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Operation Manual - Multicast
Quidway S3500 Series Ethernet Switches
Chapter 7 PIM-SM Configuration
7.1 PIM-SM Overview
7.1.1 Introduction to PIM-SM
PIM-SM (Protocol Independent Multicast-Sparse Mode) belongs to sparse mode
multicast routing protocols. PIM-SM is mainly applicable to large-scale networks with
broad scope in which group members are relatively sparse.
Different from the flood & prune principle of the dense mode, PIM-SM assumes that all
hosts do not need to receive multicast packets, unless there is an explicit request for
the packets.
PIM-SM uses the RP (Rendezvous Point) and the BSR (Bootstrap Router) to advertise
multicast information to all PIM-SM routers and uses the join/prune information of the
router to build the RP-rooted shared tree (RPT), thereby reducing the bandwidth
occupied by data packets and control packets and reducing the process overhead of
the router. Multicast data flows along the shared tree to the network segments the
multicast group members are on. When the data traffic is sufficient, the multicast data
flow can switch over to the SPT (Shortest Path Tree) rooted on the source to reduce
network delay. PIM-SM does not depend on the specified unicast routing protocol but
uses the present unicast routing table to perform the RPF check.
Running PIM-SM needs to configure candidate RPs and BSRs. The BSR is
responsible for collecting the information from the candidate RP and advertising the
information.
7.1.2 PIM-SM Operating Principle
The PIM-SM working process is as follows: neighbor discovery, building the RP-rooted
shared tree (RPT), multicast source registration and SPT switchover etc. The neighbor
discovery mechanism is the same as that of PIM-DM, which will not be described any
more.
I. Build the RP shared tree (RPT)
When hosts join a multicast group G, the leaf routers that directly connect with the hosts
send IGMP messages to learn the receivers of multicast group G. In this way, the leaf
routers calculate the corresponding rendezvous point (RP) for multicast group G and
then send join messages to the node of a higher level toward the rendezvous point
(RP). Each router along the path between the leaf routers and the RP will generate (*, G)
entries in the forwarding table, indicating that all packets sent to multicast group G are
applicable to the entries no matter from which source they are sent. When the RP
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Chapter 7 PIM-SM Configuration
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