Layer 3 Routing | 257OSPF ConfigurationFor larger networks, Open Shortest Path First (OSPF) is generally used in preference to RIP. OSPF offersseveral benefits to the administrator of a large and/or complex network:• Less network traffic:• Routing table updates are sent only when a change has occurred.• Only changed part of the table is sent.• Updates are sent to a multicast, not a broadcast, address.• Hierarchical management, allowing the network to be subdividedThe top level of the hierarchy of an OSPF network is known as an autonomous system (AS) or routingdomain, and is a collection of networks with a common administration and routing strategy. The AS isdivided into areas: intra-area routing is used when a source and destination address are in the same area,and inter-area routing across an OSPF backbone is used when they are not. An inter-area routercommunicates with border routers in each of the areas to which it provides connectivity.The S-Series operating as a router, and running OSPF, will determine the best route, using the assignedcost and the type of the OSPF route. The order for choosing a route, if more than one type of route exists,is as follows:1. Intra-area2. Inter-area3. External Type 1: The route is external to the AS.4. External Type 2: The route was learned from other protocols such as RIP.OSPF Configuration ExamplesThe examples in this section show you how to configure the S-Series first as an inter-area router and thenas a border router. They show two areas, each with its own border router connected to one inter-area router.Configuring OSPF on an S-Series operating as an inter-area routerThe first diagram shows a network segment with an inter-area router connecting areas 0.0.0.2 and 0.0.0.3.The example shows the commands used to configure the S-Series as the inter-area router in the diagram byenabling OSPF on port 0/2 in area 0.0.0.2 and port 0/3 in area 0.0.0.3.
