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Any time routes from different routing tables are compared,
things can go wrong. Unicast and multicast traffic on the Internet
and MBONE typically do not follow the same path due to the tunnels
that connect DVMRP areas through non-DVMRP areas. In Figure 8-6, we
have the following situation. Router B has a logical connection
through a tunnel to the DVMRP router. Logically, when multicast
traffic is sent by the source, the path the packets take is from the
source to the DVMRP router, from the DVMRP router through the tunnel
to router B, and then to the S1 interface of router A. Router A has
a unicast route table but no DVMRP route table because router A has
no DVMRP neighbors. When the packet arrives from router B, it does
not pass the RPF test and therefore is discarded. Router A also has
a unicast route to the source through the S0 interface, so the S0
interface is the RPF interface for the source. |
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