/mcpCGMP statistics for VLAN 3: Switch_B (enable) show cgmp sta 3 CGMP enabled CGMP statistics for vlan 3: valid rx pkts received 109 invalid rx pkts received 0 valid cgmp joins received 108 valid cgmp leaves received 1 valid igmp leaves received 1 valid igmp queries received 63 igmp gs queries transmitted 1 igmp leaves transmitted 1 failures to add GDA to EARL 0 topology notifications received 0 Switch_B (enable) IGMP Snooping IGMP snooping is another feature that allows you to directly capture IGMP frames. For IGMP snooping support on Catalyst switches, refer to Multicast Catalyst Switches Support Matrix. IGMP Snooping Overview IGMP snooping, as implied by the name, is a feature that allows the switch to "listen in" on the IGMP conversation between hosts and routers. When a switch hears an IGMP report from a host for a given multicast group, the switch adds the host's port number to the GDA list for that group. And, when the switch hears an IGMP Leave, it removes the host's port from the CAM table entry. Learning the Router Port The switch listens to the following messages in order to detect router ports with IGMP snooping: IGMP Membership query send to 01-00-5e-00-00-01 G PIMv1 hello send to 01-00-5e-00-00-02 G PIMv2 hello send to 01-00-5e-00-00-0d G DVMRP probes send to 01-00-5e-00-04 G MOSPF message send to 01-00-5e-00-05 or 06 G By enabling IGMP snooping on a switch, all the above MAC entries are added to the show cam system command output of the snooping switch. Once a router port is detected, it is added to the port list of all GDAs in that VLAN. Joining a Group With IGMP Snooping The following are two joining scenarios: Scenario A: Host A is the first host to join a group in the segment. Host A sends an unsolicited IGMP Membership report. 1.