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| 7.2 |  | Examining Multicast in a Multilayer Switched Network |
| 7.2.10 |  | CGMP operation |

CGMP is a Cisco proprietary protocol that allows switches to learn about the existence of multicast clients from Cisco routers and older IOS based switches. CGMP is based on a client/server model. The router is considered a CGMP server, with the switch taking on the client role. The basis of CGMP is that the IP multicast router sees all IGMP packets and, therefore, can inform the switch when specific hosts join or leave multicast groups. The switch then uses this information to construct a forwarding table.

When the router sees an IGMP control packet, the router creates a CGMP packet. This CGMP packet contains the request type, a join or a leave, the multicast group address, and the actual MAC address of the client. The packet is sent to a well-known address to which all switches listen. Each switch then interprets the packet and creates the proper entries in a forwarding table.

CGMP can run on an interface only if PIM is configured on the same interface. CGMP is disabled on a router by default. To enable CGMP on the router, enter the following command in the interface configuration mode.

> Router(config-if)#ip cgmp

This command enables the CGMP for IP multicast on a router. This command triggers a CGMP join message. The running configuration indicates if a specific router interface has been configured for CGMP.

In order for CGMP to operate correctly on a switch, the switch must have a network connection to a router running CGMP. A CGMP-capable IP multicast router detects all IGMP packets and informs the switch when specific hosts join or leave IP multicast groups. When the CGMP-capable router receives an IGMP control packet, the router creates a CGMP packet that contains the request type, the multicast group address, and the actual MAC address of the host. The router then sends the CGMP packet to a well-known address to which switches listen. CGMP is no longer supported on the new series of Catalyst switched including the 2950 and 3550 series. IGMP Snooping has replaced CGMP on these models.