This document discusses Bidirectional PIM (BIDIR-PIM)@ a variant of PIM Sparse-Mode that builds bidirectional shared trees connecting multicast sources and receivers. Bidirectional trees are built using a fail-safe Designated Forwarder (DF) election mechanism operating on each link of a multicast topology. With the assistance of the DF@ multicast data is natively forwarded from sources to the Rendezvous- Point (RP) and hence along the shared tree to receivers without requiring source-specific state. The DF election takes place at RP discovery time and provides the route to the RP@ thus eliminating the requirement for data-driven protocol events.