An OSPF (Open Shortest Path First) router advertises its link state (the state of its interfaces and neighbour routers) by flooding LSAs (Link State Advertisements) throughout the area to which it belongs. The area is a grouping of common OSPF networks and hosts. To know which networks are in the area, a router sends out advertisements describing what links it has in its routing table. If two routers have different links, they exchange the missing links until both routers have the same networks in the routing table. You can create the topology by using the link state information.
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Connect to the router using a console cable and the HyperTerminal program. Attach one end of the console cable to the PC and the other end to the router's console port. Next, start the router and start the HyperTerminal program on the PC. Click the "Start" icon on the lower-left side of the screen and then go to "Programs" and click the HyperTerminal selection. When the router screen comes up via HyperTerminal, type "enable" on the screen. (The router's editor will launch automatically upon router start-up.)
Type "show ip route" to view the routing table. This command gives information about the state of the routing table. Next, type "show ip OSPF database." This command presents information about the OSPF link-state database. For example, the different areas are identified: area 0, area 1 and so on. Furthermore, the network IP address of each network associated with each area is present. From this information, you now can create an OSPF topology.
Collect the IP network address from each area. Copy the information to a spreadsheet or document. You now have the topology information about the OSPF configuration.
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