3.7. RP Discovery

PIM-SM routers need to know the address of the RP for each group for which they have (*,G) state. This address is obtained automatically (e.g., embedded-RP), through a bootstrap mechanism, or through static configuration.

One dynamic way to do this is to use the Bootstrap Router (BSR) mechanism [11]. One router in each PIM domain is elected the BSR through a simple election process. All the routers in the domain that are configured to be candidates to be RPs periodically unicast their candidacy to the BSR. From the candidates, the BSR picks an RP-set, and periodically announces this set in a Bootstrap message. Bootstrap messages are flooded hop-by-hop throughout the domain until all routers in the domain know the RP-Set.

To map a group to an RP, a router hashes the group address into the RP-set using an order-preserving hash function (one that minimizes changes if the RP-Set changes). The resulting RP is the one that it uses as the RP for that group.