How to Configure OSPF in Packet Tracer: Step by Step CCNA Lab

 

Introduction

You have configured static routes. You have set up default routes. But now your network is growing, and you cannot keep adding routes manually.

This is where OSPF comes in.

OSPF (Open Shortest Path First) is a dynamic routing protocol that automatically learns routes, finds the best path, and adapts when links fail. Once you configure it, the routers do the work for you.

But here is what most beginners get wrong. They enable OSPF, see routes in the table, and think they are done. Then a link goes down, traffic stops, and they have no idea why.

In this lab, you will configure OSPF step by step, verify neighbor relationships, troubleshoot common issues, and test failover. This is the exact process network engineers use in real networks.

📌 New to CCNA? Start with Why Most Beginners Fail CCNA and OSI Model vs TCP/IP.

Your Pain Points

Many CCNA students struggle with OSPF because they only memorize commands without understanding how OSPF actually works.

Ø  Pain Point	Why It Hurts
Ø  I enabled OSPF, but neighbors won't form	You don't know why OSPF neighbors fail to establish adjacency.
Ø  I don't understand wildcard masks	The network command syntax confuses you.
Ø  I don't know why routes are missing	Some networks appear in the routing table, others don't.
Ø  I can't troubleshoot OSPF issues	You don't know which commands to use.
Ø  I am scared of CCNA exam questions	OSPF questions are common and you keep getting them wrong.

Good news: By the end of this lab, all of these problems will disappear.

 

📌 Related: Read Subnetting Made Easy and Wildcard Mask Explained.

 

What is OSPF? (The Simple Answer)

OSPF is a link-state routing protocol that calculates the shortest path to each destination using a mathematical algorithm called Dijkstra's algorithm. Unlike RIP, which uses hop count and has a limit of 15 hops, OSPF uses cost based on bandwidth and scales to large networks.

OSPF sends Hello packets to discover neighbors. Once neighbors are discovered, they form an adjacency and exchange routing information. When the network changes, OSPF recalculates the best paths automatically.

A router sends updates only when the network changes, making OSPF efficient in production networks.

 

Lab Topology

We will configure four routers in a single OSPF area. Each router connects to a local LAN and to other routers.

Device	Interface	IP Address	Subnet Mask	Purpose
R1	G0/0	192.168.1.1	255.255.255.0	Gateway for PC1
R1	S0/0/0	10.0.12.1	255.255.255.252	Link to R2
R1	S0/0/1	10.0.13.1	255.255.255.252	Link to R3
R2	S0/0/0	10.0.12.2	255.255.255.252	Link to R1
R2	S0/0/1	10.0.24.2	255.255.255.252	Link to R4
R3	S0/0/0	10.0.13.3	255.255.255.252	Link to R1
R3	G0/0	192.168.20.1	255.255.255.0	Gateway for PC2
R4	S0/0/0	10.0.24.4	255.255.255.252	Link to R2
R4	G0/0	192.168.30.1	255.255.255.0	Gateway for PC3

Step 1: Configure Basic Settings

Configure Hostnames and IP Addresses

On R1:

Router> enable

Router# configure terminal

Router(config)# hostname R1

R1(config)# interface G0/0

R1(config-if) # Ip address 192.168.1.1 255.255.255.0

R1(config-if) # no shutdown

R1(config)# interface S0/0/0

R1(config-if) # ip address 10.0.12.1 255.255.255.252

R1(config-if) # clock rate 128000

R1(config-if)# no shutdown

R1(config)# interface S0/0/1

R1(config-if)# ip address 10.0.13.1 255.255.255.252

R1(config-if)# clock rate 128000

R1(config-if)# no shutdown

On R2:

Router> enable

Router# configure terminal

Router(config)# hostname R2

R2(config)# interface S0/0/0

R2(config-if)# ip address 10.0.12.2 255.255.255.252

R2(config-if)# no shutdown

R2(config)# interface S0/0/1

R2(config-if)# ip address 10.0.24.2 255.255.255.252

R2(config-if)# clock rate 128000

R2(config-if)# no shutdown

On R3:

Router> enable

Router# configure terminal

Router(config)# hostname R3

R3(config)# interface S0/0/0

R3(config-if)# ip address 10.0.13.3 255.255.255.252

R3(config-if)# no shutdown

R3(config)# interface G0/0

R3(config-if)# ip address 192.168.20.1 255.255.255.0

R3(config-if)# no shutdown

On R4:

Router> enable

Router# configure terminal

Router(config)# hostname R4

R4(config)# interface S0/0/0

R4(config-if)# ip address 10.0.24.4 255.255.255.252

R4(config-if)# no shutdown

R4(config)# interface G0/0

R4(config-if)# ip address 192.168.30.1 255.255.255.0

R4(config-if)# no shutdown

 

Step 2: Configure OSPF on Each Router

The network command tells OSPF which interfaces to include. The wildcard mask determines which bits must match.

Method 1: Using network Command with Wildcard Mask

On R1:

R1(config)# router ospf 1

R1(config-router)# network 192.168.1.0 0.0.0.255 area 0

R1(config-router)# network 10.0.12.0 0.0.0.3 area 0

R1(config-router)# network 10.0.13.0 0.0.0.3 area 0

On R2:

R2(config)# router ospf 1

R2(config-router)# network 10.0.12.0 0.0.0.3 area 0

R2(config-router)# network 10.0.24.0 0.0.0.3 area 0

On R3:

R3(config)# router ospf 1

R3(config-router)# network 10.0.13.0 0.0.0.3 area 0

R3(config-router)# network 192.168.20.0 0.0.0.255 area 0

On R4:

R4(config)# router ospf 1

R4(config-router)# network 10.0.24.0 0.0.0.3 area 0

R4(config-router)# network 192.168.30.0 0.0.0.255 area 0

Method 2: Using Interface Subcommands

Instead of the network command, you can enable OSPF directly on each interface:

R1(config)# interface G0/0

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

R1(config)# interface S0/0/0

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

R1(config)# interface S0/0/1

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

This method is more explicit and avoids wildcard mask calculations.

Step 3: Configure Loopback Interfaces

Loopback interfaces provide stable Router IDs. OSPF prefers loopback IPs as the Router ID over physical interfaces.

R1(config)# interface loopback 0

R1(config-if)# ip address 1.1.1.1 255.255.255.255

 

R2(config)# interface loopback 0

R2(config-if)# ip address 2.2.2.2 255.255.255.255

 

R3(config)# interface loopback 0

R3(config-if)# ip address 3.3.3.3 255.255.255.255

 

R4(config)# interface loopback 0

R4(config-if)# ip address 4.4.4.4 255.255.255.255

To manually set the Router ID :

R1(config)# router ospf 1

R1(config-router)# router-id 1.1.1.1

If you change the Router ID, restart the OSPF process:

R1# clear ip ospf process

Step 4: Configure Passive Interfaces

Interfaces connected to LANs should not send OSPF Hellos. Set them as passive.

R1(config)# router ospf 1

R1(config-router)# passive-interface G0/0

 

R3(config)# router ospf 1

R3(config-router)# passive-interface G0/0

 

R4(config)# router ospf 1

R4(config-router)# passive-interface G0/0

This prevents unnecessary OSPF traffic on LAN segments.

 

Step 5: Verification Commands

Check OSPF Neighbor Relationships

R1# show ip ospf neighbor

Expected Output:

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           0   FULL/ -         00:00:37    10.0.12.2       Serial0/0/0

3.3.3.3           0   FULL/ -         00:00:35    10.0.13.3       Serial0/0/1

All neighbors should show FULL state.

Check the Routing Table

R1# show ip route

Expected Output:

O       10.0.24.0/30 [110/128] via 10.0.12.2, 00:01:23, Serial0/0/0

O       192.168.20.0/24 [110/65] via 10.0.13.3, 00:01:23, Serial0/0/1

O       192.168.30.0/24 [110/129] via 10.0.12.2, 00:01:23, Serial0/0/0

C       192.168.1.0/24 is directly connected, GigabitEthernet0/0

OSPF routes are marked with O.

Check OSPF-Enabled Interfaces

R1# show ip ospf interface brief

Check the Link-State Database

R1# sho ip ospf database

 

Step 6: Test Connectivity

From PC1 to PC2

PC1> ping 192.168.20.10

If the ping is successful, OSPF is working correctly.

From PC1 to PC3

PC1> ping 192.168.30.10

 

Step 7: Troubleshooting Common Issues

Issue 1: OSPF Neighbor Not Forming

Possible Cause: Hello/Dead timer mismatch.

Fix:

R1(config)# interface S0/0/0

R1(config-if)# ip ospf hello-interval 10

R1(config-if)# ip ospf dead-interval 40

Both routers must have the same Hello and Dead intervals.

Issue 2: Route Missing from Routing Table

Possible Cause: OSPF not enabled on the interface or wrong wildcard mask.

Fix:

R1(config)# router ospf 1

R1(config-router)# network 192.168.1.0 0.0.0.255 area 0

Check that the wildcard mask matches the interface IP correctly.

Issue 3: Router ID Conflict

Possible Cause: Two routers using the same Router ID.

Fix:

R1(config)# router ospf 1

R1(config-router)# router-id 1.1.1.1

R1# clear ip ospf process

 

Step 8: Failover Testing

Test OSPF's ability to reroute traffic when a link fails.

Step 1: Ping from PC1 to PC3 continuously.

PC1> ping 192.168.30.10 -t

Step 2: Shut down the link between R1 and R2.

R1(config)# interface S0/0/0

R1(config-if)# shutdown

Step 3: Observe the ping output.

The pings may stop briefly but should resume through the alternate path (R1 → R3 → R4 → R2 → R4? Actually R1 → R3 → R4 → R2 is not a direct path; in this topology, traffic may go R1 → R3 → R4 → R2 or R1 → R3 → R4 → R2 depending on the topology).

Wait, with this topology, if the R1-R2 link fails, R1 can reach R2 via R3-R4-R2:

R1 → R3 → R4 → R2

OSPF will automatically find this path.

Step 4: Re-enable the interface.

R1(config)# interface S0/0/0

R1(config-if)# no shutdown

Step 5: OSPF converges automatically. Verify:

R1# show ip route ospf

 

Summary: What You Learned Today

Concept	What You Now Know
OSPF	Link-state routing protocol using cost metric.
Hello Packets	Used to discover and maintain neighbor relationships.
Router ID	Unique identifier for each OSPF router.
Wildcard Mask	Defines which interfaces to enable OSPF on.
Passive Interface	Stops OSPF Hellos on LAN interfaces.
Verification	show ip ospf neighbor, show ip route, show ip ospf database.
Failover	OSPF automatically finds alternate paths.

📌 Related: Read TCP vs UDP Explained and IPv6 Addressing for Beginners for more networking basics. Your Action Step for Today

1. Open Cisco Packet Tracer and build the topology.
2. Configure OSPF on all routers.
3. Verify neighbor relationships with show ip ospf neighbor.
4. Test connectivity between PCs.
5. Test failover by shutting down a link.
6. Comment below with your results.

📚 Recommended External References

Cisco Official - OSPF Configuration Guide

For a deeper understanding of OSPF concepts and Cisco best practices, you can refer to the official documentation and trusted lab guides below. These resources cover OSPF theory, configuration commands, and troubleshooting techniques.

👉 Cisco OSPF Configuration Guide

---

Cisco Networking Academy - Packet Tracer Labs
Hands-on OSPF labs from Cisco NetAcad using Packet Tracer. Includes multi-area OSPF setup and verification steps.

👉 Cisco NetAcad Packet Tracer

---

GeeksforGeeks - OSPF Protocol Overview
A beginner-friendly explanation of OSPF working, LSA types, and key features of the Open Shortest Path First protocol.

👉 GeeksforGeeks OSPF Protocol

---

Packet Tracer Network - CCNA OSPF Lab
A complete step-by-step OSPF lab for CCNA with IP addressing table, configuration, and show commands for verification.

👉 Packet Tracer OSPF Lab

Final Thoughts

OSPF is the most widely used routing protocol in enterprise networks. It automatically learns routes, adapts to changes, and scales to large networks.

The only way to truly learn OSPF is to practice. Build the lab, configure the commands, and break things on purpose. That is how you learn.

Remember: show ip ospf neighbor is your best friend. If you see FULL, OSPF is working. If you see something else, troubleshoot.

 ➡️ Next: OSPF Neighbor Down? Common Causes and Fixes

 

 If this guide helped you, share it with someone who is struggling with OSPF.