HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 549

If the upstream neighbor is itself part of the SP tree, it prunes the downstream
router from its branch of the SP tree. If the upstream neighbor is not part of
the SP tree, it creates an (S, G) RP-bit entry to prune the downstream router
from its RP tree. (See Router D in Figure 11-8.)
A router follows these steps to prune the connection when it receives an (S,
G) RP-bit prune:
1. If the upstream router does not already have an (S, G) entry corresponding
to the prune, it creates one, completing these steps:
a. It copies the outgoing interface list from the corresponding (*, G)
entry. In effect, the upstream router is creating a new entry to control
the flow of traffic from the specific source over the RP tree.
b. The router calculates the incoming interface for the entry. Generally,
the incoming interface for an (S, G) entry is the interface through
which the router reaches the source. However, when an (S, G) entry
has its RP-bit set, its incoming interface is the interface that connects
to the upstream RP neighbor. This allows the router to use the
(S,G)RP-bit entry to forward (or to suppress) traffic received on the
RP tree.
c. The router deletes (or marks for deletion) the interface on which it
received the prune from the (S, G) entry's outgoing interface list. That
is, for as far as traffic from this source is concerned, the upstream
router prunes the downstream router from the RP tree.
When this source sends multicast packets, the router will match them
to the more specific (S, G) entry instead of to the (*, G) entry. Because
the (S,G) entry does not include the pruned interface, the router will
not forward the multicasts through it and the redundant traffic will
be suppressed.
2. Similarly, if the upstream router already has an (S, G) entry with the RP-
bit set, it deletes the interface on which it received the packet from this
entry's outgoing interface list. In this case, the upstream router had
already created an (S, G) RP-bit entry to remove other downstream
routers using the SP tree from the RP tree. The upstream router now
simply removes the router that sent the prune as well.
3. The upstream router may already have an (S, G) entry without the RP-bit
set. For example, an RP generally creates an SP tree immediately after a
source registers with it. Because the RP copies the outgoing interfaces in
the (*, G) entry to the newly created (S, G) entry, the RP continues sending
traffic over the connections in its RP tree. However, these connections
are now as part of its SP tree.
Configuring Multicast Support with PIM-SM
Overview
11-21