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11.5 Dynamic Routing Protocols
Dynamic Routing Protocols Section 11: Exploring the Functions of Routing A routing protocol is a set of processes, algorithms, and messages by which routers dynamically share their routing information. Examples of routing protocols include OSPF, EIGRP, RIP, and ISIS. Routers that are running routing protocols exchange routing update messages in order to keep their routing tables updated. As a router that is running a routing protocol becomes aware of changes to the network, it passes the information on to other routers that are running the same routing protocol. When a router receives information about new or changed routes, it updates its own routing table and, in turn, passes the information to other routers. In this way, all routers maintain accurate routing tables that are updated dynamically and can learn about routes to remote networks that are many hops way. Routing protocols not only enable routers to learn about remote networks and to quickly adapt whenever there is a change in the topology; they also enable routers to choose the best path to destination networks. Although there may be multiple paths to a given destination, a routing table holds only one path to any given destination. A routing table holds only one entry per network. To determine the best path to any given destination, routing protocols use a number called a metric. The metrics that are used by the various routing protocols differ, depending on the design of the routing algorithm that is used. The routing algorithm that is used by the protocol generates a metric for each path through the network. Metrics can be based on either a single characteristic or on several characteristics of a path. Sophisticated routing protocols can base route selection on multiple metrics, combining them into a single metric. Typically, the lower the metric value, the better the path. These metrics are most commonly used by routing protocols: Bandwidth: The data capacity of a link (the connection between two network devices) Delay: The length of time that is required to move a packet along each link from the source to the destination. The delay depends on the bandwidth of intermediate links, port queues at each router, network congestion, and physical distance. Cost: An arbitrary value that is assigned by a network administrator, usually based on bandwidth, administrator preference, or other measurement, such as load or reliability Hop count: The number of routers that a packet must travel through before reaching its destination Click the Play button to watch a short video about dynamic routing protocols. Up Next: Path Determination