Computer Networks Essentials / Network Protocols in Detail

BGP (Border Gateway Protocol)

🤔 What is BGP? (Basic Idea)

Inside a small network (like a company), protocols like RIP and OSPF work well.

But the Internet is:

  • Huge 🌍

  • Made of thousands of independent networks

  • Controlled by different organizations

📌 How do these large networks communicate with each other?

👉 Answer: BGP

🧠 Simple Definition

📌 BGP (Border Gateway Protocol) is an inter-domain routing protocol used to exchange routing information between different autonomous systems on the Internet.

🧠 One-Line Understanding

BGP decides how data travels between large networks on the Internet.

🧠 Real-Life Analogy

✈️ International Flight Routing

  • Inside a city → local roads (OSPF, RIP)

  • Between countries → international air routes (BGP)

👉 BGP = Internet’s global traffic controller

🧭 Where Does BGP Work?

  • BGP works at the Network Layer

  • Uses TCP (reliable delivery)

  • Uses Port 179

📌 This makes BGP reliable and stable.

🧩 What is an Autonomous System (AS)?

📌 An Autonomous System (AS) is a collection of IP networks controlled by a single organization that follows a common routing policy.

🧠 Examples of AS

  • ISP networks

  • Google network

  • University networks

📌 Each AS is identified by a unique ASN (Autonomous System Number).

AS15169 → Google

🔁 Why BGP is Needed (Compared to OSPF/RIP)

Protocol

Scope

RIP

Small internal networks

OSPF

Large internal networks

BGP

Between different networks (Internet)

👉 BGP is used for inter-domain routing.

🧠 Type of Routing Protocol

📌 BGP is a Path Vector Routing Protocol

Protocol

Type

RIP

Distance Vector

OSPF

Link State

BGP

Path Vector

🧠 What is Path Vector Routing?

  • Routes include:

    • Destination network

    • Complete path of AS numbers

📌 This helps prevent routing loops.

🧮 Routing Metric Used by BGP

❌ BGP does NOT use:

  • Hop count

  • Bandwidth

📌 BGP uses POLICY-BASED routing.

🔑 Important BGP Attributes

Attribute

Purpose

AS-PATH

List of ASes a route passes through

NEXT-HOP

Next router to reach destination

LOCAL-PREF

Preferred path inside AS

MED

Entry point preference

ORIGIN

Route source

👉 Shortest AS-PATH is usually preferred.

🧭 Types of BGP

🔵 1. eBGP (External BGP)

📌 Used between different ASes

Example:

  • ISP ↔ ISP

  • Company ↔ ISP

🟢 2. iBGP (Internal BGP)

📌 Used inside the same AS

Purpose:

  • Share external routes within AS

⚖️ iBGP vs eBGP

Feature

iBGP

eBGP

AS

Same AS

Different AS

Usage

Internal sharing

External routing

Hop count

Not changed

Incremented

🔄 How BGP Works (Step-by-Step)

1️⃣ BGP routers establish TCP connection

2️⃣ Exchange OPEN messages

3️⃣ Establish neighbor relationship

4️⃣ Exchange routing updates

5️⃣ Apply routing policies

6️⃣ Best path selected

📌 BGP converges slowly but safely.

📦 BGP Message Types

Message

Purpose

OPEN

Start BGP session

UPDATE

Share routing info

KEEPALIVE

Maintain connection

NOTIFICATION

Report errors

🧠 Why BGP Uses TCP?

  • Reliable delivery ✔️

  • Ordered updates ✔️

  • No packet loss ✔️

📌 Routing stability is critical for Internet.

🧠 Advantages of BGP

  • Scales to Internet size 🌍

  • Prevents routing loops

  • Supports routing policies

  • Very reliable

❌ Disadvantages of BGP

  • Complex configuration

  • Slow convergence

  • Requires expert knowledge

🧠 OSPF vs BGP

Feature

OSPF

BGP

Scope

Intra-domain

Inter-domain

Algorithm

Link State

Path Vector

Metric

Cost

Policy

Speed

Fast

Slow

Usage

Enterprise

Internet

🛡️ Security Issues in BGP

  • Route hijacking

  • Fake route advertisement

📌 Protection methods:

  • Route filtering

  • RPKI

  • Authentication

🧠 Memory Tricks

  • BGP = Big Global Paths 🌍

  • AS = Autonomous System

  • 179 = BGP port

  • Policy > Distance

⭐ One-Line Summary

🔹 BGP is a path-vector routing protocol that controls how data is routed between autonomous systems on the Internet.