8
IGMP SNOOPING
IGMP Snooping
Overview
IGMP Snooping (Internet Group Management Protocol Snooping) is a multicast
control mechanism running on Layer 2 (the link layer) of the switch. It is used for
multicast group management and control.
When receiving IGMP messages transmitted between the host and router, the
Switch 4500 uses IGMP Snooping to analyze the information carried in the IGMP
messages. If the switch hears an IGMP host report message from an IGMP host, it
will add the host to the corresponding multicast table. If the switch hears an IGMP
leave message from an IGMP host, it will remove the host from the corresponding
multicast table. The switch continuously listens to the IGMP messages to create
and maintain MAC multicast address table on Layer 2. And then it can forward the
multicast packets transmitted from the upstream router according to the MAC
multicast address table.
When IGMP Snooping is disabled, the packets are multicast to all ports, see
Figure 44.
Figure 44 Multicast packet transmission without IGMP Snooping
When IGMP Snooping operates, packets are not forwarded to all ports, see
Figure 45.
Internet / Intranet
Video stream
VOD Server
Layer 2 Ethernet Switch
Video stream
Multicast group member Non-multicast
group member
Multicast router
Video stream
Video stream
Video stream
Non-multicast
group member
Switch 4500
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