HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 550

Configuring Multicast Support with PIM-SM
Overview
N o t e
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When such a router receives an (S, G) RP-bit prune, it deletes the interface
on which it received the packet from its (S, G) entry's outgoing interface
list. This action removes the downstream router from the upstream
router's branch of the SP tree. The downstream router remains part of the
upstream router's RP tree. However, because the SP tree always overrides
an RP tree, the upstream router will not forward multicasts to it. (In Figure
11-8 Router A is part of the SP tree, but it has also pruned Router D from
this tree.)
In effect, an (S, G) entry acts as a place-holder, preventing the upstream router
from sending redundant multicasts to a router using a different connection in
SP tree than it used for the RP tree. (The upstream router will still forward
multicast traffic from a different source along the RP tree and out the interface
pruned in the (S, G) RP-bit entry.)
The RP-bit prevents the (S, G) entry from being deleted even when the entry
does not include any outgoing interfaces. This is necessary to prevent routers
from forwarding redundant multicasts. You may therefore see an (S, G) entry
in your router's multicast routing table that does not include any outgoing
interfaces.
Sending Triggered Join/Prune Packets. A router sends a triggered join/
prune packet when the state in one of its multicast routing table entries
changes. For example, the router sends a triggered join when it receives an
IGMP report for a new group and so creates a new (*, G) entry. Simply adding
or deleting an outgoing interface from an existing entry is not enough to trigger
a change: the router may be forwarding multicast traffic differently, but it still
needs the traffic.
Table 11-1 displays some of the events that trigger join/prune messages and
the type of packet the router sends as a result of the event.