When does PIM-SM switch between a Shared Tree and a Shorest Path Tree ?
In practice, either immediately or never. After the initial transmission is established, the switch-over to the Shortest Path Tree occurs once a certain number of bits (the SPT-Threshold) have been sent down the Shared Tree. This threshold seems to be almost always set to zero, so that the transition starts immediately. If the only sources for a given group are on a LAN attached to the RP, the switch-over to a SPT never occurs, as the Shared Tree is the SPT for that group. • In PIM-SM, how does a router actually becomes a rendezvous point ? Routers must be configured manually to be a candidate RP. One router in each PIM domain is elected by the other RPs to be the Bootstrap Router (BSR). The BSR floods messages announcing all of the suitable RPs (the RP-set) throughout the domain, so that all routers know the addresses of the RP-set. The RP can also be specified manually, which makes sense for a small domain with only a few routers. Note that a router can be set up to be a RP for only
