PIM ASM any-to-any - purpose of (S,G) prune RPT-bit set
Good morning experts,
Recently I was working on a scenario which isn't maybe that common and have some questions.
Consider simple PIM enabled network (ASM) where there are multiple receivers which happen to be senders at the same time.
Let's assume that there was no multicast traffic flowing up to this point and all Last Hop Routers (iosv-6, iosv-7, etc.) are configured NOT to switch to SPT.
So, at the moment, all routers on the path between LHR and RP have (*,G) entry because they are part of the shared tree.
When iosv-11 sends single multicast ping to 126.96.36.199, iosv-6 starts registering process and does something I don't understand:
*Nov 1 05:52:09.818: PIM(0): Adding register encap tunnel (Tunnel0) as forwarding interface of (10.1.0.17, 188.8.131.52).
*Nov 1 05:52:09.819: PIM(0): Insert (10.1.0.17,184.108.40.206) sgr prune in nbr 10.1.0.5's queue
*Nov 1 05:52:09.822: PIM(0): Building Join/Prune packet for nbr 10.1.0.5
*Nov 1 05:52:09.823: PIM(0): Adding v2 (10.1.0.17/32, 220.127.116.11), RPT-bit, S-bit Prune
*Nov 1 05:52:09.823: PIM(0): Send v2 join/prune to 10.1.0.5 (GigabitEthernet0/1)
*Nov 1 05:52:09.890: PIM(0): Received v2 Join/Prune on GigabitEthernet0/1 from 10.1.0.5, to us
*Nov 1 05:52:09.890: PIM(0): Join-list: (10.1.0.17/32, 18.104.22.168), S-bit set
*Nov 1 05:52:09.891: PIM(0): Add GigabitEthernet0/1/10.1.0.5 to (10.1.0.17, 22.214.171.124), Forward state, by PIM SG Join
Because it has (*,G) state for 126.96.36.199 it also sends (S,G) Prune with RPT-bit set which I guess would mean "I don't want to receive traffic from myself on the shared-tree". However, it looks like that this (S,G) Prune RPT-bit doesn't create any state on the transit routers while going to the RP. The only state that is created on iosv-1, iosv-5, iosv-4 are (S,G) entries that are result of (S,G) join sent to the source by RP.
Furthermore, I have confirmed with traffic capture that this initial multicast packet sent by iosv-11 loops back to sender, which is the result of standard PIM behavior - once RP gets PIM Register packet it performs decapsulation and sends the packet down the shared tree.
Can someone explain the purpose and impact of (S,G) prune RPT-bit set on transit routers please?