Operation Manual – Routing ProtocolH3C S7500 Series Ethernet Switches Chapter 4 OSPF Configuration4-2z Each OSPF router maintains a link state database (LSDB), which describes thetopology of the whole AS. Based on the network topology around itself, eachrouter generates link state advertisements (LSAs) and sends them to otherrouters in update packets. The LSAs a router receives from other routers formthe LSDB of the router.z An LSA describes the network topology around a router, whereas an LSDBdescribes the network topology of the whole network. Routers can easilytransform the LSDB to a weighted and directed map, which reflects the topologyof the whole network. All routers have the same map.z Each router uses the shortest path first (SPF) algorithm to calculate the shortestpath tree that shows the routes to the nodes in the autonomous system. Therouter itself is the root of the tree. External routes are leaf nodes, which aremarked by the routers from which they are advertised to record informationoutside the AS. Each router maintains a different routing table.Furthermore, to enable individual routers to broadcast their local status information(such as information about available interface and reachable neighbor) to the wholeAS, routers in the AS must establish adjacencies among them. In this case, a routechange on any router will result in multiple transmissions, which are unnecessary andbandwidth consuming. To solve this problem, designated router (DR) and backupdesignated router (BDR) are defined in OSPF. For details about DR and BDR, seesection 4.1.4 III. "DR and BDR".OSPF supports interface-based packet authentication to guarantee the security ofroute calculation. In addition, it transmits and receives packets in multicast (224.0.0.5and 224.0.0.6).4.1.3 Basic OSPF ConceptsI. Router IDTo run OSPF, a router must have a router ID. A router ID can be configured manually.If no router ID is configured, the system will automatically select an IP address fromthe IP addresses of the interfaces as the router ID. A router ID is selected in thefollowing way: if loopback interface addresses are configured, the system choosesthe latest configured IP address as the router ID; if no loopback interface is configured,the first configured IP address among the IP addresses of other interfaces will be therouter ID.II. DR and BDRFor details, see section 4.1.4 III. "DR and BDR".III. AreaIf all the routers on an ever-growing huge network run OSPF, the large number ofrouters will result in an enormous LSDB, which will consume an enormous storage
