How to Configure Single-Area OSPF on Cisco Routers (Step-by-Step)

Step 1: Configure IP Addresses

R1

R1(config)# interface loopback 0
R1(config-if)# ip address 1.1.1.1 255.255.255.255
R1(config-if)# description Router ID

R1(config)# interface gi0/0
R1(config-if)# ip address 10.1.12.1 255.255.255.252
R1(config-if)# no shutdown

R2

R2(config)# interface loopback 0
R2(config-if)# ip address 2.2.2.2 255.255.255.255

R2(config)# interface loopback 1
R2(config-if)# ip address 192.168.1.1 255.255.255.0
R2(config-if)# description User Network

R2(config)# interface gi0/0
R2(config-if)# ip address 10.1.12.2 255.255.255.252
R2(config-if)# no shutdown

R2(config)# interface gi0/1
R2(config-if)# ip address 10.1.23.2 255.255.255.252
R2(config-if)# no shutdown

R3

R3(config)# interface loopback 0
R3(config-if)# ip address 3.3.3.3 255.255.255.255

R3(config)# interface gi0/1
R3(config-if)# ip address 10.1.23.3 255.255.255.252
R3(config-if)# no shutdown

Verify connectivity:

R1# ping 10.1.12.2
R2# ping 10.1.23.3

Step 2: Enable OSPF and Configure Router ID

R1

R1(config)# router ospf 1
R1(config-router)# router-id 1.1.1.1

R2

R2(config)# router ospf 1
R2(config-router)# router-id 2.2.2.2

R3

R3(config)# router ospf 1
R3(config-router)# router-id 3.3.3.3

What we did:

• router ospf 1 - Start OSPF process 1 (number is locally significant)
• router-id x.x.x.x - Manually set Router ID

Step 3: Advertise Networks with Network Statements

Understanding Network Statements

Format:

network [network-address] [wildcard-mask] area [area-id]

What it does:

• Matches interfaces whose IP addresses fall within the specified range
• Enables OSPF on those interfaces
• Assigns them to the specified area

Wildcard Masks Explained

A wildcard mask is the inverse of a subnet mask.

Rule:

• 0 = must match
• 255 = don't care

Examples:

Shortcut:
Subtract each octet of the subnet mask from 255 to get the wildcard mask.

Example: 255.255.255.0

• 255 - 255 = 0
• 255 - 255 = 0
• 255 - 255 = 0
• 255 - 0 = 255
• Wildcard = 0.0.0.255

Configure Network Statements

R1

R1(config-router)# network 1.1.1.1 0.0.0.0 area 0
R1(config-router)# network 10.1.12.0 0.0.0.3 area 0

What this does:

• First statement: Enable OSPF on Loopback0 (exact match for 1.1.1.1)
• Second statement: Enable OSPF on Gi0/0 (10.1.12.1 falls in 10.1.12.0/30 range)

R2

R2(config-router)# network 2.2.2.2 0.0.0.0 area 0
R2(config-router)# network 192.168.1.0 0.0.0.255 area 0
R2(config-router)# network 10.1.12.0 0.0.0.3 area 0
R2(config-router)# network 10.1.23.0 0.0.0.3 area 0

R3

R3(config-router)# network 3.3.3.3 0.0.0.0 area 0
R3(config-router)# network 10.1.23.0 0.0.0.3 area 0

Alternative: Match All Interfaces

If you want to enable OSPF on all interfaces:

Router(config-router)# network 0.0.0.0 255.255.255.255 area 0

⚠️ Warning: This enables OSPF on every interface. Use carefully.

Step 4: Verify OSPF Neighbors

Check neighbor adjacencies:

R1# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2         1     FULL/  -        00:00:35    10.1.12.2       GigabitEthernet0/0

R2# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1         1     FULL/  -        00:00:34    10.1.12.1       GigabitEthernet0/0
3.3.3.3         1     FULL/  -        00:00:38    10.1.23.3       GigabitEthernet0/1

R3# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2         1     FULL/  -        00:00:36    10.1.23.2       GigabitEthernet0/1

What to look for:

• State: FULL - Adjacency is formed
• FULL/ - - Point-to-point link (no DR/BDR)
• Dead Time - Counting down from 40 seconds (default)

✅ Success! All neighbors are in FULL state.

Step 5: Verify OSPF Routes

Check Routing Table

R1:

R1# show ip route ospf
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/2] via 10.1.12.2, 00:05:23, GigabitEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/3] via 10.1.12.2, 00:03:45, GigabitEthernet0/0
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.1.23.0/30 [110/2] via 10.1.12.2, 00:05:23, GigabitEthernet0/0
O    192.168.1.0/24 [110/2] via 10.1.12.2, 00:05:23, GigabitEthernet0/0

Understanding the output:

• O = OSPF route
• [110/2] = [Administrative Distance / Metric]
  • 110 = OSPF's AD
  • 2 = Cost to reach destination
• via 10.1.12.2 = Next-hop IP
• GigabitEthernet0/0 = Exit interface

Test Connectivity

Ping all loopbacks from R1:

R1# ping 2.2.2.2 source 1.1.1.1
!!!!!
Success rate is 100 percent (5/5)

R1# ping 3.3.3.3 source 1.1.1.1
!!!!!
Success rate is 100 percent (5/5)

R1# ping 192.168.1.1 source 1.1.1.1
!!!!!
Success rate is 100 percent (5/5)

✅ Full connectivity achieved!

Step 6: Verify OSPF Configuration

Check OSPF Process

R2# show ip ospf
 Routing Process "ospf 1" with ID 2.2.2.2
 Start time: 00:12:34.567, Time elapsed: 00:15:45.678
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 Supports Link-local Signaling (LLS)
 Supports area transit capability
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 5000 msecs
 Minimum hold time between two consecutive SPFs 10000 msecs
 Maximum wait time between two consecutive SPFs 10000 msecs
 Number of external LSA 0. Checksum Sum 0x000000
 Number of opaque AS LSA 0. Checksum Sum 0x000000
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Area BACKBONE(0)
     Number of interfaces in this area is 4
     Area has no authentication
     SPF algorithm last executed 00:05:12.345 ago
     SPF algorithm executed 3 times

Key fields:

• Router ID: 2.2.2.2
• Number of areas: 1 (Area 0)
• Number of interfaces: 4 (Gi0/0, Gi0/1, Lo0, Lo1)

Check OSPF Interfaces

R2# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Lo0          1     0               2.2.2.2/32         1     LOOP  0/0
Lo1          1     0               192.168.1.1/24     1     LOOP  0/0
Gi0/0        1     0               10.1.12.2/30       1     P2P   1/1
Gi0/1        1     0               10.1.23.2/30       1     P2P   1/1

Key fields:

• State: LOOP (loopback), P2P (point-to-point)
• Nbrs F/C: Neighbors Full / Total (e.g., 1/1 = 1 full neighbor out of 1 total)

Check Detailed Interface Info

R2# show ip ospf interface gi0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.1.12.2/30, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 1
  Transmit Delay is 1 sec, State POINT_TO_POINT
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:07
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1
  Suppress hello for 0 neighbor(s)

Key fields:

• Area: 0
• Network Type: POINT_TO_POINT
• Cost: 1
• Hello timer: 10 seconds
• Dead timer: 40 seconds
• Neighbor Count: 1 (R1)

Step 7: Check OSPF Database (LSDB)

R2# show ip ospf database

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         456         0x80000003 0x00A1B2 2
2.2.2.2         2.2.2.2         345         0x80000005 0x00C3D4 4
3.3.3.3         3.3.3.3         234         0x80000002 0x00E5F6 2

What you see:

• Each router advertises a Type 1 Router LSA
• All three routers are in the LSDB
• LSDBs are synchronized (same LSAs on all routers)

Common Configuration Mistakes

Mistake 1: Wrong Wildcard Mask

Problem:

R1(config-router)# network 10.1.12.0 0.0.0.255 area 0

Impact:
This matches 10.1.12.0 - 10.1.12.255 (256 IPs). If the interface is 10.1.12.1/30, it still matches, but it's sloppy and can cause issues in more complex configs.

Fix:

R1(config-router)# network 10.1.12.0 0.0.0.3 area 0

Mistake 2: Forgetting to Include Loopbacks

Problem:
Loopback networks aren't advertised because there's no network statement for them.

Symptom:
show ip route ospf on other routers doesn't show the loopback networks.

Fix:
Add network statements for loopbacks:

R1(config-router)# network 1.1.1.1 0.0.0.0 area 0

Mistake 3: Different Area IDs

Problem:

R1(config-router)# network 10.1.12.0 0.0.0.3 area 0
R2(config-router)# network 10.1.12.0 0.0.0.3 area 1  ← Wrong area

Impact:
Neighbors won't form. Area ID mismatch prevents adjacencies.

Fix:
Ensure both sides of a link are in the same area.

Mistake 4: Wrong OSPF Process Number

Common question:
"Does the OSPF process number need to match on all routers?"

Answer:
No. The process number is locally significant. It doesn't need to match.

Example (this works):

• R1: router ospf 1
• R2: router ospf 100
• R3: router ospf 42

Neighbors will still form as long as area IDs match.

Troubleshooting OSPF Neighbors Not Forming

If neighbors aren't forming, check:

✅ IP connectivity - Can you ping the neighbor's IP?
✅ Area ID match - show ip ospf interface
✅ Subnet mask match - Mismatched masks prevent adjacency
✅ Hello/Dead timers match - show ip ospf interface
✅ Authentication - If enabled, configs must match
✅ Network statements - Are interfaces included in OSPF?
✅ Passive interfaces - Passive interfaces don't send Hellos

Debug OSPF adjacencies:

R1# debug ip ospf adj

⚠️ Turn off debug when done:

R1# undebug all

Learn more: OSPF Neighbors Not Forming Troubleshooting Guide (Article 18)

Best Practices for Single-Area OSPF

1. Use Specific Network Statements

Don't be lazy with wildcard masks. Use precise matches:

Good:

network 10.1.12.0 0.0.0.3 area 0

Bad:

network 0.0.0.0 255.255.255.255 area 0

2. Always Set Router ID Manually

Don't rely on automatic selection:

router-id 1.1.1.1

3. Use Passive Interfaces for User-Facing Networks

If a network has no other OSPF routers (e.g., user VLANs), make it passive:

R2(config-router)# passive-interface loopback 1

Learn more: OSPF Passive Interfaces (Article 9)

4. Adjust Reference Bandwidth

If you have GigabitEthernet or faster links:

Router(config-router)# auto-cost reference-bandwidth 10000

Learn more: How OSPF Calculates Cost (Article 6)

5. Document Your Config

Add descriptions to interfaces and document the OSPF design.

Summary: Configuration Checklist

Now you know how to:

✅ Enable OSPF with router ospf [process-id]
✅ Set Router ID with router-id
✅ Advertise networks with network statements
✅ Use wildcard masks correctly
✅ Verify neighbors with show ip ospf neighbor
✅ Check routes with show ip route ospf
✅ Troubleshoot common issues

Next Step:
Now that you can configure basic OSPF, learn about passive interfaces - a critical security and optimization feature. Read OSPF Passive Interfaces: When and How to Use Them next.

Screenshot Suggestions:

1. Lab topology diagram with IP addresses
2. show ip ospf neighbor output from all three routers
3. show ip route ospf showing learned routes
4. show ip ospf interface brief showing all OSPF-enabled interfaces

Internal Links:

• ← OSPF Router ID Configuration (Article 7)
• → OSPF Passive Interfaces (Article 9)
• → Interface-Based OSPF Configuration (Article 10)
• → OSPF Neighbors Not Forming (Article 18)