Dell Networking 7048 Configuration Manual page 1163

Phase 3: Shortest Path Tree
Figure 42-4. PIM-SM SPT—Part 1
• PIM-SM has the capability for last-hop routers (i.e., routers with directly
connected group members) to switch to the Shortest-Path Tree and bypass
the RP. This switchover is based upon an implementation-specific function
called SwitchToSptDesired(S,G) in the standard and generally takes a
number of seconds to switch to the SPT.
• In the above example, the last-hop router (at the bottom of the drawing)
sends an (S, G) Join message toward the source to join the SPT and bypass
the RP.
• This (S, G) Join messages travels hop-by-hop to the first-hop router (i.e.,
the router connected directly to the source), thereby creating another
branch of the SPT. This also creates (S, G) state in all the routers along
this branch of the SPT.
Managing IPv4 and IPv6 Multicast
1163