Igmp Snooping - 3Com 7700 Configuration Manual

162 C 6: M
HAPTER ULTICAST

IGMP Snooping

P
ROTOCOL
[SW7700-Ethernet1/0/1] gmrp
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
Layer 2 Switch 7700 uses IGMP Snooping to analyze the information. If the switch
hears IGMP host report message from an IGMP host, it adds the host to the
corresponding multicast table. If the switch hears 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. 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 on Layer 2. See
Figure 4.
Figure 4 Multicast Packet Transmission Without IGMP Snooping
Multicast router
Layer 2
Ethernet Switch
Video stream
Multicast
Nonmulticast
group
group
member
member
Packets are not broadcast on Layer 2 when IGMP operates. See Figure 5.
Internet/Intranet
Video stream
Video stream
Video
stream
Nonmulticast
group
member
Video stream
VOD Server