Reverse path forwarding check

General information on “Reverse path forwarding check”:

• Unlike unicast routing, multicast routing doesn’t care about the destination address
• Multicast routing depends on the reverse path because when a receiver (host) sends an (*,G) join, the multicast router needs to build the SPT (except for SSM)
• RPF allows multicast to build loop-free multicast trees
• The unicast routing table needs to be correct and fully converged for RPF to work
• Asymmetric routing within a network can lead to sub-optimal multicast trees

General information on “How RPF works”:

1. Incoming multicast packet will be examined to determine the source address
2. If the multicast packet arrives on an interface where the source address network is attached, the multicast packet will be allowed and the interface will be used as RPF interface
3. If the multicast packet arrives on an interface where the source address network is not attached, the multicast packet will be dropped