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Multicast in a single vlan on a single switch

I'm a newbie to multicasting, so forgive me.

Scenario - I have 3 Switched PDU that communicate amongst each other via multicasting. All 3 are on the same switch and in the same VLAN. 2 of the units see each other but the 3rd sees nothing. No filtering or ACLs involved.

My questions are;

1) On a Catalyst 2960G, IOS 12.2(25)SEE2, is there any configuration necessary to support multicast between ports on the same switch and vlan?

2) What debug commands can I run to see \ verify multicast traffic on the 2960?


Hall of Fame Super Blue

Re: Multicast in a single vlan on a single switch


Have you enabled IGMP snooping on the switch. IGMP snooping will allow the switch to listen for IGMP responses and match these to the relevent ports.

However if you don't have a Layer 3 interface in the multicast environment then there will be nothing to issue the queries. In simple terms if no one is asking the question then there will be no answers to listen for. The solution is to enable the igmp querier function on the switch. This allows the switch to make the queries.

The 2960 with your version of the software will do igmp queriering.


Community Member

Re: Multicast in a single vlan on a single switch

so this switch runs igmp snooping by default, and there are a couple things that need to happen to allow an IGMP snooping switch to work correctly

1) The hosts send in an igmp membership reports

2) You have an IGMP querier on the segment

#2 is usually accomplished vi running PIM on a router in the vlan.

However you might be able to trick the 2960s but setting a static mrouter port and point it at the other 2960s down stream.

Here is the key output to grab. Basically its a cam table for multicast:

show ip igmp snoop group

Community Member

Re: Multicast in a single vlan on a single switch

Thanks for the replies. Just to make sure I followed, here is what I understand.

First the config - I have a 2960 with a trunk link to a MLS 3750. On the 3750, I have a SVI interface for the VLAN the ports on the 2960 I am trying to get multicast working on for all 3 units.

On the 3750 and 2960, IGMP Snooping is enabled.

All I need to do is either enable IGMP SNOOPING QUERIER on the 3750 or on the VLAN Inerface (SVI) on the 3750, configure IP PIM.

Is this correct?

Then if I want to verify traffic, I should be able to use DEBUG IP MPACKET LIST xxx, wher xxx is an Access-List that I have PERMITS for IGMP to the group address I am interested in.

Is this correct?

Thanks for working with the newbie here.

Re: Multicast in a single vlan on a single switch

The default behaviour for a switch is (or always has been) to flood multicast & broadcast traffic. This is now inhibited on the 2960 by the default igmp snooping configuration. When you have a low level of multicast traffic you could consider switching off igmp snooping by using the global command: no ip igmp snooping

My assumption is that the switch will then revert to flooding this type of traffic.



Community Member

Re: Multicast in a single vlan on a single switch

OK, I've tried a number a variations, all based off of referencing the following link;

What I have found \ tripped over is, I can never get more than 2 of these devices to see each other at the same time. The problem isn't isolated to a fixed set of 2 devices - just limited to 2.

Is this expected?

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