RPF in multicast

[wfqk]

Four routers are connected as circle like this: A ---- B ----C ----D, and A is connected to D. So each router has the same distance cost to other. When multicast packet goes from A to C, it will have the same cost to C through B or D. My question is which interface is RPF of C? In other word, C should receive packet from B or D, or both? Thanks

[Jaderson Pessoa · Accepted solution]

Hello guy,

Look the exemple below:

Related link: https://www.fir3net.com/Networking/Protocols/what-is-multicast-reverse-path-forwarding-rpf.html

[wfqk]

Thank you for your fast reply. the below picture is from your link. Here is my question: when the cost is lower through R2 than R3, the routes go from R2, and gi0/1 is RPF. but when the cost is same at R2 and R4, the routes will have balance. that means the two interface gi0/1 and gi0/2 at R4 get the routes. If this is case, which interface is RPF? 

[Jaderson Pessoa · Accepted solution]

General Rule 2 - The RPF interface is computed as the interface with the lowest cost path (based on admin distance/metric) to the IP address of the source (or in the case of a sparse-mode (*,G) entry, the RP). If multiple interfaces have the same cost, the interface with the highest IP address is chosen as the tiebreaker.

[Jaderson Pessoa]

https://community.cisco.com/t5/other-network-architecture/multicast-rpf-check-equal-cost-paths/td-p/176389

[wfqk]

Great thank you!