The original idea in the Distance Vector Multicast Routing Protocol or the DVMRP was to use he reverse path forwarding (RPF). In RPF, a router forwards the copy that has travelled the shortest path from source to the router. To find if the packet has travelled the shortest path, the RPF uses the unicast routing table of RIP. It pretends that it needs to send a packet to source and finds if the port given by the routing table is the same from which the packet has arrived. The RPF guarantees that each network receives a copy of the multicast packet without the formation of loops.
However, the RPF does not guarantee that each network receives only one copy; a network may receive two or more copies. The reason is that the forwarding is not based on the destination address (a group address); forwarding is actually based on the source address. to eliminate this duplication, we must define only one parent router for each of the networks. We must have this restriction that a network can receive a multicast packet from a particular source only through the designated parent router. Now the policy is very clear and for each source, the router sends the packet only out of those ports for which for which it is the designated parent. This policy is known as the reverse path broadcasting or the RPB. RPB guarantees that the packet reaches every network and that every network receives only a single copy of that packet.
However, the RPF does not guarantee that each network receives only one copy; a network may receive two or more copies. The reason is that the forwarding is not based on the destination address (a group address); forwarding is actually based on the source address. to eliminate this duplication, we must define only one parent router for each of the networks. We must have this restriction that a network can receive a multicast packet from a particular source only through the designated parent router. Now the policy is very clear and for each source, the router sends the packet only out of those ports for which for which it is the designated parent. This policy is known as the reverse path broadcasting or the RPB. RPB guarantees that the packet reaches every network and that every network receives only a single copy of that packet.
