Operation Manual – MulticastH3C S7500 Series Ethernet Switches Chapter 7 PIM Configuration7-5network bandwidth that the data packets and control messages occupy is reduced, andthe processing overhead of the router is also reduced.In the receiving end, the router connected to the receiver sends a join message to theRP corresponding to the multicast group. The join message reaches the root (namely,RP) after passing each router. The passed paths become the branches of therendezvous point tree (RPT).If the sending end wants to send data to a multicast group, the first hop router will senda register message to the RP. When the register message reaches RP, the source treeestablishing is triggered. Then the multicast source sends the data to RP. When thedata reaches RP, the multicast packets are replicated and sent to the receiver over theRPT. Replication happens only in the branch of the RPT. The procedure is repeatedautomatically until the multicast packets reach the receiver.PIM-SM is independent of any specific unicast routing protocol. Instead, it performsRPF check based on the existing unicast routing table.7.1.4 Work Mechanism of PIM-SMThe working procedure of PIM-SM is:z Neighbor discoveryz DR electionz RP discoveryz RPT establishingz Multicast source registrationz RPT-to-SPT switchoverI. Neighbor discoveryThe neighbor discovery mechanism is the same as described in PIM-DM. It is alsoimplemented through Hello messages sent between each router.II. DR electionWith the help of Hello messages, DR can be elected for the shared network, such asEthernet. DR will be the unique multicast information forwarder in the network. In eitherthe network connected to the multicast source S or the network connected to thereceiver, DR must be elected only if the network is a shared network. The DR in thereceiving end sends join messages to RP and the DR in the multicast source sidesends register messages to the RP, as shown in Figure 7-3:
