3Com SuperStack 4 Configuration Manual page 158

170 C 7: M
HAPTER ULTICAST
Applying Multicast
IGMP Snooping
P
ROTOCOL
shortest path from the receiver to the source address. If a source tree is used, the
source address is the address of the source host sending the multicast packet. If a
shared tree is used, the source address is the address of the root of the shared
tree. When a multicast packet arrives at the router, if RPF check succeeds, the
packet will be forwarded according to the multicast forwarding entry. Otherwise,
the packet will be dropped.
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 (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 5500G-EI 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 39.