PIM Sparse Mode (PIM-SM)
James Aweya · 2024
Chapter 5 explores PIM-SM, a multicast routing protocol that operates under the assumption of sparse distribution of multicast traffic receivers for any group across the network. PIM-SM employs a pull model , with multicast routers and hosts using explicit Joins to request specific multicast traffic. Traffic is only sent when directly attached hosts and downstream routers express interest through IGMP Membership Report and PIM Join messages, respectively. PIM-SM builds unidirectional multicast shared distribution trees rooted at a Rendezvous Points (RP) for each multicast group, and when required, creates shortest-path trees (SPTs), also called source-rooted trees (SRTs), for each multicast traffic source. Routers on the RP tree (RPT), maintain multicast forwarding state as (*, G), * representing any source sending to group G. This chapter describes the RP’s role and various discovery methods in a PIM-SM domain. For seamless operation, all routers must map a multicast group address (G) to the same RP. Discovery mechanisms, Static RP, Embedded-RP, Auto-RP, and Bootstrap Router (BSR), distribute multicast group-to-RP mapping information. This equips PIM-SM routers with the knowledge needed to build the shared tree for requested multicast group traffic. Chapter 5 introduces PIM-SSM, delivering multicast packets via (S, G) channels. Hosts, using IGMPv3, directly request specific sources without an RP. PIM-SSM forms SPT between hosts and sources for multicast group G. Traffic on (S, G) channels requires a subscription. IGMPv3 enables source selection for multicast subscriptions.