooping involves listening to responses to IGMP queries. So something has to be making the IGMP queries for snooping to work. Normally this would be a pim enabled L3 interface but as you are not using multicast routing then something else needs to make those queries.

You can use the "igmp snooping querier" function where it is available on catalyst switches. Looks like the 2960 does support this

Jon Thu, 04/17/2008 - 05:20


Thanks for that. I had come across this before, but for some reason thought I wouldn't have to go down this route!

Ill revisit and see how I get on.



sateeshk10 Wed, 10/15/2008 - 17:13


Sorry to intreput you..I had a query about multicast

I want enable multicast in my network ..Below is my requirements

1) I am planning to enable multicast routing in my core switch

2) ACCess switch1 connted to core SW trgh trunk

3)My multicast server has connected to access swtich. (Vlan 10)

What config is required in access swtich to pass multicast traffic?

If access sw2 wants to comunicate trgh vlan 20( configured in core switch only)

Access sw2 also connected to core switch as trunk.

pls let me know the config...which i need to be done in acess swtcihes and core swtich.




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