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19.4 PIM-SM 19.4.1 Introduction to PIM-SM PIM-SM (Protocol Independent Multicast，Sparse Mode) is a sparse mode multicast protocol, the
mode is protocol independent. It is mainly used in large scale networks with group members
relatively scattered in large ranges. In contrast to the flooding-prune method in dense mode,
PIM-SM protocol assumes no hosts are receiving the multicast packets, PIM-SM routers will send
multicast packets to a host only when the host explicitly request for the packets.
By setting rendezvous points (RP) and bootstrap routers, PIM-SM announces multicast information
to all PIM-SM routers and builds up RP-rooted shared tree with the router join/prune information.
As a result, the bandwidth occupied by data packets and control packets can be reduced, and router
processing overhead can be lowered. Multicast data move along the shared tree to the network
segments of the multicast group members. When the data traffic reachs a certain level, the multicast
stream can be toggled to source-based shortest path tree to reduce network lag. PIM-SM is
independent of specific unicast routing protocol, but uses the existing unicast routing table for RPF
check.
1. How PIM-SM works
PIM-SM workflow is mainly comprised of the following parts: neighbor discovery, RP shared tree
generation, multicast source registration and SPT toggle, etc. The neighbor discovery mechanism is
the same as PIM-DM and is omitted here.
(1) RP shared tree (RPT) generation
When a host joins a multicast group G, the leaf route directly connected with the host learns the
presence of recipient of multicast group G through IGMP packets. The router then calculates the
corresponding rendezvous point (RP) for the multicast group G, and sends a join message to the
upstream node in the RP direction. Each router between the leaf router and the RP will create a (*, G)
entry in their forwarding table, indicating packets sent by any source to multicast group G applies to
this entry. When RP receives a packet sending to multicast group G, the packet will move along the
established route to reach the leaf router and the host. This completes a RP-rooted RPT.
(2) Multicast source registration.
When multicast source S sends a multicast packet to multicast group G, the PIM-SM multicast
router directly connected to it will see the multicast packet as a registration packet and unicast to the
appropriate RP. If multiple PIM-SM multicast routers exist in the network, the designated router
(DR) is responsible for the forwarding of this multicast packet.
(3) SPT toggle
When multicast router finds the multicast packets from RP destined to G in a speed exceeding the
threshold, the multicast router will send a join message to the upstream node in the source S
direction and cause the toggling from RPT to SPT.
2. Pre-PIM-SM configuration work
(1) Configure candidate RP
In PIM-SM networks, multiple RPs are allowed, they are referred to as the candidate RP (C-RP).
Each C-RP is responsible for the forwarding of multicast packet destined to a certain range of
addresses. Configuring multiple C-RP enables RP load balance. All C-RPs are of the same priority.