3Com S7906E - Switch, Configuration Manual

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Switchover to SPT
In an IPv6 PIM-SM domain, an IPv6 multicast group corresponds to one RP and one RPT. Before the
SPT switchover takes place, the DR at the IPv6 multicast source side encapsulates all 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 multicast data and sends the 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 will 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 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 receiving the first IPv6 multicast packet, the receiver-side DR initiates an SPT switchover process,
as follows:
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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.
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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. Upon receiving this prune message, the RP sends a prune message toward the
IPv6 multicast source (suppose only one receiver exists). Thus, SPT switchover is completed.
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Finally, 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 the similar assert mechanism as IPv6 PIM-DM does. Refer to Assert .