3Com MSR 50 Series Configuration Manual page 1099

the destination address. The corresponding routing entry explicitly defines the
RPF interface and the RPF neighbor.
2 Then, the router selects one from these two optimal routes as the RPF route. The
selection is as follows:
If configured to use the longest match principle, the router selects the longest
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match route from the two; if these two routes have the same mask, the route
selects the route with a higher priority; if the two routes have the same priority,
the router selects the multicast static route.
If not configured to use the longest match principle, the router selects the
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route with a higher priority; if the two routes have the same priority, the router
selects the multicast static route.
n
The above-mentioned "packet source" can mean different things in different
situations
For a packet traveling along the shortest path tree (SPT) from the multicast
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source to the receivers or the source-based tree from the multicast source to
the rendezvous point (RP), "packet source" means the multicast source.
For a packet traveling along the rendezvous point tree (RPT) from the RP to the
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receivers, "packet source" means the RP.
For a bootstrap message from the bootstrap router (BSR), "packet source"
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means the BSR.
For details about the concepts of SPT, RPT and BSR, refer to
on page 1161.
Assume that unicast routes exist in the network and no multicast static routes
have been configured on Router C, as shown in
travel along the SPT from the multicast source to the receivers.
Figure 325 RPF check process
Source
192 .168 .0.1/24
Multicast packets
IP Routing Table on Router C
Destination/Mask
192.168.0.0/24
A multicast packet from Source arrives to POS5/0 of Router C, and the
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corresponding forwarding entry does not exist in the multicast forwarding
table of Router C. The Router C performs an RPF check, and finds in its unicast
routing table that the outgoing interface to 192.168.0.0/24 is POS5/1. This
Multicast Routing and Forwarding Overview
Router B
POS 5/1
Router A
POS 5/1
Interface Router C
POS5/1
"PIM Configuration"
Figure 325. Multicast packets
Receiver
POS5/0
Receiver
POS5/0
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