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124 CHAPTER 6: MULTICAST PROTOCOL
IGMP Snooping IGMP Snooping (Internet Group Management Protocol Snooping) is a multicast
control mechanism running on layer 2. It is used for multicast group management
and control.
IGMP Snooping runs on the link layer. When receiving the IGMP messages, the
Switch 7750 uses IGMP Snooping to analyze the information. If the switch hears
an IGMP host report message from an IGMP host, it adds the host to the
corresponding multicast table. If the switch hears IGMP leave a 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 a MAC
multicast address table on Layer 2. It can then 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 28.
Figure 28 Multicast Packet Transmission Without IGMP Snooping
Packets are not forwarded to all ports when IGMP operates. See Figure 29.
VOD Server
Video stream
Video stream
Video stream
Video
stream
Video stream
Multicast
group
member
Nonmulticast
group
member
Nonmulticast
group
member
Layer 2
Ethernet Switch
Multicast router
Internet/Intranet