TP-Link T3700G-28TQ T3700G-28TQ V1 UG - Page 238
Neighbor Discovering, SPT Building
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Generally speaking, the packet forwarding tree in the dense mode is Source Tree (a forwarding tree with multicast source as the root, and multicast members as the branches). As the Source Tree is a forwarding tree with the shortest path from the multicast source to the receivers, it is also called Shortest Path Tree (SPT). The working process of PIM DM can be summarized as follows: Neighbor Discovering SPT Building Grafting Neighbor Discovering In PIM domain, routers periodically sends PIM Hello packets to all the PIM routers with the multicast address 224.0.0.13 to discover PIM neighbors, maintain the PIM neighboring relationships between the routers, thus to build and maintain the SPT. SPT Building The SPT building process is also the "flood-and-prune" process: (1) When the multicast source S is sending multicast packets to multicast group G in PIM DM domain, the multicast packets will firstly be flooded: After the multicast packet passes the router's RPF check, the router will create a corresponding (S, G) entry and forward this packet to all the nodes downstream in the network. All the routers in the PIM DM domain will create the (S, G) entry after this flooding process. (2) Then branches without receivers downstream are pruned. The downstream branches with no receivers will send prune message to the upstream node to delete the corresponding interface in the output interface list of the multicast forwarding entry (S, G), and the multicast packets will no longer forwarded to the pruned branches. Note: The entry (S, G) contains the multicast source address S, the multicast group G, the list of output interfaces and input interfaces. The prune process is initiated by the leaf router, as shown in Figure 11-11, the leaf router without receivers (such as the router directly connected to Host A) performs the prune actively, and the prune process will last until there are only necessary branches in the PIM DM domain. These branches form the SPT. 227