HP FlexNetwork 7500 Series Configuration Manual page 317

Figure 98 IPv6 multicast source registration
As shown in
The IPv6 multicast source S sends the first multicast packet to the IPv6 multicast group G.
1.
When receiving the multicast packet, the source-side DR that directly connects to the IPv6
multicast source encapsulates the packet into a register message and unicasts the message to
the RP.
After the RP receives the register message, it decapsulates it and forwards it down to the RPT.
2.
Meanwhile, it sends an (S, G) source-specific join message toward the IPv6 multicast source.
The routers along the path from the RP to the IPv6 multicast source constitute an SPT branch.
Each router on this branch creates an (S, G) entry in its forwarding table.
The subsequent IPv6 multicast data from the IPv6 multicast source are forwarded to the RP
3.
along the SPT. When the IPv6 multicast data reaches the RP along the SPT, the RP forwards
the data to the receivers along the RPT. Meanwhile, it unicasts a register-stop message to the
source-side DR to prevent the DR from unnecessarily encapsulating the data.
Switchover to SPT
In an IPv6 PIM-SM domain, only one RP and one RPT provide services for a specific IPv6 multicast
group. Before the switchover to SPT occurs, the source-side DR encapsulates all IPv6 multicast data
in register messages and sends them to the RP. After receiving these register messages, the RP
decapsulates them and forwards them to the receiver-side DR along the RPT.
IPv6 multicast forwarding along the RPT has the following weaknesses:
•
Encapsulation and decapsulation are complex on the source-side DR and the RP.
•
The path for an IPv6 multicast packet might not be the shortest one.
•
The RP might be overloaded by IPv6 multicast traffic bursts.
To eliminate these weaknesses, IPv6 PIM-SM allows an RP or the receiver-side DR to initiate the
switchover to SPT when the traffic rate exceeds a specific threshold:
•
The RP initiates the switchover to SPT:
The RP periodically checks the multicast packet forwarding rate. If the RP finds that the traffic
rate exceeds the specified threshold, it sends an (S, G) source-specific join message toward
the IPv6 multicast source. The routers along the path from the RP to the IPv6 multicast source
constitute an SPT branch. The subsequent IPv6 multicast data is forwarded to the RP along the
SPT without being encapsulated into register messages.
Figure 98, the IPv6 multicast source registers with the RP as follows:
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