Hello Heng
In the first version of PIM-SM, all routers were required to be manually configured with the IP address of the RP. This does not scale well for a large complex network.
After this, Cisco implemented the Auto-RP feature which automates the distribution of group-to-RP mappings in a PIM network. To make Auto-RP work, there must be an RP mapping agent which receives the RP announcement messages from the RPs and arbitrates conflicts. The RP mapping agent then sends the consistent group to RP mappings to all other routers by dense mode flooding. This way, all routers automatically discover which RP to use for the groups they support.
I hope this has been helpful!
Laz