Data Communications Networking Devices - 4th Ed.pdf

580 ________________________________________________ LAN INTERNETWORKING DEVICESOperation exampleTo illustrate the operation of a link state routing protocol let us return to theinternetwork con®guration previously illustrated in Figure 5.16. Figure 5.23indicates the initial network map for router R1. This map lists the destination of allnetworks on the internet from router R1, their distance and route. Note that ifmultiple routes exist to a destination each route is listed, as doing so de®nes acomplete network topology as well as allowing alternate routes to be selected if linkstatus information indicates that one or more routes cannot be used.Let us assume that at a particular point in time the link status messages generatedby the routers in the internet are as indicated in Figure 5.24. As indicated in Figure5.16, L1 represents the link connecting routers R1 and R2, L2 connects routers R2and R3, L3 connects routers R1 and R3, L4 connects routers R3 and R4, while L5connects routers R1 and R4. Note that both routers R2 and R3 determined that linkL2 is down. Using this information Router R1 would then update the statuscolumn for its network map. Since link L2 is down, all routes that require a dataFigure 5.23Router R1 initial link state network mapFigure 5.24Link status messages