Ubigate iBG2016 Configuration Guide© SAMSUNG Electronics Co., Ltd. 105CHAPTER 5. OSPFThe Open Shortest Path First(OSPF) protocol is an interior gatewayprotocol(IGP) that routes packets within a single autonomous system(AS).OSPF uses link-state information to make routing decisions, making routecalculations using the shortest path first(SPF) algorithm(also referred to as theDijkstra algorithm). Each router running OSPF floods link-state advertisementsthroughout the AS that contain information about that router’s attached interfacesand routing metrics. Each router takes the information in these link-stateadvertisements and creates a complete routing table for the network.OSPF was designed for the TCP/IP environment and as a result explicitlysupports IP sub netting and the tagging of externally derived routinginformation. OSPF also provides for the authentication of routing updates.OSPF routes IP packets based solely on the destination IP address contained inthe IP packet header. OSPF quickly detects topological changes, such as whenrouter interfaces become unavailable, and calculates new loop-free routesquickly and with a minimum of routing overhead traffic.Each interface running OSPF is assigned a cost, which is a unitless numberbased on factors such as throughput, round-trip time, and reliability, which areused to determine how easy or difficult it is to reach a destination. If two ormore routes to a destination have the same cost, OSPF distributes trafficequally among the routes, a process that is called load balancing.Each router maintains a database that describes the topology of the AS.Each OSPF router has an identical topological database so that all routers inthe area have a consistent view of the network. All routers maintainsummarized topologies of other areas within an AS. Each router distributesinformation about its local state by flooding link-state advertisementsthroughout the AS. When the AS topology changes, OSPF ensures that thecontents of all routers’ topological databases converge quickly.
