Davide Cherubini - PhD Thesis - UniCA Eprints

2.1 Understanding IS-ISOSI also defines an exterior (interdomain) routing protocol, that is the InterdomainRouting Protocol (IDRP 1 ), designed to exchange routing informationbetween autonomous systems.How does it work?IS-IS and OSPF, are the most used IP routing protocols within the backbonenetworks and the router packet-forwarding decision is taken using only the destinationaddress specified in the packet header.In order to determine the routes to all reachable destinations, IS-IS routers exchangelink state information with their nearest neighbours. These topologyinformation together with a metric value associated to every link, are floodedthroughout the AS, so that every router within the AS has a complete knowledgeof the topology of the AS.Starting from the knowledge of the full topology of the “network”(the socalledlink state database), each router routes the traffic toward a destinationnode, along shortest paths. In fact, based on the graph, each router constructsa tree consisting of the shortest paths to all destinations, and with itself as theroot.The calculation of the paths in IS-IS, and then the building of each routingtable, is based on the “shortest path first” algorithm developed by Edgar W.Dijkstra [32] (see figure 2.1(a)). If the network topology changes, the protocolrecalculates the routes, using the Dijkstra’s algorithm.The Dijkstra’s Algorithm• C(i, j) = cost of link connecting nodes i and j• D(v) = current cost value of the path toward the destination node v• p(v) = predecessor node along the path toward v• N = set of destination nodes with a known shortest path1 IETF introduced the Border Gateway Protocol (BGP) as exterior/border routing protocolthat is widely used in in backbone networks as well as in the Internet.5