IGMP snooping -odd problem

Answered Question
Mar 4th, 2009

We have a router with 3 subinterfaces

sub 100 for PC's

sub 200 for Phones

sub 300 for Call manager

This links to a Cisco 3560 (no routing) trunked.

On this 3560 we have a wireless access point and 3 PC's. The wireless access point has 2 wireless handheld devices that are used to connect to vehicles for diagnostics. The PC's (hard wired) run an application that are able to connect to the handheld devices and pull information into a report in the PC's application. Here is what our problem is and mind you I know some about multicast on a network however not entirely sure how this application works and who the sender/reciever is.

If we disable IP IGMP SNOOPING on the switch we are able to see the handheld devices. With IGMP snooping on, we cannot see the handheld devices during the discovery when the application is fired up.

Whats odd is the PC's (hardwired) and the access point are all on the same switch and VLAN 100.

All sub interfaces on the routers have ip pim sparse-dense-mode enabled and ip multicast routing is enabled on the router as well.

I have been attempting to troubleshoot the problem but having a hard time determining where to start or to even know what the problem may be. I suspect the developer of this application has done something wrong as disabling igmp snooping on a switch shouldn't be needed to make this work.

If you want output please let me know.

Correct Answer by Giuseppe Larosa about 7 years 11 months ago

Hello Glenn,

yes the AP can pass IGMP vers 2 packets they are multicast frames

And yes enabling IGMP version3 on the switch (and on the multicast router if it is a different device) can be a solution

Hope to help


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Giuseppe Larosa Wed, 03/04/2009 - 08:24

Hello Glenn,

if I understand correctly the PC uses some form f multicast to discover the wireless handhelds.

the question is that if no IGMP messages are seen for the multicast group in use the switch will block the traffic when igmp snooping is enabled in an attempt to optimize multicast traffic.

you should capture the messages on the wired PC and on the wired port of the access point to see what is going on.

you can use a span session to a third port where you place a PC with a protocol analyzer like wireshark

if you don't see any igmp messages for the involved group the switch behaviuor is correct.

To be noted that disabling ip igmp snooping for a single vlan is possible and could be an acceptable solution.

Hope to help


glenthms Mon, 03/09/2009 - 13:19

Guiseppe thanks for responding as always. I had our customer take the wireless handheld today and hard wire it into the same Cisco switch that the PC is connected to. When we fire up the handheld you never see the IGMP report from the handheld and the device never gets added to recieve the mulitcast from the PC @ So My question is at this point, is this a design flaw on the developers of the handheld and should I demand the build IGMP into their application? Or is this something I shouldn't make a big deal of and just disable IGMP snooping on VLAN 100?

I also recieved this on the uplink to the wireless access point that I could not interpret.

29w0d: IGMPSN: Rx IGMPv3 Report on Fa0/34 when Querier is not IGMPv3, Vlan 100.

does that make any sense? I looked it up but not finding much on cisco site.

Giuseppe Larosa Mon, 03/09/2009 - 13:34

Hello Glenn,

as we suspected the handheld doesn't behave correctly.

The only doubt is if the application is missing IGMP support at all or the developer was relying on the access point.

The handheld does not use IGMP but the wired port of interest is that of the access point in the application scenario.

the message you see says that the Access Point is configured to send an IGMP version 3 (source specific) report but your switch and the multicast router are configured for IGMP version2.

If there are only few ports in this vlan100 you can use as a temporary fix igmp snooping disabling on vlan100.

if this is not true you can create an ad hoc vlan to put inside only the access-point in this way you have no counter effects of igmp snooping disable.

There is no waste of bandwidth if there is only one port in a specific vlan.

in the long term The developer can do the following:

have the handheld or the access point to send IGMP version 2 reports.

I think the AP has been configured by someone to support IGMP version3 or at least the AP vendor has decided to have IGMP vers.3 as the default version (this is possible too).

Hope to help


glenthms Mon, 03/09/2009 - 14:07

Well here is another kicker, they have a laptop setup wirelessly and I show IGMP reports v2 coming in from the laptop now. So the AP is passing IGMP v1 v2. Would enabling IGMP v3 on the switch also be a solution to this problem?

Correct Answer
Giuseppe Larosa Mon, 03/09/2009 - 15:04

Hello Glenn,

yes the AP can pass IGMP vers 2 packets they are multicast frames

And yes enabling IGMP version3 on the switch (and on the multicast router if it is a different device) can be a solution

Hope to help


JamesLuther Wed, 03/04/2009 - 08:46


This happens when the application isn't sending the correct igmp join messages.

igmp snooping will restrict the flooding of multicast packets to ports it has seen an igmp join from.

You can disable igmp snooping globally or on a vlan basis (on the SVI). If the vlan is just a layer 2 vlan you can create an SVI in the shutdown state and still disable igmp snooping.



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