Dell PowerConnect 8024 User Configuration Manual page 1176

– The RP sends a source group (S, G) Join back towards the source to
create a branch of an (S, G) Shortest-Path Tree (SPT). This results in
the (S, G) state being created in the entire router path along the SPT,
including the RP.
Figure 43-3. PIM-SM Sender Registration—Part 2
• As soon as the SPT is built from the Source router to the RP, multicast
traffic begins to flow unencapsulated from source S to the RP.
• Once this is complete, the RP Router will send a "Register Stop" message
to the first-hop router to tell it to stop sending the encapsulated data to
the RP.
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Managing IPv4 and IPv6 Multicast