580
Pruned
PIM prunes interface, the interface
receives Prune messages
Asserted Asserted
state
Outgoing
Multicast data finally exported from
interface is index number, index is 2 in this
case. It can check interface information in
detail by commanding show ip pim
interface
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 bandwidth
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
