H3C Technologies H3C S12500-X Series Switches - Manual (Page 46)

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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 priority, 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
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 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 on an interface,
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 to the
multicast forwarding table. The process goes as follows:
{
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 forwarding entry matches the packet and the receiving interface that received the packet is the
incoming interface of the forwarding entry, the router forwards the packet out of all the outgoing
interfaces.
• If a forwarding entry matches the packet but the receiving interface is not the incoming interface of
the forwarding entry, the multicast packet undergoes an RPF check.
{
If the RPF interface is the incoming interface, it indicates 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 indicates that 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.