Bidirectional Shared-Trees violates current (*, G) RPF rules, as it accepts traffic from one Reverse Path Forwarding (RPF) interface only. To avoid forwarding multicast packet looping, bidir-PIM introduces a new mechanism called the designated forwarder (DF) election.

The designated forwarder (DF) election takes place for all PIM routers on every network segment and point-to-point link. The procedure selects one router as the DF for every RP of bidirectional groups. The designated forwarder is responsible for forwarding multicast packets received on that network.