Learning cisco ospf start from zero - Lazy Guy’s Tech Hub

Teaching Guide: Configuring OSPF with Authentication on a Cisco Router

Level: Beginner (Zero IT Knowledge)
Topic: Basic OSPF in Area 0 with Peer Authentication
Date: March 09, 2025

What is OSPF?

OSPF stands for Open Shortest Path First. It’s a way routers talk to each other to share information about networks.
Think of it like a GPS for routers—it helps them find the best path to send data.
We’ll set it up in Area 0, which is like the main hub where all OSPF routers connect.

What is Authentication?

Authentication is like a secret password between routers.
It makes sure only trusted routers can join the OSPF “conversation.”
We’ll use MD5 authentication, which is a secure way to check the password.

Reference Link:

Learn more about OSPF basics: Visit Cisco’s official OSPF guide at https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html.
Understand MD5 authentication: Check out this tutorial at https://networklessons.com/ospf/ospf-authentication.
Download Cisco Packet Tracer for practice: Get it from https://www.netacad.com/courses/packet-tracer.

What You’ll Learn:

How to turn on OSPF on a router.
How to add a network to OSPF in Area 0.
How to set up a password (authentication) so only trusted routers connect.
How to check if it works.

Sample Scenario

Imagine you have two routers (Router1 and Router2) connected by a cable. We’ll configure Router1 to use OSPF in Area 0 and add authentication. Router2 would need a similar setup to “talk” to Router1.

Router1 IP: 192.168.1.1 (on its GigabitEthernet0/0 port)
Network: 192.168.1.0/24 (a group of IP addresses)
Password: “cisco123” (our secret key)

Step-by-Step Configuration for Router1

Step 1: Get into Configuration Mode

When you turn on a Cisco router, you see a prompt like Router>.
Type these commands to start changing settings:

enable
configure terminal

What it does: enable gives you permission to change things, and configure terminal lets you edit the router’s settings.

Step 2: Start OSPF

OSPF needs a number to identify it (like a name tag). We’ll use “1”:

router ospf 1

Give the router a unique ID (like a nickname, e.g., 1.1.1.1):

router-id 1.1.1.1

What it does: This turns on OSPF and gives Router1 an ID so other routers know who it is.

Step 3: Tell OSPF Which Network to Share

We want Router1 to tell others about the network 192.168.1.0:

network 192.168.1.0 0.0.0.255 area 0

What it does:
192.168.1.0 is the network address.
0.0.0.255 is a “wildcard mask” (it means all IPs from 192.168.1.0 to 192.168.1.255).
area 0 puts this network in the main OSPF area.

Step 4: Turn On Authentication for Area 0

Add a password requirement for all routers in Area 0:

area 0 authentication message-digest

What it does: This says, “Every router in Area 0 must use a password with MD5 (a secure method).”

Step 5: Set Up the Router’s Port

Go to the port connected to Router2 (e.g., GigabitEthernet0/0):

interface GigabitEthernet0/0

Give it an IP address:

ip address 192.168.1.1 255.255.255.0

Turn on authentication for this port:

ip ospf authentication message-digest

Set the password (key) for this port:

ip ospf message-digest-key 1 md5 cisco123

Turn the port on:

no shutdown

What it does:
ip address sets the port’s address.
message-digest uses MD5 for the password.
key 1 md5 cisco123 means key number 1 uses the password “cisco123.”
no shutdown makes the port active.

Step 6: Exit and Save

Go back to the main prompt and save your work:

exit
end
write memory

What it does: write memory saves the config so it’s not lost when the router restarts.

Full Config for Router1

Here’s everything together:

enable
configure terminal
router ospf 1
 router-id 1.1.1.1
 network 192.168.1.0 0.0.0.255 area 0
 area 0 authentication message-digest
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 cisco123
 no shutdown
exit
end
write memory

Practice Task

Set Up Router2:

Use the same steps, but change:
- router-id to 2.2.2.2
- ip address to 192.168.1.2 255.255.255.0
- Keep the same password (cisco123) and key number (1).

Connect the Routers:

In a simulator (like Packet Tracer), connect GigabitEthernet0/0 on Router1 to GigabitEthernet0/0 on Router2 with a cable.

How to Check if It Works

After configuring both routers, type these commands on Router1:

show ip ospf neighbor

You should see Router2 listed with its ID (2.2.2.2) and “FULL” status.
If it’s empty, something’s wrong (e.g., password mismatch).

show running-config

This shows your config—check for typos!

Why It Matters

OSPF: Helps routers share maps of the network so data gets where it needs to go.
Authentication: Keeps your network safe from untrusted routers.

Beginner Tips

Practice: Use a simulator like Cisco Packet Tracer to try this.
Mistakes to Avoid:
If the password or key number doesn’t match on both routers, they won’t connect.
Don’t forget no shutdown—the port won’t work without it!
Next Steps: Try adding more routers or networks to Area 0.