| 6.2 |  | Dynamic Routing Overview |
| 6.2.5 |  | Distance vector routing protocol features |

| Distance vector routing algorithms pass periodic copies of a routing table from router to router. These regular updates between routers communicate topology changes. Distance vector based routing algorithms are also known as Bellman-Ford algorithms. Each router receives a routing table from its directly connected neighbor routers. Router B receives information from Router A. Router B adds a distance vector number (such as a number of hops), which increases the distance vector. Then Router B passes this new routing table to its other neighbor, Router C. This same step-by-step process occurs in all directions between neighbor routers. The algorithm eventually accumulates network distances so that it can maintain a database of network topology information. However, distance vector algorithms do not allow a router to know the exact topology of an internetwork as each router only sees its neighbor routers. Each router that uses distance vector routing begins by identifying its own neighbors. The interface that leads to each directly connected network is shown as having a distance of 0. As the distance vector network discovery process proceeds, routers discover the best path to destination networks based on the information they receive from each neighbor. Router A learns about other networks based on the information that it receives from Router B. Each of the other network entries in the routing table has an accumulated distance vector to show how far away that network is in a given direction. Routing table updates occur when the topology changes. As with the network discovery process, topology change updates proceed step-by-step from router to router. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. The routing tables include information about the total path cost as defined by its metric and the logical address of the first router on the path to each network contained in the table. An analogy of distance vector could be the signs found at a highway intersection. A sign points towards a destination and indicates the distance to the destination. Further down the highway, another sign points toward the destination, but now the distance is shorter. As long as the distance is shorter, the traffic is following the best path.
| Web Links Routing Protocols - Distance Vector http://www.firewall.cx/ index.php?c=distance_vector