/mcpcreate a CGMP Join. Routers running CGMP need to see all IGMP reports, so proxy reporting must be disabled. Any reports sent to the router should only be those strictly needed for IGMP snooping. Multicast Source-Only Network If the segment contains only one multicast server (multicast source) and no client, you might end up with a situation where you do not have any IGMP packets in that segment, but you do have a lot of multicast traffic. In this case, the switch simply forwards the traffic from that group to everybody in the segment. Fortunately, a switch running IGMP snooping is able to detect these multicast streams and adds a multicast entry for that group with only the router port. These entries are flagged internally as mcast_source_only and are aged out each 5 minutes, or when the router port goes away. Note that even after this aging, the address is relearned within a few seconds if the traffic continues. Within the relearning period, momentary flooding can occur in the VLAN. In order to avoid this and retain the entries, use the set igmp flooding enable | disable command. After the flooding is disabled, the switch does not age out the source-only entries. Limitations As with CGMP, GDAs that map to a MAC that falls in the range 01-00-5e-00-00-xx is never pruned by IGMP snooping. Configuration of IGMP Snooping on Cisco Switches To enable/disable IGMP snooping, issue the following command: set igmp <enable/disable> G To configure the multicast router (static), issue the following command: set multicast router <mod/port> G clear multicast router <mod |port | all> G To monitor and check IGMP statistics, issue the following commands: show igmp statistics <VLAN_id> G show multicast router G Practical Example of IGMP Snooping The setup for this example is similar to that CGMP testing, used earlier in this document. The only difference is that port 3/2 and 3/3 are both connected to the same VLAN and are both client- configured to join group 224.10.10.10. The following example explains several manipulations, looks at what the switch does, and examines the resulting output. In the following example, Switch_B is a Catalyst 5500 running IGMP snooping, and Router_A is the multicast router connected to port 3/1. Enable IGMP snooping on the switch and see the result by issuing the debug command. Notice that each set of entries has been added to the show cam sys command output, allowing for the detection of the router port through PIM, MOSPF, and so on. Switch_B (enable) set igmp en MCAST-IGMP: Set Sys Entries 1.