BGP

networking

Stands for: Border Gateway Protocol

The protocol that decides how traffic is routed between the large networks that make up the internet.

Border Gateway Protocol (RFC 4271) is how autonomous systems tell each other which destinations they can reach and via what path. It runs the internet's inter-network routing, and because it trusts what it is told, securing it (see RPKI) is an ongoing project.

BGP is the protocol that makes the internet a network of networks rather than one network. Inside your own domain an interior protocol computes best paths against a shared view; between autonomous systems there is no shared view and no common authority, so BGP exchanges reachability and lets each operator apply its own policy.

That policy dimension is the part that surprises engineers used to shortest-path routing. BGP does not choose the fastest route. It chooses the route your business relationships prefer: transit costs money, peering is usually free, and a customer route is preferred over a peer route over a provider route, regardless of latency. Traffic that takes a geographically absurd path is often following commercial logic exactly as configured.

The fragility is that BGP was built on trust. An autonomous system announcing a prefix it does not own can attract that traffic globally, which has happened repeatedly by accident and occasionally on purpose. RPKI adds cryptographic origin validation and is the meaningful defence, but adoption is partial, so a substantial part of internet routing still depends on operators filtering each other correctly.

Also known as: bgp, autonomous system, as-path

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