HP 6125G HP 6125G & 6125G/XG Blade Switches IP Multicast Configuration - Page 116

3. If a tie exists in route metric to the source, the router with a higher IP address of the downstream interface wins. PIM-SM overview PIM-DM uses the flood-and-prune principle to build SPTs for multicast data distribution. Although an SPT has the shortest path, it is built with a low efficiency. Therefore, the PIM-DM mode is not suitable for largeand medium-sized networks. PIM-SM is a type of sparse mode multicast protocol. It uses the pull mode for multicast forwarding and is suitable for large-sized and medium-sized networks with sparsely and widely distributed multicast group members. The basic implementation of PIM-SM is as follows: • PIM-SM assumes that no hosts need to receive multicast data. In the PIM-SM mode, routers must specifically request a particular multicast stream before the data is forwarded to them. The core task for PIM-SM to implement multicast forwarding will build and maintain rendezvous point trees (RPTs). An RPT is rooted 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 the receivers. • When a receiver is interested in the multicast data addressed to a specific multicast group, the router connected to this receiver sends a join message to the RP that corresponds to that multicast group. The path along which the message goes hop by hop to the RP forms a branch of the RPT. • When a multicast source sends multicast streams to a multicast group, the source-side designated router (DR) first registers the multicast source with the RP by sending register messages to the RP by unicast until it receives a register-stop message from the RP. The arrival of a register message at the RP triggers the establishment of an SPT. Then, the multicast source sends subsequent multicast packets along the SPT to the RP. After reaching the RP, the multicast packet is duplicated and delivered to the receivers along the RPT. NOTE: Multicast traffic is duplicated only where the distribution tree branches, and this process automatically repeats until the multicast traffic reaches the receivers. The working mechanism of PIM-SM is summarized as follows: • Neighbor discovery • DR election • RP discovery • RPT building • Multicast source registration • Switchover to SPT • Assert Neighbor discovery PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For more information, see "Neighbor discovery." DR election PIM-SM also uses hello messages to elect a DR for a multi-access network (such as Ethernet). The elected DR will be the only multicast forwarder on this multi-access network. 105