In case of Inter-AS MVPN Option B, routing information towards the source PE is not available in a remote AS domain, since IGP routes are not exchanged between ASes. Routers in an AS other than that of a source PE, have no routes available to reach the source PE and thus PIM JOINs would never be sent upstream. To enable setup of MDT towards a source PE, BGP next-hop (ASBR) information from that PE's MDT-SAFI advertisement is used to fake a route to the PE. If the BGP next-hop is a PIM neighbor, the PIM JOINs would be sent upstream. Otherwise, the PIM JOINs would be sent to the immediate IGP next-hop (P) to reach the BGP next-hop. Since the IGP next-hop does not have a route to source PE, the PIM JOIN would not be propagated forward unless it carried extra information contained in RPF Vector.
In case of Inter-AS MVPN Option C, unicast routing information towards the source PE is available in a remote AS PEs/ASBRs as BGP 3107 tunnels, but unavailable at remote P routers. If the tunneled next-hop (ASBR) is a PIM neighbor, the PIM JOINs would be sent upstream. Otherwise, the PIM JOINs would be sent to the immediate IGP next-hop (P) to reach the tunneled next-hop. Since the IGP next-hop does not have a route to source PE, the PIM JOIN would not be propagated forward unless it carried extra information contained in RPF Vector.
To enable setup of MDT towards a source PE, PIM JOIN thus carries BGP next hop information in addition to source PE IP address and RD for this MVPN. For option-B, both these pieces of information are derived from MDT-SAFI advertisement from the source PE. For option-C, both these pieces of information are obtained from the BGP tunneled route.
The RPF vector is added to a PIM join at a PE router when configure router pim rpfv option is enabled. P routers and ASBR routers must also have the option enabled to allow RPF Vector processing. If the option is not enabled, the RPF Vector is dropped and the PIM JOIN is processed as if the PIM Vector were not present.
For further details about RPF Vector processing please refer to [RFCs 5496, 5384 and 6513]