4.4.5. OSPF Configuration
Basic Configuration:
Router(config)# router ospf 1
Router(config-router)# router-id 1.1.1.1
Router(config-router)# network 192.168.1.0 0.0.0.255 area 0
Router(config-router)# network 10.0.0.0 0.255.255.255 area 0
Router(config-router)# passive-interface GigabitEthernet0/1
Interface-level Configuration (alternative):
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip ospf 1 area 0
OSPFv3 (IPv6) has no network command. It is enabled only per interface, with the ipv6 keyword in place of ip:
Router(config)# ipv6 unicast-routing
Router(config)# ipv6 router ospf 1
Router(config-rtr)# router-id 1.1.1.1
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ipv6 ospf 1 area 0
Key OSPF Commands:
| Command | Purpose |
|---|---|
network ... area 0 | Enable OSPF on interfaces |
passive-interface | Stop sending Hellos (but still advertise network) |
default-information originate | Advertise a default route into OSPF, only if this router has one in its table (add always to advertise it regardless) |
ip ospf cost | Manually set interface cost |
Verification:
Router# show ip ospf neighbor ! Neighbors and their adjacency state
Router# show ip ospf interface brief ! OSPF interfaces with area, cost, state
Router# show ip ospf interface Gi0/0 ! Timers, network type, cost, DR/BDR
Router# show ip ospf database ! The LSDB (LSAs), not the routing table
Router# show ip route ospf ! OSPF routes actually installed
Router# show ip protocols ! Process, router ID, networks, passive interfaces
Example show ip ospf neighbor output:
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/DR 00:00:32 192.168.1.2 Gi0/0
3.3.3.3 1 FULL/BDR 00:00:35 192.168.1.3 Gi0/0
⚠️ Exam Trap: OSPF uses wildcard masks in the network command, not subnet masks. Wildcard = inverse of subnet mask (0.0.0.255 = 255.255.255.0).
Router roles and route types in output: In single-area OSPF all routers are normally in area 0 (the backbone), so there are no ABRs (routers joining area 0 to another area), no inter-area summarization and no virtual links. A router that injects external routes, by redistribution or default-information originate, is an ASBR and advertises them as Type 5 External LSAs (Type 7 inside an NSSA). In show ip route, O (type intra-area) means the destination is in this router's own area, O IA is inter-area, and O E1/O E2 are external.
