Cisco Systems 3750E manual Leaving a Multicast Group, 24-5

Chapter 24 Configuring IGMP Snooping and MVR

Understanding IGMP Snooping

If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group

(Figure 24-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table as shown in Table 24-2. Note that because the forwarding table directs IGMP messages only to the CPU, the message is not flooded to other ports on the switch. Any known multicast traffic is forwarded to the group and not to the CPU.

Figure 24-2 Second Host Joining a Multicast Group

Router A

1

PFC

CPU

0

VLAN

Forwarding

table

2	3	4	5
Host 1	Host 2	Host 3	Host 4

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Table 24-2	Updated IGMP Snooping Forwarding Table
Destination Address		Type of Packet	Ports
224.1.2.3		IGMP	1, 2, 5

Leaving a Multicast Group

The router sends periodic multicast general queries, and the switch forwards these queries through all ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wishes to receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch forwards multicast group traffic only to those hosts listed in the forwarding table for that IP multicast group maintained by IGMP snooping.

When hosts want to leave a multicast group, they can silently leave, or they can send a leave message. When the switch receives a leave message from a host, it sends a group-specific query to learn if any other devices connected to that interface are interested in traffic for the specific multicast group. The switch then updates the forwarding table for that MAC group so that only those hosts interested in receiving multicast traffic for the group are listed in the forwarding table. If the router receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 24-5