3Com Switch 7750 Configuration Guide

43
IGMP SNOOPING CONFIGURATION
Overview
IGMP Snooping
Fundamentals
Internet group management protocol snooping (IGMP Snooping) is a multicast
control mechanism running on Layer 2 switch. It is used to manage and control
multicast groups.
When the IGMP messages transferred from the hosts to the router pass through
the Layer 2 switch, the switch uses IGMP Snooping to analyze and process the
IGMP messages, as shown in Table 333.
By listening to IGMP messages, the switch establishes and maintains MAC
multicast address tables at data link layer, and uses the tables to forward the
multicast packets delivered from the router.
As shown in Figure 106, multicast packets are broadcasted at Layer 2 when IGMP
Snooping is disabled and multicast at Layer 2 when IGMP Snooping is enabled.
Figure 106 Multicast packet transmission with or without IGMP Snooping being enabled
Table 333 IGMP message processing on the switch
Received message type Sender Receiver Switch processing
IGMP host report
message
Host Switch Add the host to the corresponding
multicast group.
IGMP leave message Host Switch Remove the host from the multicast
group.
Multicast packet transmission
without IGMP Snooping
Source
Multicast router
Host A
Receiver
Host B
Host C
Receiver
Multicast packets
Layer 2 switch
Multicast packet transmission
when IGMP Snooping runs
Source
Multicast route
r
Host A
Receiver
Host B
Host C
Receiver
Layer 2 switch