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receiving the prune message, the upstream node deletes the interface connected with this downstream

node from the outgoing interface list and determines whether it has receivers for that IPv6 multicast

group. If not, the router continues to forward the prune message to its upstream router.

Multicast source registration

The purpose of IPv6 multicast source registration will inform the RP about the existence of the IPv6

multicast source.

Figure 91 IPv6 multicast source registration

Source

Server

Host A

Host B

Host C

Receiver

IPv6 multicast packets

SPT

Join message

RPDR

As shown in Figure 91, the IPv6 multicast source registers with the RP as follows:

1. The IPv6 multicast source S sends the first IPv6 multicast packet to IPv6 multicast group G. Upon

receiving the multicast packet, the DR that directly connects to the multicast source encapsulates the

packet in a register message. Then it sends the message to the corresponding RP by unicast.

2. When the RP receives the register message, it extracts the multicast packet from the register

message and forwards the multicast IPv6 multicast packet down the RPT, and 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.

3. The subsequent IPv6 multicast data from the IPv6 multicast source travels along the established SPT

to the RP, and 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:

The RP is configured to initiate an SPT switchover as described in this section. Otherwise, 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 the SPT

switchover occurs, the DR at the IPv6 multicast source side encapsulates all multicast data destined to the

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