Data Communications Networking Devices - 4th Ed.pdf

5.2 ROUTERS ____________________________________________________________________ 579The preceding example provides a general indication of how RIP enables nodesto learn the topology of a network. In addition, if a link should fail, the conditioncan be easily compensated for as, similar to bridge table entries, those of routers arealso time-stamped and the periodic transmission of distance vector informationwould result in a new route replacing the previously computed one.Link state protocolAlink state routing protocol addresses the traf®c problem associated with largenetworks that use a vector distance routing protocol. It does this by transmittingrouting information only when there is a change in one of its links. Aseconddifference between vector distance and link state protocols concerns the manner inwhich a route is selected when multiple routes are available between destinations.For a vector distance protocol the best path is the one that has fewest intermediaterouters or hops between destinations. In comparison, a link state protocol can usemultiple paths to provide traf®c balancing between locations. In addition, a linkstate protocol permits routing to occur based upon link delay, capacity andreliability. This provides the network manager with the ability to specify a varietyof route development situations.SPF algorithmsLink state routing protocols are implemented through the use of a class ofalgorithms known as Shortest Path First SPF). Unfortunately, the name associatedwith this class of algorithms is a misnomer as routing is not based upon theshortest path.The use of SPF algorithms requires each participating router to have completeknowledge of the network topology. Each router participating in an SPF algorithmthen performs two tasksÐstatus testing of neighboring routers and periodicallytransmitting link status information to other routers.To test neighboring routers a short message is periodically transmitted. If theneighbor replies, the link is considered to be up. Otherwise, the absence of a replyafter a prede®ned period of time indicates that the link is down.To provide link status information each router will periodically broadcast amessage which indicates the status of each of its links. Unlike the vector distanceprotocol in which routes are speci®ed, an SPF link status message simply indicateswhether or not communications are possible between pairs of routers. Usinginformation in the link status message, routers are able to update their networkmap.In comparison to vector distance protocols in which tables are required to beexchanged, link state protocols such as SPF algorithms exchange a much lowervolume of information in the form of link status queries and replies. Then, SPFparticipating routers simply broadcast a status of each of its links that other routersuse to update their network map. This routing technique permits each router tocompute routes independently of other routers and eliminates the potential fortable ¯ooding that can occur when a vector state protocol is used to interconnect alarge number of networks.