Igmp Snooping; Overview - 3Com 7757 Configuration Manual

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Overview

IGMP Snooping

Fundamentals
IGMP S
NOOPING
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.
Table 333 IGMP message processing on the switch
Received message type Sender
IGMP host report
message
IGMP leave message
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
Multicast packet transmission
without IGMP Snooping
Multicast router
Source
Layer 2 switch
Host A
Receiver
Host B
Multicast packets
C
ONFIGURATION
Receiver
Host Switch
Host Switch
Multicast packet transmission
when IGMP Snooping runs
Source
Host A
Host C
Receiver Receiver
Switch processing
Add the host to the corresponding
multicast group.
Remove the host from the multicast
group.
Multicast router
Layer 2 switch
Host C
Receiver
Host B