HP 6125G HP 6125G & 6125G/XG Blade Switches IP Multicast Configuration - Page 114

Neighbor discovery In a PIM domain, a PIM router discovers PIM neighbors, maintains PIM neighboring relationships with other routers, and builds and maintains SPTs by periodically multicasting hello messages to all other PIM routers (224.0.0.13) on the local subnet. NOTE: Every PIM-enabled interface on a router sends hello messages periodically, and thus learns the PIM neighboring information pertinent to the interface. SPT building The process of building an SPT is the flood-and-prune process. 1. In a PIM-DM domain, when a multicast source S sends multicast data to multicast group G, the multicast packet is first flooded throughout the domain. The router first performs RPF check on the multicast packet. If the packet passes the RPF check, the router creates an (S, G) entry and forwards the data to all downstream nodes in the network. In the flooding process, an (S, G) entry is created on all the routers in the PIM-DM domain. 2. Then, nodes without receivers downstream are pruned. A router having no receivers downstream sends a prune message to the upstream node to "tell" the upstream node to delete the corresponding interface from the outgoing interface list in the (S, G) entry and stop forwarding subsequent packets addressed to that multicast group down to this node. An (S, G) entry contains the multicast source address S, multicast group address G, outgoing interface list, and incoming interface. For a given multicast stream, the interface that receives the multicast stream is referred to as "upstream," and the interfaces that forward the multicast stream are referred to as "downstream." A prune process is first initiated by a leaf router. As shown in Figure 36, a router without any receiver attached to it (the router connected with Host A, for example) sends a prune message. This prune process goes on until only necessary branches are left in the PIM-DM domain. These branches constitute the SPT. Figure 36 SPT building 103