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* Purpose: This figure introduces the distance vector routing algorithm, the first of the classes of routing protocols, and outlines how it operates. Emphasize: Distance vector algorithms do not allow a router to know the exact topology of an internetwork. This information is somewhat analogous to the information found on signs at a highway intersection. A sign points toward a road leading away from the intersection and indicates the distance to the destination. Further down the highway, another sign also points toward the destination, but now the distance to the destination is shorter. As long as each successive point on the path shows that the distance to the destination is successively shorter, the traffic is following the best path. * Layer 3 of 3 Emphasize: Layer 3 adds the final entries received some time later that have distances of 2 from routers A and C. * 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. * Slide 1 of 4 Purpose: This figure describes the first of the general problems that a distance vector protocol could face without the corrective influence of some countermeasure. Emphasize: Layer 1 shows the original state of the network and routing tables. All routers have consistent knowledge and correct routing tables. In this example, the cost function is hop count, so th
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