H3C Technologies H3C SecPath F1000-E - Manual (Page 173)

10
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:
• The DR at the source side and the RP need to implement complicated encapsulation and
decapsulation of IPv6 multicast packets.
• IPv6 multicast packets are delivered along a path that is not necessarily the shortest one.
• 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 when the traffic rate exceeds the threshold:
1. The RP initiates an SPT switchover process
The RP can periodically check the passing-by IPv6 multicast packets. Once it finds that the traffic rate
exceeds a configurable threshold, the RP will send 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.
NOTE:
For more information about the SPT switchover initiated by the RP, see 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:
• 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. 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.
• 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. For more information, see Assert .
Introduction to Administrative Scoping in IPv6 PIM-SM
Division of IPv6 PIM-SM domains
Typically, an IPv6 PIM-SM domain contains only one BSR, which is responsible for advertising RP-set
information within the entire IPv6 PIM-SM domain. The information for all multicast groups is forwarded
within the network scope administered by the BSR. We call this IPv6 non-scoped BSR mechanism.