HP 6125XLG Ip Multicast Configuration Manual page 64

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 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
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.
•
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.
For more information about the concepts of SPT, RPT, source-side RPT, RP, and BSR, see
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 for
a matching entry:
If no match is found, the router determines the RPF route back to the packet source. It also creates
•
a forwarding entry with the RPF interface as the incoming interface and makes the following
judgments:
If the receiving interface is the RPF interface, the RPF check succeeds and the router forwards
the packet out of all the outgoing interfaces.
If the receiving interface is not the RPF interface, the RPF check fails and the router discards the
packet.
If a match is found and the receiving interface is the incoming interface of the forwarding entry, the
•
router forwards the packet out of all the outgoing interfaces.
If a match is found but the receiving interface is not the incoming interface of the forwarding entry,
•
the router determines the RPF route back to the packet source. Then, the router makes the following
judgments:
If the RPF interface is the incoming interface, 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, the forwarding entry has expired, and the
router replaces the incoming interface with the RPF interface. In this case, if the receiving
interface is the RPF interface, the router forwards the packet out of all outgoing interfaces.
Otherwise, it discards the packet.
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"Configuring