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.

 

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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 239.239.1.1, iosv-6 starts registering process and does something I don't understand:

[code]
*Nov  1 05:52:09.818: PIM(0): Adding register encap tunnel (Tunnel0) as forwarding interface of (10.1.0.17, 239.239.1.1).
*Nov  1 05:52:09.819: PIM(0): Insert (10.1.0.17,239.239.1.1) 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, 239.239.1.1), 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, 239.239.1.1), S-bit set
*Nov  1 05:52:09.891: PIM(0): Add GigabitEthernet0/1/10.1.0.5 to (10.1.0.17, 239.239.1.1), Forward state, by PIM SG Join
[/code] 

Because it has (*,G) state for 239.239.1.1 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?

 

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