HP MSR2003 Configuration Manual page 51

If the router uses the longest prefix match principle, the router selects the matching route as the
RPF route. If the routes have the same mask, the router selects the route that has the highest
priority as the RPF route. If the routes have the same priority, the router selects a route as the RPF
route in the order of static multicast route, MBGP route, and unicast route.
For more information about the route preference, see Layer 3—IP Routing Configuration
Guide.
If the router does not use the longest prefix match principle, the router selects the route that has
the highest priority as the RPF route. If the routes have the same priority, the router selects a route
as the RPF route in the order of static multicast route, MBGP route, and unicast route.
In RPF checks, a "packet source" means different things in different situations:
For a packet that travels along the SPT from the multicast source to the receivers or to the RP, the
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packet source is the multicast source.
For a packet that travels along the RPT from the RP to the receivers, the packet source is the RP.
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• For a packet that travels along the source-side RPT from the multicast source to the RP, the packet
source is the RP.
For a bootstrap message from the BSR, the packet source is the BSR.
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For more information about the concepts of SPT, RPT, source-side RPT, RP, and BSR, see "Configuring
PIM."
RPF check implementation in multicast
Implementing an RPF check on each received multicast packet brings a big burden to the router. The use
of a multicast forwarding table is the solution to this issue. When the router creates a multicast forwarding
entry for a multicast packet, it sets the RPF interface of the packet as the incoming interface of the
forwarding entry. After the router receives a multicast packet, it looks up its multicast forwarding table:
If no match is found, the router first determines the RPF route back to the packet source and the RPF
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interface. Then, it creates a forwarding entry with the RPF interface as the incoming interface and
performs one of the following actions:
If the interface that received the packet is the RPF interface, the RPF check succeeds and the
router forwards the packet out of all the outgoing interfaces.
If the interface that received the packet is not the RPF interface, the RPF check fails and the router
discards the packet.
If a match is found and the matching forwarding entry contains the receiving interface, the router
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forwards the packet out of all the outgoing interfaces.
If a match is found but the matching forwarding entry does not contain the receiving interface, the
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router determines the RPF route back to the packet source. Then, the router performs one of the
following actions:
If the RPF interface is the incoming interface, it means that the forwarding entry is correct, but
the packet traveled along a wrong path. The router discards the packet.
If the RPF interface is not the incoming interface, it means that the forwarding entry has expired.
The router replaces the incoming interface with the RPF interface. In this case, if the interface
that received the packet is the RPF interface, the router forwards the packet out of all outgoing
interfaces. Otherwise, it discards the packet.
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