HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 548

Configuring Multicast Support with PIM-SM
Overview
11-20
If a group's prune list includes the specific source, the router deletes (or
schedules for deletion) the interface from the corresponding (S, G) entry's
outgoing interface list.
Receiving (S, G) RP-bit Prunes. The prune list for a group may include a
specific source marked with an RP-bit. The RP-bit indicates that even though
this prune includes a specific source, it deals with the RP tree.
A router would send such a prune to its upstream RP neighbor when the
upstream connection in its SP tree differs from that in its RP tree. The prune
instructs the upstream router to stop forwarding multicasts to the down-
stream router: this router is using a different connection to reach the source.
For example, in Figure 11-8, Router E has established an RP tree for multicast
traffic for a particular group before a source begins transmitting such traffic.
After Router E begins to receive multicasts, it constructs an SP tree to the
source. The RP and the SP tree diverge at Router C. Router C would send a
(S, G) RP-bit prune to Router D.
Multicast
Host
RP Tree SP Tree
Router A
RP
RP Tree
(S, G) RP-bit Prune
Router D
Figure 11-8. Sending an (S, G) RP-bit Prune
SP Tree
Router B
DR
SP Tree
Router C
RP Tree
RP Tree SP Tree
Router E
SP Tree
Multicast
Source
RP Tree
Multicast
Hosts
SP Tree
Multicast
Host