---
title: What is BGP? (and why it can break the whole internet)
description: BGP is the protocol that decides how traffic travels between the world's networks — the internet's routing map. When it goes wrong, large chunks of the internet go dark. Here's how it works.
canonical: https://watchfor.io/blog/what-is-bgp
---

# What is BGP? (and why it can break the whole internet)

2026-01-03 · WatchFor Team · Networking · networking, basics

BGP is the protocol that decides how traffic travels between the world's networks — the internet's routing map. When it goes wrong, large chunks of the internet go dark. Here's how it works.

Every so often, a big chunk of the internet vanishes for an hour — a major site or even a whole country becomes unreachable — and the post-mortem blames three letters: **BGP**. It's one of the most important protocols on the internet and one of the least known. Let's fix that.

## What BGP does

The internet isn't one network — it's tens of thousands of separate networks (ISPs, clouds, companies) stitched together. Each is called an **Autonomous System (AS)**. **BGP (Border Gateway Protocol)** is how these networks **tell each other which destinations they can reach**, so traffic can find a path from any AS to any other.

In short: **BGP is the internet's routing map.** It's how a packet leaving your ISP knows which series of networks to hop through to reach a server on the other side of the world.

## How it works (simply)

| Step | What happens |
| --- | --- |
| **Advertise** | Each network announces "I can reach these IP ranges" |
| **Exchange** | Networks share these announcements with their neighbours |
| **Choose** | Each network picks the best path to each destination |
| **Route** | Traffic follows those chosen paths, hop to hop |

It's a giant, constantly-updating conversation between networks about who can reach what — and the paths it produces are what [traceroute](/blog/ping-traceroute-mtr) reveals when you trace a route.

## Why it's fragile

Here's the catch: BGP largely runs on **trust**. Historically, if a network *claims* it can reach a set of IPs, others tend to believe it. That's efficient — and dangerous.

| Failure mode | What happens |
| --- | --- |
| **Route leak / misconfiguration** | A network accidentally announces routes it shouldn't, black-holing traffic |
| **BGP hijack** | A network (maliciously or by error) claims someone else's IPs, diverting traffic |
| **Withdrawal** | A network stops announcing its routes — and effectively disappears |

> **The famous outages** — where a big platform or whole region drops off the internet — are often a BGP **misconfiguration**: a network accidentally withdrew or mis-announced routes, and suddenly nobody could find it. The servers were fine; the *map* was wrong.

Improvements like **RPKI** (which cryptographically validates route ownership) are steadily making BGP safer, but its trust-based core is why these incidents still happen.

## Do you need to deal with BGP?

For most people: **no, but you should recognise its symptoms.** You don't configure BGP unless you run network infrastructure. But when you see:

- A site unreachable from *some* regions but fine from others,
- A [traceroute](/blog/ping-traceroute-mtr) that dies or detours wildly at an early hop,
- "The server is up but unreachable" with no server-side fault,

…a routing/BGP issue upstream is a real possibility — and it's usually **out of your hands** (your ISP's or a transit provider's). Recognising it saves you from chasing a phantom server bug.

## A monitoring angle

Because BGP problems are often **regional**, monitoring from [multiple locations](/blog/global-monitoring-locations) is how you spot them: "down from Europe, up from the US" is a classic routing-problem signature that a single-location check would completely miss.

## The bottom line

| | In one line |
| --- | --- |
| **What** | The protocol that routes traffic between the world's networks. |
| **Role** | The internet's constantly-updating routing map. |
| **Fragility** | Trust-based — misconfigs and hijacks can black-hole traffic. |
| **You** | Recognise the regional symptoms; the fix is usually upstream. |

BGP is the invisible glue holding the internet's networks together — and when it slips, the effects are dramatic and global. You won't touch it day to day, but knowing it exists turns a baffling "half the world can't reach us" into a recognisable routing problem.

Related: [ping, traceroute & MTR](/blog/ping-traceroute-mtr), [anycast explained](/blog/anycast-explained), [why monitor globally](/blog/global-monitoring-locations).

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Canonical page: https://watchfor.io/blog/what-is-bgp · All articles: https://watchfor.io/blog/all.md · Site guide: https://watchfor.io/llms.txt