Publication date: Jan., 2009
Revision A1
66
3-7. Multicast
The function, is used to establish the multicast groups to forward the
multicast packet to the member ports, and, in nature, avoids wasting the bandwidth
while IP multicast packets are running over the network. This is because a switch
that does not support IGMP or IGMP Snooping can not tell the multicast packet from
the broadcast packet, so it can only treat them all as the broadcast packet. Without
IGMP Snooping, the multicast packet forwarding function is plain and nothing is
different from broadcast packet.
A switch supported IGMP Snooping with the functions of query, report and
leave, a type of packet exchanged between IP Multicast Router/Switch and IP
Multicast Host, can update the information of the Multicast table when a member
(port) joins or leaves an IP Multicast Destination Address. With this function, once a
switch receives an IP multicast packet, it will forward the packet to the members
who joined in a specified IP multicast group before.
The packets will be discarded by the IGMP Snooping if the user transmits
multicast packets to the multicast group that had not been built up in advance.
