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16.3.4.2 PIM-DM Troubleshooting Help
In configuring and using PIM-DM protocol, the PIM-DM protocol may fail to run properly
due to reasons such as physical connection failure or wrong configurations. The user
should ensure the following:
Good condition of the physical connection.
All interface and link protocols are in the UP state (use “show interfaces status”
command).
Next, enable PIM-DM protocol on the interface (use the “ip pim dense-mode”
command).
Multicast protocols use unicast routes to perform RPF check, for this reason, the
unicast route correctness must be ensured.
16.4 PIM-SM
16.4.1 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 reaches 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 use 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