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314 CHAPTER 18: MULTICAST PROTOCOL
Applying Multicast IP multicast technology effectively solves the problem of packet forwarding from
single-point to multi-point. It implements highly-efficient data transmission from
single-point to multi-point in IP networks and can save a large amount of network
bandwidth and reduce network loads. New value-added services that use multicast
can be delivered, including direct broadcasting, Web TV, distance learning, distance
medicine, net broadcasting station and real-time audio/video conferencing.
■ Multimedia and streaming media applications
■ Communications at training and corporate sites
■ Data repository and finance (stock) applications
■ Any “point-to-multipoint” data distribution
With the increase of multimedia services on IP networks, multicast has huge market
potential.
IGMP Snooping 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
5500 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 80.
Figure 80 Multicast packet transmission without IGMP Snooping
When IGMP Snooping operates, packets are not forwarded to all ports, see Figure 81.
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 5500