304
c.
Sends an (S, G) join message hop-by-hop toward the IPv6 multicast source.
The routers along the path from the RP to the IPv6 multicast source form an SPT branch. Each router
on this branch generates an (S, G) entry in its forwarding table. The source-side DR is the root of
the SPT, and the RP is the leaf of the SPT.
4.
The subsequent IPv6 multicast data from the IPv6 multicast source travels along the established SPT
to the RP. Then, the RP forwards the data along the RPT to the receivers. When the IPv6 multicast
traffic arrives at the RP along the SPT, the RP sends a register-stop message to the source-side DR
by unicast to stop the source registration process.
NOTE:
In this section, the RP is configured to initiate a switchover to SPT. If the RP is not configured with switchover
to SPT, the DR at the IPv6 multicast source side keeps encapsulating multicast data in register messages,
and the registration process will not stop unless no outgoing interfaces exist in the (S, G) entry on the RP.
Switchover to SPT
In an IPv6 PIM-SM domain, an IPv6 multicast group corresponds to one RP and one RPT. Before a
switchover to SPT occurs, the source-side DR encapsulates all multicast data destined to the multicast
group in register messages and sends these messages to the RP. After receiving these register messages,
the RP extracts the multicast data and sends the multicast data down the RPT to the receiver-side DRs. 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 these issues, IPv6 PIM-SM allows an RP or the receiver-side DR to initiate an SPT switchover
process when the traffic rate exceeds the threshold:
•
The RP initiates an SPT switchover process:
The RP can periodically examine 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 switchover to SPT initiated by the RP, see "
."
•
The receiver-side DR initiates an SPT switchover process:
a.
After discovering that the traffic rate exceeds a configurable threshold, the receiver-side DR
initiates an SPT switchover process, as follows:
b.
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, establishing an SPT branch.
c.
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 (suppose only one receiver exists) to implement SPT
switchover.
IPv6 multicast data is directly sent from the source to the receivers along the SPT.
