PIM
acronymnetworkingISP & telecom
Stands for: Protocol Independent Multicast
Protocol Independent Multicast - the routing of multicast: building the delivery trees that carry one stream to many networks, using whatever unicast routing table already exists.
The 'protocol independent' is the design: PIM does not compute its own topology - it borrows the unicast table (OSPF's, IS-IS's, BGP's) for its reverse-path checks. PIM Sparse Mode (RFC 7761), the mode that won, starts every group on a shared tree rooted at a rendezvous point - the meeting place where sources and receivers find each other - written (*,G); once traffic flows, receivers' routers can switch to the shortest-path tree straight to the source, written (S,G). Dense mode's flood-and-prune survives mostly in labs, and SSM drops the rendezvous point entirely when receivers know their source. In practice, multicast troubleshooting is PIM troubleshooting: where is the RP, does the RPF check pass, and which tree is this packet on.
Also known as: pim, pim-sm, pim-dm, ssm, rendezvous point, rp, shared tree, source tree