Manual

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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.
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.
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.
SPT Switch
When the Multicast router finds that the rate of the Multicast packet from RP with destination
address G exceeds threshold, the Multicast router will send Join message to the next upper
lever nodes in the source direction, which results in the switch from RPT to SPT.
2. Preparation before PIM-SM configuration
Configuration Candidate RP
More than one RPs (candidate RP) can exist in PIM-SM network and each C-RP (Candidate
RP) takes charge of transmitting Multicast packets with destination address in a certain range.
To configure more than one candidate RPs can implement RP load share. No master or slave
is differentiated among RPs. All Multicast routers work out the RP corresponding to some
Multicast group based on the same algorithm after receiving the candidate RP message
announced by BSR.
Note that one RP can serve more than one Multicast groups and all Multicast groups. Each
Multicast group can only correspond to one unique RP at any moment. It can’t correspond to