The RP router serves as the information exchange point for the other routers. All routers in a PIM domain must provide mapping to an RP router. It is the only router that needs to know the active sources for a domain—the other routers just need to know how to get to the RP. In this way, the RP matches receivers with sources.
The RP router is downstream from the source and forms one end of the SPT. As shown in Figure 1, the RP router is upstream from the receiver and thus forms one end of the RPT.
The benefit of using the RP as the information exchange point is that it reduces the amount of state in non-RP routers. No network flooding is required to provide non-RP routers information about active sources.