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DRs at the receiver side. The RP acts as a transfer station for all IPv6 multicast packets. The whole process
involves the following issues:
• The DR at the source side and the RP need to implement complicated encapsulation and
de-encapsulation of IPv6 multicast packets.
• IPv6 multicast packets are delivered along a path that might not be the shortest one.
• An increase in IPv6 multicast traffic heavily burdens the RP, increasing the risk of failure.
To solve the issues, IPv6 PIM-SM allows an RP or the DR at the receiver side to initiate a switchover to SPT:
1. The RP initiates a switchover to SPT.
The RP can periodically check the passing-by IPv6 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 IPv6
multicast source to establish an SPT between the DR at the source side and the RP. Subsequent IPv6
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 a switchover to SPT
After receiving the first IPv6 multicast packet, the receiver-side DR initiates an SPT switchover
process, as follows:
{ The receiver-side DR sends an (S, G) join message hop by hop toward the IPv6 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 IPv6 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 IPv6 multicast source (supposing only one receiver exists) to implement SPT switchover.
{ IPv6 multicast data is directly sent from the source to the receivers along the SPT.
IPv6 PIM-SM builds SPTs through SPT switchover more economically than IPv6 PIM-DM does through the
flood-and-prune mechanism.
Assert
IPv6 PIM-SM uses a similar assert mechanism as IPv6 PIM-DM does. For more information, see "Assert."
IPv6 BIDIR-PIM overview
In some many-to-many applications, such as multi-side video conference, there might be multiple
receivers interested in multiple IPv6 multicast sources simultaneously. With IPv6 PIM-DM or IPv6 PIM-SM,
each router along the SPT must create an (S, G) entry for each IPv6 multicast source, consuming a lot of
system resources. IPv6 BIDIR-PIM is introduced to address this problem. Derived from IPv6 PIM-SM, IPv6
BIDIR-PIM builds and maintains bidirectional RPTs, each of which is rooted at an RP and connects IPv6
multiple multicast sources with multiple receivers. Traffic from the IPv6 multicast sources is forwarded
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