Networking - BGP (Border Gateway Protocol

What is BGP?

BGP (Border Gateway Protocol) is the protocol that controls how data is routed between different autonomous systems (AS) on the internet.

  • An Autonomous System (AS) is a collection of IP networks under a single organization or administrative domain.

  • BGP is often called the “Internet’s routing protocol” because it decides the best paths for data to travel across the global internet.

Why BGP is Important

  • The internet is made of thousands of networks (ISPs, companies, universities).

  • BGP ensures that data finds the most efficient and reliable path between these networks.

  • It prevents routing loops and enables redundancy.

How BGP Works

  1. Exchanging Routing Information:

    • BGP routers share their network routes with neighboring BGP routers.

    • Each router maintains a routing table with the best path to each network.

  2. Path Selection:
    BGP chooses the path based on multiple factors:

    • AS Path → Prefer shorter sequences of AS hops.

    • Policies → Administrative preferences (business agreements, costs).

    • Other attributes → Local preference, MED (Multi-Exit Discriminator), etc.

  3. Updating Routes:

    • BGP updates only when there’s a change (path added/removed).

    • Uses TCP (port 179) for reliable communication between BGP peers.

Types of BGP

  1. External BGP (eBGP)

    • Between routers of different autonomous systems.

    • Example: ISP A ↔ ISP B.

  2. Internal BGP (iBGP)

    • Between routers inside the same AS.

    • Ensures all internal routers know about external routes.

Features of BGP

  • Scalable → Handles thousands of routes on the internet.

  • Policy-based routing → Allows administrative control over routing.

  • Loop Prevention → Tracks AS paths to avoid routing loops.

Real-Life Example

  • When you visit a website hosted in another country:

    • Your ISP uses BGP to determine the best path through multiple networks to reach the web server.

Key Point

BGP doesn’t care about speed or latency directly — it primarily focuses on policy and reachability.
Other protocols (like OSPF, EIGRP) handle routing inside a network, but BGP handles routing between networks (the internet).

 In short: BGP is like the GPS for the internet.

  • Each network (AS) tells its neighbors which roads (routes) it can take.

  • BGP decides the “best” path for your data to reach its destination, even across continents.