H3C Technologies H3C S7500E Series Switches - Manual (Page 363)

13-12
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:
z The DR at the source side and the RP need to implement complicated encapsulation and
decapsulation of IPv6 multicast packets.
z IPv6 multicast packets are delivered along a path that is not necessarily the shortest one.
z 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:
z 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.
z 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.
z 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 .