Cisco Systems, Inc. Cisco Systems, Inc. Cisco Systems, Inc. Juniper Networks PIM Flooding Mechanism and Source Discovery (PFM-SD) is a mechanism for source discovery within a PIM domain. PIM signaling over BIER has been defined, allowing for BIER to interoperate with PIM. This document defines PFM-SD over BIER, such that PFM-SD can be used by PIM in a PIM domain to discover sources that are reachable via BIER. Also, this document provides PFM-SD extensions to discover the BIER ingress router closest to the source. This can be used by BIER overlays, such as PIM signaling over BIER, to determine which router to signal.