R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

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{ The router looks up its static multicast routing table by using the IP address of the packet source
as the source address, and automatically chooses an optimal static multicast route. The route
explicitly defines the RPF interface and the RPF neighbor.
2. The router selects one of the two optimal routes as the RPF route according to the following
principles:
{ 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 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 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, or 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 "Configuring
PI
M."
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 routing
entry and 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 forwarding entry matches the packet, the packet undergoes an RPF check. The router creates
a multicast routing entry with the RPF interface as the incoming interface and adds the entry into the
multicast forwarding table. The process goes as follows:
{ 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 forwarding entry matches the packet and the interface that received the packet is the incoming
interface of the forwarding entry, the router forwards the packet out of all the outgoing interfaces.