Accton Technology ES4626 - Manual (Page 622)

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16.3 PIM-SM
16.3.1 Introduction to PIM-SM
PIM-SM（Protocol Independent Multicast, Sparse Mode）is Protocol Independent
Multicast Sparse Mode. It is a Multicast Routing Protocol in Sparse Mode and mainly
used in big scale network with group members distributed relatively sparse and
wide-spread. Unlike the Flooding&Prune of Dense Mode, PIM-SM Protocol assumes no
host needs receiving Multicast data packets. PIM-SM router transmits Multicast Data
Packets to a host only if it presents explicit requirement.
By setting RP (Rendezvous Point) and BSR (Bootstrap Router), PIM-SM announce
Multicast packet to all PIM-SM routers and establish RPT (RP-rooted shared tree) based
on RP using Join/Prune message of routers. Consequently the network bandwith
occupied by data packets and message control is cut down and the transaction cost of
routers decreases. Multicast data get to the network segment where the Multicast group
members are located along the shared tree flow. When the data traffic reaches a certain
amount, Multicast data stream can be switched to the shortest path tree SPT based on
the source to reduce network delay. PIM-SM doesn’t rely on any specific Unicast Routing
Protocol but make RPF Check using existing Unicast routing table.
1. PIM-SM Working Principle
The central working processes of PIM-SM are: Neighbor Discovery, Generation of
RP Shared Tree (RPT), Multicast source registration, SPT Switch, etc. We won’t describe
the mechanism of Neighbor Discovery here since it is same as that of PIM-DM.
(1) Generation of RP Shared Tree (RPT)
When a host joins a Multicast Group G, the leaf router that is connected to this host
directly finds out through IGMP message that there is a receiver of Multicast Group G,
then it works out the corresponding Rendezvous Point RP for Multicast Group G, and
send join message to upper lever nodes in RP direction. Every router on the way from
the leaf router to RP will generate a (*, G) table entry, where a message from any source
to Multicast group applies to this entry. When RP receives the message sent to Multicast
Group G, the message will get to the leaf router along the set up path and reach the host.
In this way the RPT with RP as root is generated.
(2) Multicast Source Registration
When a Multicast Source S sends a Multicast packet to Multicast Group G, the
PIM-SM Multicast router connected to it directly will take charge of encapsulating the
Multicast packet into registered message and unicast it to corresponding RP. If there are
more than one PIM-SM Multicast routers on a network segment, then DR (Designated
Router) takes charge of sending the Multicast packet.