DELL N3000 User Manual

Managing IPv4 and IPv6 Multicast

• 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.