Multicast RP - RPF concept

Hi Group,

Just trying to clarify a bit in the RPF check mechanism in PIM Sparse
mode..Suppose 3 routers R1, R2 and R3 are connected linearly .. with R1 as
the source of the multicast and R3 as the RP.. Not concerned with the
receivers in this example.. If R1 sends traffic towards the receiver
(downstream R3) , the unicast register reaches R2 and R2 does a RPF check on
the source of the traffic i.e R1 as well as the RP i. R3 .. Now as I have
read it when a router receives a multicast packet it does a RPF check on the
source of the packet. But in this case the RP (i.e R3 ) has not yet sent any
traffic towards R2 so R2 has not received any packet from the RP.. Yet R2
does a check on the RP address.. My question is have I understood this
correctly that the RPF check for the RP address is done without actually any
multicast traffic being received from the RP ...while for other scenarios
the multicast packet has first to be received from a source and then the RPF
check is performed ..

I have a scenario where R2 has two connections to R3 (RP ) and one of the
connections has PIM enabled while the RP is reachable through the other
connection ..Essentially, the RPF check on R2 fails for the RP and when I
debug the packets there is no packet which was sent to the RP .. the check
failed on R2 straightaway ...

Hope I have put the question properly !!

Regards,
Ravi

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Received on Tue Jul 07 2009 - 12:15:21 ART