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• Multicast packets are delivered along a path that might not be the shortest one.
• An increase in multicast traffic adds a great burden on the RP, increasing the risk of failure.
To solve the issues, PIM-SM allows an RP or the DR at the receiver side to initiate the switchover to SPT.
1. The RP initiates the switchover to SPT.
The RP can periodically check the passing-by IPv4 multicast packets. If it finds that the traffic rate
exceeds a configurable threshold, the RP sends an (S, G) join message hop by hop toward the
multicast source to establish an SPT between the DR at the source side and the RP. Subsequent
multicast data travels along the established SPT to the RP.
For more information about the switchover to SPT initiated by the RP, see "Multicast source
regi
stration."
2. The receiver-side DR initiates the switchover to SPT.
After receiving the first multicast packet, the receiver-side DR initiates the switchover to SPT, as
follows:
{ The receiver-side DR sends an (S, G) join message hop by hop toward the multicast source.
When the join message reaches the source-side DR, all the routers on the path have installed
the (S, G) entry in their forwarding table, and thus an SPT branch is established.
{ When the multicast packets travel to the router where the RPT and the SPT deviate, the router
drops the multicast packets received from the RPT and sends an RP-bit prune message hop by
hop to the RP. After receiving this prune message, the RP sends a prune message toward the
multicast source (suppose only one receiver exists). Thus, the switchover to SPT is completed.
{ Multicast data is directly sent from the source to the receivers along the SPT.
PIM-SM builds SPTs through the switchover to SPT more economically than PIM-DM does through the
flood-and-prune mechanism.
Assert
PIM-SM uses a similar assert mechanism as PIM-DM does. For more information, see "Assert."
BIDIR-PIM overview
In some many-to-many applications, such as multi-side video conference, there might be multiple
receivers interested in multiple multicast sources simultaneously. With PIM-DM or PIM-SM, each router
along the SPT must create an (S, G) entry for each multicast source, consuming a lot of system resources.
BIDIR-PIM addresses the problem. Derived from PIM-SM, BIDIR-PIM builds and maintains bidirectional
RPTs, each of which is rooted at an RP and connects multiple multicast sources with multiple receivers.
Traffic from the multicast sources is forwarded through the RPs to the receivers along the bidirectional RPTs.
Each router needs to maintain only one (*, G) multicast routing entry, saving system resources.
BIDIR-PIM is suitable for networks with dense multicast sources and dense receivers.
The working mechanism of BIDIR-PIM is summarized as follows:
• Neighbor discovery
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