Dell PowerConnect 8024 User Configuration Manual page 1178

Figure 43-5. PIM-SM SPT—Part 2
• Finally, special (S, G) RP-bit Prune messages are sent up the Shared Tree to
prune off this (S, G) traffic from the Shared Tree.
If this were not done, (S, G) traffic would continue flowing down the
Shared Tree resulting in duplicate (S, G) packets arriving at the receiver.
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Managing IPv4 and IPv6 Multicast