PIM
Protocol Independent Multicast — multicast routing protocol that builds on top of unicast routing table (independent from specific routing protocol, hence the name). Two variants: Sparse-Mode (SM) standard, Dense-Mode (DM) old.
PIM-SM: explicit join model, hosts join via IGMP, router sends PIM join upward to rendezvous-point (RP). Multicast traffic flows from RP downward. SSM (Source-Specific Multicast) skips RP, host specifies source IP. PIM-DM: flood-and-prune (deprecated, poor scaling). RP config is design decision: static RP, Auto-RP, BSR (Bootstrap Router), MSDP for inter-domain. Modern: BIER replaces PIM state-heavy design with stateless bit-mask routing.