年计算机网络考研辅导讲座网络层上.ppt

* Layer 1 of 3: Purpose: This figure continues the concept of how a router using a distance vector protocol generally discovers the best path to destinations from each router neighbor. Emphasize: Layer 1 shows the topology consisting of four networks and three routers. Routing tables inside each router begin with entries for the 0 distance to directly connected networks. * Layer 1 of 3: Emphasize: Layer 2 adds routing entries received some time later about noncontiguous networks that have distances of 1 from the given routers. * Layer 1 of 3: Emphasize: Layer 3 adds the final entries received some time later that have distances of 2 from routers A and C. * Layer 1 of 3: Purpose: This figure continues the concept of how a router using a distance vector protocol generally performs its routing information update process when the network topology changes. Emphasize: This layer shows the bullet point, the router on the right, and, on the right, a topology change; routing tables will need updating to reflect this topology change. * Layer 2 of 3: Emphasize: Layer 2 adds the updated routing table that router A sends out after it processes the topology change. * Layer 3 of 3: Layer 3 adds router B, which receives the updated routing table from router?A. In turn, router B will perform its own process to update its routing table given this new topology update from router A. Distance vector updates occur step by step. Typically, a router sends updates by multicasting its table on each configured port, but other methods, such as sending the table only to preconfigured neighbors, are employed by some routing algorithms. Multicast is used by the RIP2, OSPF, and EIGRP routing protocols. RIP and IGRP use broadcast. The routing table can be sent routinely and periodically, or whenever a change in the topology is discovered. Updates sent when changes occur are called triggered updates. * Purpose: This figure introduces the link-state routing algorithm, the second of th