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Multicast works by sending messages or data to IP multicast group addresses. Routers then
forward copies (unlike broadcasts, which are not forwarded) of the packet out every interface
that has hosts subscribed to that group address. This is where multicast differs from broadcast
messages—with multicast communication, copies of packets, in theory, are sent only to subscribed
hosts. When I say in theory, this means that the hosts will receive, for example, a multicast
packet destined for 224.0.0.9 (this is an EIGRP packet and only a router running the
EIGRP protocol will read these). All hosts on the broadcast LAN (Ethernet is a broadcast
multi-access LAN technology) will pick up the frame, read the destination address, and immediately
discard the frame, unless they are in the multicast group. This saves PC processing, not
LAN bandwidth. Multicasting can cause severe LAN congestion, in some instances, if not
implemented carefully.
There are several different
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