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The RPF interface is selected by examining the IP routing
table, an example of which is given in Listing 6-1. From the sample
routing table, we can determine the RPF interface for any multicast
source. Remember that the multicast source is a unicast class A, B,
or C address and not a multicast class D address. For example, if
the router receives a multicast packet on the serial 1 interface
from the source 130.10.9.1, should the packet be forwarded? By
examining the routing table in Listing 6-1 we find that the unicast
route back to 130.10.9.1 is through interface serial 0 so the packet
did not arrive on the RPF interface. For this case, the multicast
packet would be dropped and no further processing would occur. We
can determine the RPF interface for each known source network by
examining the routing table. Each route listed contains a forwarding
interface, which is also the RPF interface. How would the router
handle multicast traffic from sources not in the routing table? For
this situation the default route would be used. |
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