Understand Reverse-Path Forwarding (RPF)

Reverse-Path Forwarding (RPF) is an algorithm used for forwarding multicast datagram's.
It functions as follows:

• If a router receives a datagram on an interface that it uses to send unicast packets to the source, the packet has arrived on the RPF interface.

 (then)

• If the packet arrives on the RPF interface, a router forwards the packet out the interfaces that are present in the outgoing interface list of a multicast routing table entry.

(else)

• If the packet does not arrive on the RPF interface, the packet is silently discarded to prevent loops.


PIM uses both source trees and RP-rooted shared trees to forward datagram's; the RPF check is performed differently for each.

• If a PIM router has source-tree state (that is, an (S,G) entry is present in the multicast routing table), the router performs the RPF check against the IP address of the source of the multicast packet.

• If a PIM router has shared tree state (and no explicit source-tree state), it performs the RPF check on the RP's address (which is known when members join the group).

Sparse-mode PIM uses the RPF lookup function to determine where it needs to send Joins and Prunes. (S,G) Joins (which are source-tree states) are sent toward the source.
(*,G) Joins (which are shared-tree states) are sent toward the RP.

DVMRP and dense-mode PIM use only source trees and use RPF as described above.