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I have a network formed by some Cisco switches (a 2960 at the center, some 2950s for distribution). The network uses VLANs, and there are trunks between the various switches. Computers in the same VLAN can happily talk across multiple switches. Inter-VLAN routing is handled by a firewall (Microsoft TMG 2010), but this is not relevant, because the traffic I'm concerned with is confined to a single VLAN.

There is an application which uses IGMP to broadcast messages between multiple computers (as I said above, in the same VLAN), and it doesn't work.

What should I do on those switches to allow the application to work?

I had the same problem with an AiroNet access point, and I was able to solve it using the command no ip igmp snooping; however, this command seems to not have the same effect on the switches.

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You use command no ip igmp snooping in global configuration or in vlan configuration? –  Gnouc Jun 19 '13 at 11:57
    
Global. On the AP it made it work (although I'm not sure if now IGMP traffic would be able to exit from the AP at all... bit since the switches are dropping it anyway, I'm unable to test). –  Massimo Jun 19 '13 at 13:27
    
May I confuse. But I think if you turn off IGMP snooping, the IGMP traffic will be dropped by switch? –  Gnouc Jun 19 '13 at 13:34
    
This is entirely possible, I don't know what that command actually does. What I know is, typing it on the AP made the devices connected to that AP able to talk IGMP between them. Typing it on the switches did, apparently, nothing. –  Massimo Jun 19 '13 at 13:45
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Yeah, I found this solution in cisco web. The keyword is mrouter port.

Updated Here is solution from website:

The IGMP querier is a relatively new feature on Layer 2 switches. When a network/VLAN does not have a router that can take on the multicast router role and provide the mrouter discovery on the switches, you can turn on the IGMP querier feature. The feature allows the Layer 2 switch to proxy for a multicast router and send out periodic IGMP queries in that network. This action causes the switch to consider itself an mrouter port. The remaining switches in the network simply define their respective mrouter ports as the interface on which they received this IGMP query.

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Ok, this makes (some) sense. But in a scenario where my switches are layer-2 only (except for a manegement IP address on a single VLAN), and inter-VLAN routing is performed by a firewall, how should I set the mrouter port? Do all switches need one or only one of them? And where/how? –  Massimo Jun 19 '13 at 14:17
    
The article also says that disabling IGMP snooping on all switches should be a viable solution (although non-optimal). It certainly works on APs. But it doesn't seem to work on the switches... –  Massimo Jun 19 '13 at 18:01
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