PIM devices in a domain must be able to map each multicast group to the correct RP address. The BSR protocol for PIM-SM provides a dynamic, adaptive mechanism to distribute group-to-RP mapping information rapidly throughout a domain. With the IPv6 BSR feature, if an RP becomes unreachable, it will be detected and the mapping tables will be modified so that the unreachable RP is no longer used, and the new tables will be rapidly distributed throughout the domain.
Every PIM-SM multicast group needs to be associated with the IP or IPv6 address of an RP. When a new multicast sender starts sending, its local DR will encapsulate these data packets in a PIM register message and send them to the RP for that multicast group. When a new multicast receiver joins, its local DR will send a PIM join message to the RP for that multicast group. When any PIM device sends a (*, G) join message, the PIM device needs to know which is the next device toward the RP so that G (Group) can send a message to that device. Also, when a PIM device is forwarding data packets using (*, G) state, the PIM device needs to know which is the correct incoming interface for packets destined for G, because it needs to reject any packets that arrive on other interfaces.
A small set of devices from a domain are configured as candidate bootstrap routers (C-BSRs) and a single BSR is selected for that domain. A set of devices within a domain are also configured as candidate RPs (C-RPs); typically, these devices are the same devices that are configured as C-BSRs. Candidate RPs periodically unicast candidate-RP-advertisement (C-RP-Adv) messages to the BSR of that domain, advertising their willingness to be an RP. A C-RP-Adv message includes the address of the advertising C-RP, and an optional list of group addresses and mask length fields, indicating the group prefixes for which the candidacy is advertised. The BSR then includes a set of these C-RPs, along with their corresponding group prefixes, in bootstrap messages (BSMs) it periodically originates. BSMs are distributed hop-by-hop throughout the domain.
Bidirectional BSR support allows bidirectional RPs to be advertised in C-RP messages and bidirectional ranges in the BSM. All devices in a system must be able to use the bidirectional range in the BSM; otherwise, the bidirectional RP feature will not function.