Operation Manual – IPv4 RoutingH3C S5500-EI Series Ethernet Switches Chapter 3 OSPF Configuration3-2z Loop-free: Computes routes with the shortest path first (SPF) algorithm accordingto the collected link states, so no route loops are generated.z Area partition: Allows an AS to be split into different areas for ease of managementand the routing information transmitted between areas is summarized to reducenetwork bandwidth consumption.z Equal-cost multi-route: Supports multiple equal-cost routes to a destination.z Routing hierarchy: Supports a four-level routing hierarchy that prioritizes theroutes into intra-area, inter-area, external Type-1, and external Type-2 routes.z Authentication: Supports interface-based packet authentication to guarantee thesecurity of packet exchange.z Multicast: Supports packet multicasting on some types of links.3.1.1 Basic ConceptsI. Autonomous SystemA set of routers using the same routing protocol to exchange routing informationconstitute an Autonomous System (AS).II. OSPF route computationOSPF route computation is described as follows:z Based on the network topology around itself, each router generates Link StateAdvertisements (LSA) and sends them to other routers in update packets.z Each OSPF router collects LSAs from other routers to compose a LSDB (LinkState Database). An LSA describes the network topology around a router, so theLSDB describes the entire network topology of the AS.z Each router transforms the LSDB to a weighted directed graph, which actuallyreflects the topology architecture of the entire network. All the routers have thesame graph.z Each router uses the SPF algorithm to compute a Shortest Path Tree that showsthe routes to the nodes in the autonomous system. The router itself is the root ofthe tree.III. Router IDTo run OSPF, a router must have a Router ID, which is a 32-bit unsigned integer, theunique identifier of the router in the AS.You may assign a Router ID to an OSPF router manually. If no Router ID is specified,the system automatically selects one for the router as follows:z If the loopback interfaces are configured, select the highest IP address amongthem.z If no loopback interface is configured, select the highest IP address amongaddresses of active interfaces on the router.
