User's Manual

48-8
48.2.4 PIM-DM Troubleshooting
In configuring and using PIM-DM Protocol, PIM-DM Protocol might not operate normally caused by physical
connection or incorrect configuration. Therefore, the user should pay attention to the following issues:
 To assure that physical connection is correct
 To assure the Protocol of Interface and Link is UP (use show interface command)
 To assure PIM Protocol is enabled in Global Mode (use ipv6 pim multicast-routing )
 Enable PIM-DM Protocol on the interface (use ipv6 pim dense-mode command)
 Multicast Protocol requires RPF Check using Unicast routing; therefore the correctness of Unicast
routing must be assured beforehand
If all attempts including Check are made but the problems on PIM-DM can’t be solved yet, then use debug
commands such as debug pim please, and then copy DEBUG information in 3 minutes and send to
Technology Service Center.
48.3 PIM-SM
48.3.1 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 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