- 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.
- Shortest Path Trees (SPT)
- 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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