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Day 12 235Example 12-1Configuring OSPF NetworksR1(config)#router ospf 1R1(config-router)#network 172.16.1.16 0.0.0.15 area 0R1(config-router)#network 192.168.10.0 0.0.0.3 area 0R1(config-router)#network 192.168.10.4 0.0.0.3 area 0R2(config)#router ospf 1R2(config-router)#network 10.10.10.0 0.0.0.255 area 0R2(config-router)#network 192.168.10.0 0.0.0.3 area 0R2(config-router)#network 192.168.10.8 0.0.0.3 area 0R3(config)#router ospf 1R3(config-router)#network 172.16.1.32 0.0.0.7 area 0R3(config-router)#network 192.168.10.4 0.0.0.3 area 0R3(config-router)#network 192.168.10.8 0.0.0.3 area 0Router IDThe router ID plays an important role in OSPF. It is used to uniquely identify each router in the OSPFrouting domain. Cisco routers derive the router ID based on three criteria in the following order:1. Use the IP address configured with the OSPF router-id command.2. If the router ID is not configured, the router chooses the highest IP address of any of itsloopback interfaces.3. If no loopback interfaces are configured, the router chooses the highest active IP address ofany of its physical interfaces.The router ID can be viewed with several commands including show ip ospf interfaces, show ipprotocols, and show ip ospf.Two ways to influence the router ID are to configure a loopback address or configure the routerID. The advantage of using a loopback interface is that, unlike physical interfaces, it cannot fail.Therefore, using a loopback address for the router ID provides stability to the OSPF process.Because the OSPF router-id command is a fairly recent addition to Cisco IOS Software (Release12.0[1]T), it is more common to find loopback addresses used for configuring OSPF router IDs.Example 12-2 shows the loopback configurations for the routers in our topology.Example 12-2Loopback ConfigurationsR1(config)#interface loopback 0R1(config-if)#ip address 10.1.1.1 255.255.255.255R2(config)#interface loopback 0R2(config-if)#ip address 10.2.2.2 255.255.255.255R3(config)#interface loopback 0R3(config-if)#ip address 10.3.3.3 255.255.255.255