Hi,
The command (Ip pim spt-threshhold), informs the router to switch to the shortest path tree in certain condition.
All routers that have active Joining Group on RP Multicast domain, defaults to switch to the shortest path tree as soon as they recieve thier multicast stream.
Configuring "IP pim spt-threshold" would influence the router to switch or not to the shortest path tree under some conditions.
The "Deignated Router" on a segment is elected to forward pim Join, prune as well as IGMP query messages. The "DR" is chosen based on the least cost toward the root. If the Costs are the same, the Highest IP address is prefered.
HTH
Mohamed
