384Figure 103 Building an SPT in IPv6 PIM-SSMAs shown in Figure 103, Hosts B and C are IPv6 multicast information receivers. They send an MLDv2report message to the respective DRs to announce that they are interested in the information about thespecific IPv6 multicast source S and that sent to the IPv6 multicast group G.The DR that has received the report first determines whether the IPv6 group address in this message fallsinto the IPv6 SSM group range and then does the following:• If the IPv6 group address in the message does fall into the IPv6 SSM group range, the IPv6 PIM-SSMmodel is built. The DR sends a channel subscription message hop by hop toward the IPv6 multicastsource S. An (S, G) entry is created on all routers on the path from the DR to the source. Thus, an SPTis built in the network, with the source S as its root and receivers as its leaves. This SPT is thetransmission channel in IPv6 PIM-SSM.• If the IPv6 group address in the message does not fall into the IPv6 SSM group range, the DR followsthe IPv6 PIM-SM process. The receiver-side DR sends a (*, G) join message to the RP, and thesource-side DR registers the IPv6 multicast source.Relationship among IPv6 PIM protocolsIn an IPv6 PIM network, IPv6 PIM-DM cannot work with IPv6 PIM-SM, IPv6 BIDIR-PIM, or IPv6 PIM-SSM.However, IPv6 PIM-SM, IPv6 BIDIR-PIM, and IPv6 PIM-SSM can work together. When they work together,which one is chosen for a receiver trying to join a group depends, as shown in Figure 104.SourceServerHost AHost BHost CReceiverReceiverIPv6 multicast packetsSPTSubscribe messageDRDRRP
