/mcpIncoming interface: FastEthernet0.2, RPF nbr 0.0.0.0 Outgoing interface list: FastEthernet0.3, Forward/Dense, 00:00:39/00:00:00 Router_A#show ip igmp groups IGMP Connected Group Membership Group Address Interface Uptime Expires Last Reporter 239.10.10.10 Fa0.3 00:02:48 00:02:04 10.3.3.2 Router_A# Cisco Group Management Protocol For CGMP support on Catalyst switches, refer to Multicast Catalyst Switches Support Matrix. CGMP Frames and Message Types CGMP was first implemented by Cisco to restrain multicast traffic in a L2 network. Because a switch is, by essence, not capable of looking at L3 packets, it cannot distinguish an IGMP packet. With CGMP, the router provides the interface between the hosts. The routers "talk" IGMP, and the switches "talk" CGMP. CGMP frames are Ethernet frames with the destination MAC address 01-00-0c-dd-dd-dd, and with a Subnetwork Access Protocol (SNAP) header with the value 0x2001. The CGMP frames contain the following fields: Version: 1 or 2. G Message Type: Join or Leave. G Count: The number of multicast/unicast address pairs in the message. G GDA: The 48-bit MAC address of the multicast group. G Unicast Source Address (USA): The 48-bit MAC unicast address of the devices that wants to join the GDA. G Note: The value of the count field determines how many times the last two fields display. By default, the processors of a switch (called NMP in Catalyst) only listen to multicast addresses when the show cam system command is issued. When you enable CGMP on a switch, the address 01-00-0c-dd-dd-dd is added to the show cam system command output. The table below lists all of the possible CGMP messages. GDA USA Join/Leav e Meanin g Mcast MAC Client MAC Join Add port to group. Mcast MAC Client MAC Leave Delete port from group. 00-00-00-00-00- 00 Router MAC Join Assign router