3Com 10014298 manual Igmp Snooping

124CHAPTER 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
			Video stream
	Multicast router		Internet/Intranet
		Video stream
	Layer 2		VOD Server
	Ethernet Switch
	Video stream	Video	Video stream

stream

Multicast	Nonmulticast	Nonmulticast
group	group	group
member	member	member

Packets are not forwarded to all ports when IGMP operates. See Figure 29.