LAN, Switching and Routing: Multicast advice needed on Cat 6513 IOS 12.2SXF5

Hi folks

I have a vlan interface configured with ip igmp snooping querier, this interface serves a vlan hosting alot of multicast senders and receivers. No multicast routing between vlans is needed at his time. I'm starting to get complaints from our developers that some applications listening on multicast channels loose their traffic from time to time. What I've gathered so far is that some applications on some servers loose their channel connectivity while the same application on another server still works. It seems the servers just drops from the multicast channels.

Multicast is not my best area of knowledge and I have neither full insight into the applications having these problems. What I understand so far from reading the IOS manuals igmp snooping querier is a "passive" method of connecting a multicast sender and listener without reporting much back to the servers/applications. When I present this theory to the developers they request that I set up some sort of "active" multicast control on the 6513, something that checks back to the applications or at least is better at keeping the traffic flowing. A show ip igmp int vlan XX reads IGMP disabled on interface but IGMP snooping globally enabled. Would enabling IGMP on the vlan interface solve the problem? Have I got it all wrong or is there something I can do to improve the multicast reliability?

Regards Fredrik Hofgren

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IGMP is used in two different contexts on a 6500. One for layer 3 (multicast routing) and another context for layer 2 (multicast switching) IGMP is enabled by default for layer 2, and is enabled for layer 3 by turning on "ip multicast-routing".

IGMP is a protocol used by multicast sources and receivers to signal that the are the source, or want to receive a specific multicast group. A multicast source, will periodically send an "IGMP source" message. To receive a specific multicast stream, a receiver will send an "IGMP join" message.

In layer 3, the IGMP message is processed by the router to arrange for the multicast source to be delivered to it.

In layer 2, IGMP messages are "snooped" passively by the switch to determine which ports need to receive a multicast stream If IGMP were not enabled, then the multicast traffic received by the switch would be transmitted out EVERY port, just like a broadcast.

I **think** that you can enable "ip multicast-routing" on the switch with really turning on multicast. IP multicast routing is done using PIM (Protocol Independent Multicast). While ip multicast-routing will enable multicast support on the switch PIM must be enable on each layer 3 interface using an "ip pim " command. If an "ip pim" command is not enabled on the interface, it will not route multicast traffic, which is what you want in this case. This will enable layer 3 IGMP on the layer 3 vlans and give you some visibility into what is going on with IGMP.

You should also read the RFCs dealing with IGMP, and read up on PIM and multicast from Cisco. Cisco has a multicast resources page here.

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Hi Fredrik "igmp snooping querier" is used in cases where L3 multicast routing is not necessary. In your case, since you are not doing L3 multicast routing, "igmp snooping querier" should suffice. I do not understand why multicast channels would lose their channels from time to time using "igmp snooping querier". As you mentioned, the alternative approach to "igmp snooping querier" is enabling PIM on the vlan. Sreeni

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