Multicast RPF neighbor to source not expected

[andersi]

When using 12.1(14) software, the RPF neighbor against an (S,G) mroute is not the next hop to source in the unicast routing table, instead it appears to be the DR for the LAN that leads towards the source. It is also marked with an asterisk. In previous IOS versions the RPF neighbor is the next unicast hop to source. What does this mean and in what software version did this change?

[chris-jennings]

The RPF neighbour you see is in fact the IP address of an upstream PIM neighbour to the source that was the winner of an an assert proccess.

This operation is in-fact correct and is documented in RFC 2362 (PIM Sparse Mode). It allows for PIM to listen to asserts from upstream neighbours and use the winner as an RPF neighbour even though it may differ from the unicast RPF neighbor. This may come in handy in some odd situations where the unicast RPF neighbour is not suitable see

http://www.cisco.com/warp/public/619/hsrpmcast.html

for an example of why you might need this feature.

It does appear that IOS has only implemented this feature in 12.1, I suspect it arrived around 12.1.5 along with PIM DM State Refresh.