Cisco Systems 3560 Manual De Usuario

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.

Immediate Leave is only supported on IGMP Version 2 hosts.

The switch uses IGMP snooping Immediate Leave to remove from the forwarding table an interface that 
sends a leave message without the switch sending group-specific queries to the interface. The VLAN 
interface is pruned from the multicast tree for the multicast group specified in the original leave message. 
Immediate Leave ensures optimal bandwidth management for all hosts on a switched network, even 
when multiple multicast groups are simultaneously in use.

224.1.2.3

IGMP

1, 2, 5

Forwarding

table

CPU

Host 1

Host 2

Host 3

Host 4

Router A

PFC

VLAN

1

0

2

3

4

5

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