Hello Madhu
This is an excellent question, and it shows that you are thinking critically and combining multiple concepts in your mind. If you take a look at the Multicast RPF (Reverse Path Forwarding) lesson, you will note that when the RPF check fails, it is because there is multicast traffic being received from an RPF neighbor that is not specified in the multicast routing table. Multicast traffic is received from IP address “X” when the RPF neighbor is actually “Y” in the multicast routing table.
In the example you are referring to, you will notice that you have two entries in the multicast routing table, one because R4 has joined the RTP, but also one because R4 has joined the STP specifically for traffic sourced from S1. This results in the following two scenarios:
- For multicast traffic coming from R2, any source will do, and since the IP address of R2 is in the multicast routing table, the RPF check will succeed.
- Similarly, for multicast traffic coming from R1 that is sourced from S1, this too will pass the RPF check because the IP address of R1 is in the multicast routing table (STP traffic) and the source IP is specified.
So in this scenario, the RPF check succeeds because the “RPF nbr” indicator in the routing table is satisfied.
I hope this has been helpful!
Laz