Administrative Distance, Access Lists, Route Manipulation, and IP Multicast - CCIE Routing and Switching Exam Certification Guide - page 492

On segments with more than one multicast routers, normally one becomes the designated
querier. The designated querier sends the IGMP membership query messages on the segment.
The designated querier is the router with the lowest IP address on the segment that is running a
multicast routing protocol.

CGMP

CGMP is a Cisco proprietary protocol that controls multicast traffic at Layer 2. Because a
Layer-2 switch is not aware of Layer-3 IGMP messages, it cannot restrain multicast packets
from being sent to all ports.

As shown in Figure 10-5, with CGMP, the LAN switch can speak with the IGMP router to find
out the MAC addresses of the hosts that want to receive the multicast packets. With CGMP,
switches distribute multicast sessions to the switch ports that have group members.

CGMP

Figure 10-5

Without CGMP

With CGMP

A B C D A B C D

When a router receives an IGMP report, it processes the report and then sends a CGMP message
to the switch. The switch can then forward the multicast messages to the port with the host that
is receiving the multicast traffic. CGMP Fast-Leave Processing allows the switch to detect
IGMPv2 leave messages that are sent by hosts on any of the supervisor engine module ports.
When the IGMPv2 leave message is sent, the switch can then disable multicast for the port.

You enable CGMP on the router with the ip cgmp command. On the switch, you use the set
cgmp enable command.

IGMP Snooping
IGMP Snooping is another method for switches to control multicast traffic at Layer 2. It listens
to IGMP messages between the hosts and routers. If a host sends an IGMP query message to
the router, the switch adds the host to the multicast group and permits that port to receive