For correct operation, every PIM-SM router within a PIM domain must be able to map a particular global-scope multicast group address to the same RP. If this is not possible, then black holes can appear (this is where some receivers in the domain cannot receive some groups). A domain in this context is a contiguous set of routers that all implement PIM and are configured to operate within a common boundary.
The bootstrap router (BSR) mechanism provides a way in which viable group-to-RP mappings can be created and distributed to all the PIM-SM routers in a domain. Each candidate BSR originates bootstrap messages (BSMs). Every BSM contains a BSR priority field. Routers within the domain flood the BSMs throughout the domain. A candidate BSR that hears about a higher-priority candidate BSR suppresses its sending of further BSMs for a period of time. The single remaining candidate BSR becomes the elected BSR and its BSMs inform the other routers in the domain that it is the elected BSR.
It is adaptive, meaning that if an RP becomes unreachable, it is detected and the mapping tables are modified so the unreachable RP is no longer used and the new tables are rapidly distributed throughout the domain.