3Com Switch 8800 Advanced Software V5 Configuration Guide

680 CHAPTER 46: IPV6 PIM CONFIGURATION
upstream node deletes its link with this downstream node from the outgoing
interface list and checks whether it itself 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 is to inform the RP about the
existence of the IPv6 multicast source.
Figure 208 IPv6 multicast source registration
As shown in Figure 208, the IPv6 multicast source registers with the RP as follows:
1 When the IPv6 multicast source S sends the first IPv6 multicast packet to an IPv6
multicast group G, the DR directly connected with the multicast source, upon
receiving the multicast packet, encapsulates the packet in a register message, and
sends the message to the corresponding RP by unicast.
2 When the RP receives the register message, on one hand, it extracts the multicast
packet from the register message and forwards the multicast IPv6 multicast packet
down the RPT, and, on the other hand, sends an (S, G) join message hop by hop
toward the IPv6 multicast source. Thus, the routers along the path from the RP to
the IPv6 multicast source form an SPT branch. Each router on this branch
generates a (S, G) entry in its forwarding table. The IPv6 multicast source is the
root, while 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.
Switchover from RPT to SPT
Initially, multicast traffic flows along an RPT from the RP to the receivers. Because
the RPT is not necessarily the tree that has the shortest path, the receiver-side DR
Source
Server
Host A
Host B
Host C
Receiver
Receiver
IPv6 multicast packets
SPT
Join message
Register message
RPDR