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Embed code for: 23.4 Distance Vector and Link-State Routing Protocols
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23.4 Distance Vector and Link-State Routing Protocols
Distance Vector and LinkState Routing Protocols Section 23: Implementing Dynamic Routing Within an AS, most IGP routing can be classified as conforming to one of these algorithms: Distance vector: The distance vector routing approach determines the direction (vector) and distance (a metric), such as the hop count in the case of RIP to any link in the internetwork. Pure distance vector protocols periodically send complete routing tables to all connected neighbors. This mode of operation is important for defining a distance vector routing protocol. In large networks, these routing updates can become enormous, causing significant traffic on the links. The only information that a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Different distance vector routing protocols may use different kinds of metrics. Distance vector routing protocols do not have an actual map of the network topology. For a router, the view of the network is based on the information that is provided by its neighbors. Linkstate: The linkstate approach, which uses the SPF algorithm, creates an abstraction of the exact topology of the entire internetwork, or at least of the partition in which the router is situated. A linkstate router uses the linkstate information to create a topology map and to select the best path to all destination networks in the topology. All linkstate routers use an identical "map" of the network and calculate the shortest paths to reach the destination networks in relation to where they are on that map. Unlike their distance vector counterparts, complete routing tables are not exchanged periodically. Instead eventbased "triggered" updates that contain only specific linkstate information are sent. Periodic keepalives that are small and efficient, in the form of hello messages, are exchanged between directly connected neighbors to establish and maintain reachability to that neighbor. Advanced distance vector: The advanced distance vector approach combines aspects of the linkstate and distance vector algorithms. EIGRP is a Cisco proprietary routing protocol that combines the advantages of linkstate and distance vector routing protocols. EIGRP may act like a linkstate routing protocol, because it uses a Hello protocol to discover neighbors and form neighbor relationships and because only partial updates are sent when a change occurs. However, it is still based on the key distance vector routing protocol principle that information about the rest of the network is learned from directly connected neighbors. There is no single best routing algorithm for all internetworks. All routing protocols provide the information differently. Click the Play Button to watch a short video about dynamic routing protocols: distance vector versus link state. Up Next: Routing Decision Criteria