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The situation becomes very interesting if the destination
address is a multicast or Class D IP address. This is the first
general difference between unicast and multicast—there may be
multiple receivers with the same address, possibly on different
networks, as shown in Figure 5-2. Each host that wants to receive
multicast traffic for group 225.65.10.154 will use IGMP to inform
the local router using a Join message. When the multicast packet
reaches router A, the router determines that the packet is multicast
because the address is Class D. The IGMP table is consulted and
router A sees that at least one host on a directly attached network
(172.16.2.0) has joined the group so the packet is forwarded onto
that network. If downstream hosts are to receive the multicast
traffic, then router A must forward the traffic on the serial
interface and so must router B. If there are no downstream
receivers, then it does not make sense for router A to forward the
traffic to B because this is a waste of valuable
bandwidth. |
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