H3C LS-3100-52P-OVS-H3 Operation Manual - page 1151
1-11
Switchover to SPT
In a IPv6 PIM-SM domain, a IPv6 multicast group corresponds to one RP and RPT. Before the SPT
switchover takes place, the DR at the IPv6 multicast source side encapsulates all IPv6 multicast data
destined to the multicast group in register messages and sends these messages to the RP. Upon
receiving these register messages, the RP abstracts the IPv6 multicast data and sends the IPv6
multicast data down the RPT to the DRs at the receiver side. The RP acts as a transfer station for all
IPv6 multicast packets. The whole process involves three issues as follows:
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The DR at the source side and the RP need to implement complicated encapsulation and
decapsulation of IPv6 multicast packets.
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IPv6 Multicast packets are delivered along a path that is not necessarily the shortest one.
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When the IPv6 multicast traffic increases, a great burden is added to 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 an SPT
switchover process:
1) The RP initiates an SPT switchover process
Upon receiving the first IPv6 multicast packet, 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. The
subsequent IPv6 multicast data from the multicast source travel along the established SPT to the RP.
For details about the SPT switchover initiated by the RP, refer to
Multicast source registration
.
2) The receiver-side DR initiates an SPT switchover process
Upon discovering that the traffic rate exceeds a configurable threshold, the receiver-side DR initiates an
SPT switchover process, as follows:
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First, the receiver-side DR sends an (S, G) join message hop by hop toword the multicast source S.
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.
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When subsequent IPv6 multicast packets arrive at the router at the junction of the RPT and SPT,
the router drops those transmitted along the RPT and sends an RP-bit prune message containing
the RP bit hop by hop to the RP. Upon receiving this prune message, the RP sends a prune
message toward the IPv6 multicast source (suppose only one receiver exists), thus to implement
SPT switchover.
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 the similar assert mechanism as IPv6 PIM-DM does. Refer to
Assert
.