IT Certification

RPF Check:

In order to forward a multicast datagram, a  router checks if the packet is

received on the interface it would use to forward a unicast packet back to the source
==>If the RPF check succeeds, the router forwards the packet
==>If the RPF check fails, the packet silently discarded

Reverse Path Forwarding

IP multicasting uses reverse path forwarding (RPF) to verify that a router receives a

multicast packet on the correct incoming interface. The RPF algorithm allows a router to

accept a multicast datagram only on the interface from which the router would send a

unicast datagram to the source of the multicast datagram.

Figure 4-1 illustrates reverse path forwarding in a network where all routers run dense-

mode multicasting protocols. Routers that receive a multicast datagram associated with a

group for which they have no hosts return prune messages upstream toward the source of

the datagram. Upstream routers do not forward subsequent multicast datagrams to routers

from which they receive prune messages. This technique creates a source-rooted tree

(SRT), also known as a shortest-path tree (SPT), — a structure that connects the source

of a datagram to subnetworks of a multicast group through the shortest path. For more

information on dense-mode protocols, see PIM DM, later in this chapter.

When all routers in a network are running sparse-mode multicast protocols, the routers

forward a multicast datagram only to other routers with downstream members of the

groups associated with the datagram. Routers running sparse-mode protocols forward

multicast traffic only when explicitly requested to do so, whereas routers running dense-

mode protocols forward multicast traffic except when explicitly requested not to do so. For

more information on sparse-mode protocols, see PIM SM, later in this chapter.

RPF may take place through static routes, dynamic routes, or local subnets. You can define

static routes for this purpose and view information associated with RPF routes.

Figure 4-1 Reverse path forwarding in a dense-mode environment

RPF check is not just a matter of fact used in multicast inside, often also used in a safe inside it to prevent smurf attacks, for example, if a router from one interface receives a message when checking the source address in accordance with routing table, if source address in the routing table of the interface and enter the same, then the normal forwarding, or discarded, which can effectively avoid the smurf attack.