IGMP Snooping: Advanced Traffic Management
Life isn’t perfect. Therefore, not everyone uses Cisco routers, even though they might use Cisco switches. What do you do if the user has non-Cisco routers in the system along with Catalysts? Or for that matter, what do other vendor products do with multicast frames? You cannot use CGMP in a non-Cisco environment because it is a Cisco proprietary feature.
Many other vendors use a software feature where the switch processor examines each frame transferred through the switch. The processor software looks for IGMP join and leave messages. If the processor sees such a message, it modifies its local bridge table. Too often, though, the processor is underpowered to keep up with an active multicast environment.
Cisco has a similar feature, but one that does not depend upon the switch processor. If you have a NetFlow Feature Card (NFFC) installed in your Catalyst with a Supervisor III module, you can use the IGMP Snooping feature. Because Cisco’s IGMP Snooping occurs on the NFFC, it can keep up with busy multicast systems without impairing switching performance. The NFFC offloads the snooping activities from the Supervisor processor.
In the multicast environments described in previous sections, a router participates in the IGMP messages and informs Catalyst switches, through CGMP, about any necessary modifications to the bridge tables. With IGMP Snooping, the Catalyst monitors the IGMP exchanges between routers and workstations, and autonomously determines any need to modify the bridge tables.
If you have a Catalyst 5000 Supervisor III engine card with an NFFC installed, and are running Supervisor engine software release 4.1 or later, you can enable IGMP Snooping. This is useful if you have non-Cisco routers in your system, but still want to control multicast distribution in your network. Further, you must have the Catalyst attached to a multicast router either directly or indirectly.
To enable IGMP Snooping, make sure that you disable CGMP first. You cannot have both CGMP and IGMP Snooping operating concurrently in a Catalyst. Use the command set igmp enableto enable IGMP Snooping. As with CGMP, the switch needs to know about the multicast router. But with IGMP Snooping, you must manually configure the setting with the set multicast router mod_num/port_num command. This is necessary because the routers are not issuing CGMP announcements for the switch to automatically discover the multicast router.
The Catalyst needs to know where the router is so that it can forward multicast source data and IGMP membership reports to all multicast routers on the segment. Use the show igmp statistics[vlan number] command to see how many of what IGMP packet type the Catalyst received and transmitted.