HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 538

Configuring Multicast Support with PIM-SM
Overview
11-10
The RP follows this process to generate an SP tree to the source. (See
Figure 11-3):
1. A source registers with the RP and the RP generates an SP tree to draw
the multicast traffic towards itself and down the RP tree.
The RP initially receives encapsulated multicast traffic from a new source
in unicast register packets. The RP decapsulates these packets and for-
wards them along the RP tree. It also creates an (S, G) entry and transmits
a join towards the source.
2. The RP receives a multicast on the SP tree.
The source's DR receives the SP tree join from the RP. As well as unicast-
ing register packets to the RP, the DR now forwards multicast packets to
the RP. When the RP receives the first multicast packet, it sets the SPT-bit
for the (S, G) entry. (Note that the RP, as the root of the shared tree, does
not have an incoming interface for the (*, G) entry. Therefore, the incom-
ing interface for the RP and the SP tree are always different.)
3. The RP sends a register-stop to the source's DR.
When the RP receives another unicast register packet, it sends a register-
stop to source's DR. The DR stops sending the encapsulated multicasts,
but continues sending multicasts to the RP because the RP is part of its
SP tree.