HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 541

1. The router creates the (S, G) entry, but continues to accept traffic from
the RP tree.
An (S, G) entry's SPT-bit signals that the router is using the SP tree
exclusively. When the router first creates the (S, G) entry, it clears the SPT-
bit so that the multicast stream will not be disrupted while the SP tree is
established. The router continues to accept packets from its upstream RP
neighbor for as long as the (S, G) entry has a cleared SPT-bit.
2. The router joins an SP tree.
The router sends a join for the specific source and group out the incoming
interface for the (S, G) entry. The upstream SP neighbor, in turn, creates
an (S, G) entry, determines the forwarding interface for its best connection
to the source, and transmits a source-specific join out that interface. This
process continues until this branch of the SP tree has reached the root,
the source's DR.
3 Router D joins SP Tree
Source of
Multicast
Group X
4 Router D prunes itself from
RP tree
Source of
Multicast
Group X
Figure 11-5. Switching to an SP Tree (Edge Router)
Configuring Multicast Support with PIM-SM
RP Tree
Router A
DR
Multicast
Router A
DR
Multicast
Overview
Router B
RP for Group X
Router C
Multicast
Intermediate
Router D
Edge Router
Host Y
Router B
RP for Group X
Router C
Router D
Edge Router
Host Y
11-13