Single-Area OSPFv2 Configuration

Enabling OSPF is a two-decision job. The process is created with router ospf <process-id>, and that ID is locally significant — pick any number, it need not match neighbours. The second decision, which interfaces participate, has two valid styles that coexist happily.

R1(config)# router ospf 1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 0
R1(config-router)# network 10.1.12.0 0.0.0.3 area 0

The statement does not advertise a subnet directly. It is a match rule: OSPF examines every local interface, and any interface whose address falls within the given address and wildcard is enabled for OSPF in the stated area. So network 10.1.1.0 0.0.0.255 area 0 enables OSPF on whichever local interface sits in 10.1.1.0/24.

R1(config)# interface GigabitEthernet0/1
R1(config-if)# ip ospf 1 area 0

This form is unambiguous — you can see OSPF is on this interface without reverse-engineering a wildcard. Prefer it when you want zero doubt about which interfaces run OSPF; prefer network statements when one line cleanly covers a range. Either way, area 0 is the backbone, and every router in a single-area design lives in it.

OSPF chooses its Router ID at process start: a manual router-id first, else the highest loopback address, else the highest active physical address. Letting it default means an interface flap or a new loopback can silently change the router's identity, and duplicate RIDs across routers cause real faults. Set it explicitly with router-id 1.1.1.1 inside the process.

On a LAN facing end hosts there is no OSPF neighbour to talk to, yet sending Hellos out that interface wastes CPU and — worse — invites a rogue router to peer with you. passive-interface GigabitEthernet0/0 stops Hellos on that port while still advertising its connected network into OSPF.

R1(config)# router ospf 1
R1(config-router)# router-id 1.1.1.1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 0
R1(config-router)# network 10.1.12.0 0.0.0.3 area 0
R1(config-router)# passive-interface GigabitEthernet0/0
R1(config-router)# end

R1# show ip protocols
Routing Protocol is "ospf 1"
  Router ID 1.1.1.1
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Routing for Networks:
    10.1.1.0 0.0.0.255 area 0
    10.1.12.0 0.0.0.3 area 0
  Passive Interface(s):
    GigabitEthernet0/0
  Routing Information Sources:
    Gateway         Distance      Last Update
    2.2.2.2              110      00:03:11

show ip protocols is the fastest confirmation that the configuration is right: it echoes the Router ID, the exact network statements with their wildcards, the passive interfaces, and the neighbours it is learning from. Cross-check which interfaces are actually running OSPF with show ip ospf interface brief.

Interface    PID   Area    IP Address/Mask    Cost  State Nbrs F/C
Gi0/0        1     0       10.1.1.1/24        1     DR    0/0
Gi0/1        1     0       10.1.12.1/30       1     P2P   1/1

Gi0/1 shows one neighbour Full over Count — the transit adjacency. Gi0/0 shows 0/0 because it is passive: it is in OSPF and advertised, but forms no adjacency. That 0/0 on a LAN is expected, not a fault, and reading it as one sends people hunting for a problem that does not exist.

• Enable with network <address> <wildcard> area <id>, a match rule, or per interface with ip ospf <pid> area <id>.
• The wildcard is the inverse of the subnet mask: /24 gives 0.0.0.255, /30 gives 0.0.0.3.
• Router ID: manual, then highest loopback, then highest active address. Changes need clear ip ospf process.
• passive-interface stops Hellos on a LAN but still advertises the connected route — never use it on a transit link.
• Verify the configuration with show ip protocols and show ip ospf interface brief before looking at routes.