Border Gateway Protocol

BGP is the postal service of the internet. It doesn't move the data; it determines the path the data will take across 75,000+ independent networks.

Autonomous Systems (AS)

The internet is a network of networks. Each of these networks is called an Autonomous System, identified by an ASN (Autonomous System Number). Google is AS15169, Cloudflare is AS13335, and AT&T is AS7018.

BGP's job is to exchange reachability information between these ASNs. When you request a website, BGP calculates the shortest AS-path to get your packets there. Tier 1 networks sit at the top of this hierarchy.

The Global Routing Table

A router running full BGP needs to keep track of every IP prefix on the internet. As of late 2023, the IPv4 global routing table contains over 950,000 active routes. IPv6 adds another 170,000+.

Year	Approx. IPv4 Routes	Milestone
2001	100,000	Early broadband expansion
2008	250,000	Router memory limit concerns (512k day approaches)
2014	512,000	"512k Day" - Older routers crash globally due to memory limits
2023	950,000+	IPv4 exhaustion drives extreme route fragmentation

This massive table requires immense router memory (TCAM) and constant processing power to calculate best paths on the fly. See how this impacts transit costs.

Tool: BGP Route Analyzer (AS Path parser)

Paste an AS path to parse it and estimate the business relationships based on standard Valley-Free routing principles.

JavaScript Required: An AS path is a sequence of numbers (e.g., "7018 3356 15169"). Routes typically flow Customer -> Provider -> Provider -> Customer. A route going Customer -> Provider -> Customer is a "valley" and indicates a leak.

Common Mistakes in BGP Understanding

Assuming shortest path means lowest latency: BGP calculates the shortest AS Path (fewest network hops), not geographic distance or lowest millisecond latency. A 2-hop route via satellite will be chosen over a 3-hop route on direct fiber. Check theoretical latency.
Trusting the routing table: By default, BGP operates on absolute trust. If a network in Russia claims it is the best path to YouTube, routers believe it. This causes BGP Hijacks.

FAQ

How do we fix BGP security?

The industry is slowly adopting RPKI (Resource Public Key Infrastructure), which uses cryptography to verify that an AS is actually authorized to announce a specific IP prefix. Read our RPKI adoption report.

What happens during a BGP leak?

Traffic intended for one destination is sucked into a black hole or routed through an intermediary, causing massive outages or allowing traffic interception. The 2021 Facebook outage was caused by a configuration error that withdrew their BGP routes, making them disappear from the internet.