BGP: The Internet's Routing Protocol

What BGP Is

BGP (Border Gateway Protocol) is the routing protocol that connects the internet's independently-run networks together. While OSPF routes traffic within a single organization's network, BGP routes traffic between entirely separate organizations - internet service providers, large companies and universities all running their own independent networks.

Autonomous Systems: The Internet's Building Blocks

Each independently operated network on the internet is called an Autonomous System (AS) and is assigned a unique AS number. BGP's entire job is figuring out the best path between different Autonomous Systems, not the internal details within any one of them.

[ AS 100 - ISP A ] --- BGP --- [ AS 200 - ISP B ] --- BGP --- [ AS 300 - ISP C ]

Path Vector: A Third Routing Approach

BGP uses a "path vector" approach: rather than a simple hop count (distance vector) or a full network map (link state), each route advertisement includes the actual list of Autonomous Systems it has passed through. This lets BGP make policy-based decisions - like avoiding a specific AS entirely - not just find the mathematically shortest path.

Why BGP Prioritizes Policy Over Pure Distance

Unlike OSPF, which purely optimizes for lowest cost, BGP routing decisions are heavily influenced by business relationships and policy: an ISP might prefer routing traffic through a paying customer's network over a technically shorter path through a competitor, since real-world internet routing is also a business decision, not just a technical one.

Why BGP Is Called "The Protocol That Runs the Internet"

Every time you access a website hosted by a completely different provider than your own ISP, BGP is what determined the path your traffic took across potentially dozens of independently-operated networks to get there and back - making it arguably the single most critical protocol holding the modern internet together.