• Reverse Path Forwarding Check
    • PIM does not exchange its own topology.
    • PIM relies on IGP for loop free path selection.
    • RPF check is an extra data plane loop prevention technique.
  • RPF check performed on all incoming multicast packets.
    • If incoming multicast interface == outgoing unicast interface, RPF check passed.
    • If incoming multicast interface DOES NOT == outgoing unicast interface, RPF check fails, packet dropped.
  • RPF check changes depending on type of tree.
    • Shortest Path Trees (SPT)
      • Performs check against source
      • Used in (S,G) trees.
        • Pim Dense Mode
        • Pim Sparse Mode from RP to Source
        • Pim Sparse mode from Receiver to Source after SPT Switchover (S-bit set)
        • Pim Source Specific Multicast (SSM)
    • Shared Trees (RPT)
      • Perform RPF check against RP.
      • Used in (*,G) trees
        • Pim Sparse Mode from Receiver to RP before SPT switchover.
  • RPF check also performed on RP against Register messages.
    • RP must have route back to source that is being registered.
    • If no route, (S, G) state cannot be created.
  • Verify/Troubleshooting commands:
    • show ip mroute
    • show ip mroute count
    • show ip rpf
    • mtrace
    • debug ip pim
    • debug ip mfib pak
  • RPF controls how trees must be built, not how it can be built.
    • PIM join is sent out RPF interface for both (*,G) and (S,G)
    • Changing RPF results in traffic engineering for multicast.
    • You can override Unicast route table if there are static mroutes entered.
      • ip mroute x.x.x.x x.x.x.x x.x.x.x

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