IPv6 PIM Overview 1239■ IPv6 PIM-SM assumes that no hosts need to receive IPv6 multicast data. In theIPv6 PIM-SM mode, routers must specifically request a particular IPv6 multicaststream before the data is forwarded to them. The core task for IPv6 PIM-SM toimplement IPv6 multicast forwarding is to build and maintain rendezvous pointtrees (RPTs). An RPT is rooted at a router in the IPv6 PIM domain as thecommon node, or rendezvous point (RP), through which the IPv6 multicastdata travels along the RPT and reaches the receivers.■ When a receiver is interested in the IPv6 multicast data addressed to a specificIPv6 multicast group, the router connected to this receiver sends a join messageto the RP corresponding to that IPv6 multicast group. The path along which themessage goes hop by hop to the RP forms a branch of the RPT.■ When a multicast source sends an IPv6 multicast packet to an IPv6 multicastgroup, the router directly connected with the multicast source first registers themulticast source with the RP by sending a register message to the RP byunicast. The arrival of this message at the RP triggers the establishment of anSPT. Then, the multicast source sends subsequent IPv6 multicast packets alongthe SPT to the RP. Upon reaching the RP, the IPv6 multicast packet is duplicatedand delivered to the receivers along the RPT.n IPv6 multicast traffic is duplicated only where the distribution tree branches, andthis process automatically repeats until the IPv6 multicast traffic reaches thereceivers.How IPv6 PIM-SM Works The working mechanism of IPv6 PIM-SM is summarized as follows:■ Neighbor discovery■ DR election■ RP discovery■ Embedded RP■ RPT establishment■ IPv6 Multicast source registration■ Switchover from RPT to SPT■ AssertNeighbor discoveryIPv6 PIM-SM uses exactly the same neighbor discovery mechanism as IPv6 PIM-DMdoes. Refer to “Neighbor discovery” on page 1236.DR electionIPv6 PIM-SM also uses hello messages to elect a designated router (DR) for amulti-access network. The elected DR will be the only multicast forwarder on thismulti-access network.In the case of a multi-access network, a DR must be elected, no matter thisnetwork connects to IPv6 multicast sources or to receivers. The DR at the receiverside sends join messages to the RP; the DR at the IPv6 multicast source side sendsregister messages to the RP.
