Bidirectional PIM (BIDIR-PIM) is operationally very similar to Protocol Independent Multicast – Sparse Mode (PIM-SM). Like PIM sparse-mode, bidirectional PIM also has unconditional forwarding of source traffic toward the RP upstream on the shared tree, but has no registering process for sources. This ensures all routers send traffic using strictly (*, G) multicast routing entries. This elimination of source-specific states allows much better scalability in the case of many different sources.
Devices signal membership in a bidirectional PIM group using explicit Join messages. Sources send multicast traffic up the shared tree toward the Rendezvous Point (RP). The RP then passes traffic down the tree to any receivers on each branch. Note that for these packets passed downstream, there is no fundamental difference between BIDIR-PIM and PIM-SM. The unique behavior is with the traffic that passes from the various sources upstream to the RP.
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