Distance vector algorithms are exam

Distance vector algorithms are examples of dynamic routing protocols. These algorithms allow each device in the network to automatically build and maintain a local IP routing table.
The principle behind distance vector routing is simple. Each router in the internetwork maintains the distance or cost from itself to every known destination. This value represents the overall desirability of the path. Paths associated with a smaller cost value are more attractive to use than paths associated with a larger value. The path represented by the smallest cost becomes the preferred path to reach the destination.

This information is maintained in a distance vector table. The table is periodically advertised to each neighboring router. Each router processes these advertisements to determine the best paths through the network.
The main advantage of distance vector algorithms is that they are typically easy to implement and debug. They are very useful in small networks with limited redundancy. However, there are several disadvantages with this type of protocol:
_ During an adverse condition, the length of time for every device in the network to produce an accurate routing table is called the convergence time. In large, complex internetworks using distance vector algorithms, this time can be excessive. While the routing tables are converging, networks are susceptible to inconsistent routing behavior. This can cause routing loops or other types of unstable packet forwarding.
_ To reduce convergence time, a limit is often placed on the maximum number of hops contained in a single route. Valid paths exceeding this limit are not usable in distance vector networks.
_ Distance vector routing tables are periodically transmitted to neighboring devices. They are sent even if no changes have been made to the contents of the table. This can cause noticeable periods of increased utilization in reduced capacity environments.
Enhancements to the basic distance vector algorithm have been developed to reduce the convergence and instability exposures. We describe these enhancements in 5.3.5, “Convergence and counting to infinity” on page 185.
RIP is a popular example of a distance vector routing protocol.