Back to OSPF Guide Single-area OSPF lab

Single-Area OSPF Configuration Lab

This lab configures OSPF Area 0 on three Cisco routers connected in a triangle. You will first assign interface IP addresses, then enable OSPF with router IDs and wildcard-mask network statements.

Area 0 R1 R2 R3 Wildcard Masks Router ID Neighbor Verification

Single-Area OSPF Configuration Lab: Quick Summary

All three routers participate in OSPF process 1 and belong to Area 0. The links are simple /24 Ethernet networks: R1-R2 uses 192.168.12.0/24, R1-R3 uses 192.168.13.0/24, and R2-R3 uses 192.168.23.0/24.

OSPF process 1
OSPF area 0
Subnet mask 255.255.255.0
Wildcard mask 0.0.0.255

Single-Area OSPF Configuration Lab: Table of Contents

  1. Topology and Addressing
  2. Base Interface Configuration
  3. OSPF Area 0 Configuration
  4. How the Network Command Works
  5. Verification Commands
  6. Troubleshooting Checklist

1. Topology and Addressing

The topology uses three point-to-point style Ethernet links. Each router has two active interfaces, and every link is advertised into OSPF Area 0.

OSPF triangle topology connecting routers R1, R2 and R3 across three subnets
R1, R2 and R3 form a triangle. This gives each router two OSPF neighbors after all links are configured.
Link Network Router Interface IP Address
R1 to R2 192.168.12.0/24 R1 Ethernet0/1
R2 Ethernet0/1		 192.168.12.1
192.168.12.2
R1 to R3 192.168.13.0/24 R1 Ethernet0/2
R3 Ethernet0/1		 192.168.13.1
192.168.13.3
R2 to R3 192.168.23.0/24 R2 Ethernet0/2
R3 Ethernet0/2		 192.168.23.2
192.168.23.3

2. Base Interface Configuration

Start by configuring hostnames, interface IP addresses and enabling the router interfaces with no shutdown. This step only builds IP connectivity; OSPF is added in the next section.

R1 Configuration

hostname R1
!
interface Ethernet0/1
 ip address 192.168.12.1 255.255.255.0
 no shutdown
!
interface Ethernet0/2
 ip address 192.168.13.1 255.255.255.0
 no shutdown
!
end

R2 Configuration

hostname R2
!
interface Ethernet0/1
 ip address 192.168.12.2 255.255.255.0
 no shutdown
!
interface Ethernet0/2
 ip address 192.168.23.2 255.255.255.0
 no shutdown
!
end

R3 Configuration

hostname R3
!
interface Ethernet0/1
 ip address 192.168.13.3 255.255.255.0
 no shutdown
!
interface Ethernet0/2
 ip address 192.168.23.3 255.255.255.0
 no shutdown
!
end
Before OSPF: Verify directly connected links with show ip interface brief and simple pings between neighboring routers.

3. OSPF Area 0 Configuration

Enable OSPF with router ospf 1, set a stable router ID, and advertise each connected /24 network into Area 0 using a wildcard mask.

R1 OSPF

configure terminal
router ospf 1
 router-id 1.1.1.1
 network 192.168.12.0 0.0.0.255 area 0
 network 192.168.13.0 0.0.0.255 area 0
end

R2 OSPF

configure terminal
router ospf 1
 router-id 2.2.2.2
 network 192.168.12.0 0.0.0.255 area 0
 network 192.168.23.0 0.0.0.255 area 0
end

R3 OSPF

configure terminal
router ospf 1
 router-id 3.3.3.3
 network 192.168.13.0 0.0.0.255 area 0
 network 192.168.23.0 0.0.0.255 area 0
end
  • Process ID: The value 1 is locally significant. It does not need to match between routers, but keeping it consistent makes the lab easier to read.
  • Router ID: Manually setting router IDs keeps neighbor tables predictable: R1 is 1.1.1.1, R2 is 2.2.2.2 and R3 is 3.3.3.3.
  • Area: All networks are in Area 0, so this is a single-area OSPF design.

4. How the Network Command Works

The OSPF network statement selects local interfaces that match the address and wildcard mask, then advertises those connected networks into the chosen OSPF area.

Command Part Meaning Example
network 192.168.12.0 Matches the R1-R2 subnet. R1 Ethernet0/1 and R2 Ethernet0/1
0.0.0.255 Wildcard mask for a /24 subnet. It is the inverse of 255.255.255.0. Match all addresses from .0 through .255
area 0 Places the matched interface into OSPF Area 0. Backbone/single-area lab
Simple rule: A subnet mask tells the router which bits are network bits. A wildcard mask tells OSPF which bits to ignore while matching interfaces.

5. Verification Commands

After the OSPF commands are applied, each router should form two neighbor adjacencies because every router connects to the other two routers in the triangle.

show ip ospf neighbor

Confirms neighbor IDs, adjacency state, dead timer, neighbor address and interface. The expected state is FULL.

show ip protocols

Confirms OSPF process 1, router ID, networks advertised and administrative distance 110.

show ip route ospf

Shows routes learned through OSPF. R1 should learn 192.168.23.0/24, R2 should learn 192.168.13.0/24, and R3 should learn 192.168.12.0/24.

show ip ospf interface brief

Lists OSPF-enabled interfaces, process ID, area, cost and neighbor count in a compact format.

Expected Neighbor Result

R1#show ip ospf neighbor

Neighbor ID     State           Address         Interface
2.2.2.2         FULL            192.168.12.2    Ethernet0/1
3.3.3.3         FULL            192.168.13.3    Ethernet0/2

6. Troubleshooting Checklist

If a neighbor does not reach FULL state, check the basics before changing the routing design.

  • Interfaces: Confirm every Ethernet interface is up/up with show ip interface brief.
  • IP addressing: Make sure both sides of each link are in the same /24 subnet.
  • Wildcard masks: Use 0.0.0.255 for these /24 networks, not 255.255.255.0.
  • Timers and area: OSPF neighbors must agree on area ID, hello/dead timers, authentication and network type.
  • Router IDs: Router IDs must be unique. If you change a router ID after OSPF starts, clear the process or reload the router.