What is OSPF? Beginner Guide

In networking, one protocol always steals the spotlight — Open Shortest Path First. If you’re learning networking or preparing for the CCNA, you’ve probably already heard its name. And let’s be honest — the moment a new interview comes up, everyone runs to revise this protocol.

This blog is beginner-friendly, written in a casual tone so even if you’re just starting out, you won’t feel overwhelmed. I’ll also share how I learned this protocol, got rejected in interviews, and then cleared two back-to-back — so stay with me till the end.

What We’ll Cover:

• Introduction
• Areas
• LSAs (Link-State Advertisements)
• Router Types
• States
• Timers
• Packet Types
• Interview Tips

Let’s go!

🔍 What is OSPF?

OSPF stands for Open Shortest Path First. It’s a link-state routing protocol and is a key part of the CCNA syllabus. When routers use a link-state protocol, each one builds a complete map of the network, then uses that map to calculate the shortest path to a destination.

This calculation is done using the Shortest Path First (SPF) algorithm, which was developed by Dutch scientist Edsger Dijkstra — yes, remember that name for interviews!

📅 Open Shortest Path First Protocol Versions:

• OSPFv1 (1989): Obsolete
• OSPFv2 (1998): Supports IPv4, most commonly used
• OSPFv3 (2008): Supports both IPv4 and IPv6

When we say “Open Shortest Path First Protocol” in this blog, we mean OSPFv2.

🌐 Areas

In a large network, It uses areas to segment routers. If you’re working in a small network, Area 0 (backbone) may be enough. But in real-world enterprise networks, areas make this protocol scalable.

Types of OSPF Areas:

• Backbone Area (Area 0): The core of the It’s network. All other areas must connect to it.
• Regular Area: Supports all LSA types.
• Stub Area: Doesn’t accept external routes (Type 5 LSAs). Ideal for small branch offices.
• Totally Stubby Area: Even more restricted — only allows a default route.
• Not-So-Stubby Area (NSSA): Allows limited external routes (Type 7 LSAs). Used with BGP or other protocols.

👉 These are common interview questions, so know them well.

🛰️ LSA Types

LSA stands for Link-State Advertisement — the way routers share information.

LSA Type	Name	Purpose
Type 1	Router LSA	Info about router’s own links
Type 2	Network LSA	Info about multi-access networks
Type 3	Summary LSA	Routes between areas (by ABR)
Type 4	ASBR Summary LSA	Routes to external networks
Type 5	External LSA	External routes (e.g., from BGP)
Type 7	NSSA External LSA	External routes in NSSA

🧠 Router Types

• Internal Router: All interfaces in the same area
• Area Border Router (ABR): Connects two or more areas
• Autonomous System Boundary Router (ASBR): Connects It’s to external routing protocols
• Designated Router (DR) and Backup DR (BDR): Elected in multi-access networks to reduce LSA flooding

🔄 States

This routers go through seven states before forming full adjacency:

1. Down – No communication
2. Init – Hello packet sent
3. 2-Way – Hello packet received, DR/BDR election starts
4. ExStart – Master/slave negotiation
5. Exchange – DBD packets exchanged
6. Loading – Requesting full LSAs
7. Full – Synchronized LSDB

If you’re preparing for interviews, memorize this flow.

📦 Packet Types

It uses five types of packets:

• Hello Packet – To find and maintain neighbors
• DBD (Database Description) – Contains LSA summaries
• LSR (Link State Request) – Requests specific LSAs
• LSU (Link State Update) – Sends LSAs in reply
• LSAck (Acknowledgement) – Confirms LSU receipt

⏲️ OSPF Timers

• Hello Timer: Sent every 10 seconds (default)
• Dead Timer: If no Hello received in 40 seconds, neighbor is considered down

🎯 Interview Bonus Tip

If you’re asked to choose between protocols, check their administrative distance:

• OSPF: 110
• RIP: 120
• EIGRP: 90

Lower = better preference.

💬 Final Words

OSPF is a huge topic, and no single post can cover everything. But this guide should give you enough to:

• Pass your interviews
• Understand OSPF conceptually
• Build your confidence

I’ll write advanced posts like “OSPF Troubleshooting,” “OSPF vs RIP,” and “OSPF Configuration Lab” soon. If you want OSPF interview questions, just comment or message — I’ll reply personally.

Let’s keep learning and growing together 🚀 and Feel free to contact me.