Operation Manual – Multicast ProtocolH3C S3610&S5510 Series Ethernet Switches Chapter 6 PIM Configuration6-5data is forwarded to them. The core task for PIM-SM to implement multicastforwarding is to build and maintain rendezvous point trees (RPTs). An RPT isrooted at a router in the PIM domain as the common node, or rendezvous point(RP), through which the multicast data travels along the RPT and reaches thereceivers.z When a receiver is interested in the multicast data addressed to a specificmulticast group, the router connected to this receiver sends a join message to theRP corresponding to that multicast group. The path along which the messagegoes hop by hop to the RP forms a branch of the RPT.z When a multicast source sends a multicast packet to a multicast group, the routerdirectly connected with the multicast source first encapsulates the packet in aregister message, and sends the message to the corresponding RP by unicast.The arrival of this message at the RP triggers the establishment of an SPT. Then,the multicast source sends the multicast packet along the SPT to the RP. Uponreaching the RP, the multicast packet is duplicated and delivered to the receiversalong the RPT.6.1.4 How PIM-SM WorksThe working mechanism of PIM-SM is summarized as follows:z Neighbor discoveryz DR electionz RP discoveryz RPT buildingz Multicast source registrationz Switchover from RPT to SPTz AssertI. Neighbor discoveryPIM-SM uses exactly the same neighbor discovery mechanism as PIM-DM does. Referto “Neighbor discovery”.II. DR electionPIM-SM also uses hello messages to elect a designated router (DR) for a multi-accessnetwork. The elected DR will be the only multicast forwarder on this multi-accessnetwork.A DR must be elected for a multi-access network, no matter this network connects tomulticast sources or to receivers. The DR at the receiver side sends join messages tothe RP; the DR at the multicast source side sends register messages to the RP.
