BGP and routing policy

Why it matters

Network-facing roles and networking research expect you to explain how Internet routing actually works, read an incident report about a leak or hijack, and reason about anycast, multi-homing and cloud interconnects. In general infrastructure work, BGP misconfigurations cause some of the most visible outages.

Core ideas

Autonomous systems and path vectors. The Internet is a collection of autonomous systems (ASes), each a network under one administration with its own AS number. BGP speakers exchange routes for prefixes over TCP sessions; each route carries attributes including the AS_PATH, the list of ASes it has traversed. A router discards routes that already contain its own AS (loop prevention) and selects among the rest. Unlike link-state protocols inside a network (OSPF, IS-IS), no router has a global map.

Policy before path length. Route selection applies local preference first (set by the operator, usually from business relationships), then AS-path length, then tie-breakers such as MED and the internal cost to the exit. Export policy decides which routes are announced to whom. The common model: an AS prefers routes from customers (who pay it), then from peers (settlement-free), then from providers (whom it pays); it exports customer routes to everyone but peer and provider routes only to customers. These rules keep paths "valley-free" and stop networks from carrying traffic they are not paid for.

More-specific wins in forwarding. Forwarding uses longest-prefix match, so a more-specific announcement attracts traffic even when a legitimate less-specific route exists. Traffic engineering uses this deliberately (announcing more-specifics, or prepending one's own AS to make a path look longer); mistakes and attacks exploit it.

Leaks and hijacks. A route leak is an announcement propagated beyond its intended scope, for example a customer re-announcing one provider's routes to another provider, which can pull traffic through a network unable to carry it. A hijack is the announcement of someone else's prefix, or a more-specific of it, as if one originated it. Defenses include prefix filters built from registry data, maximum-prefix limits on sessions, RPKI route origin validation (signed statements of which AS may originate a prefix) and baseline practices promoted by initiatives such as MANRS. Origin validation checks only the origin AS, not the rest of the path.

Common misconceptions

• "BGP picks the shortest path." Local preference and export policy come first; AS-path length only breaks ties among equally preferred routes.
• "RPKI stops all hijacks." It rejects announcements from the wrong origin AS, but not leaks or forged paths that keep the correct origin.
• "BGP converges quickly." Path exploration and timers can take seconds to minutes, during which traffic may be dropped or misrouted.
• "Anycast gives automatic failover." Only if a failing site withdraws its routes; a broken site that keeps announcing keeps attracting traffic.

How to approach it

Learn the decision order and the export rules, then explain one public leak or hijack post-mortem in those terms. Look up real AS paths for a prefix you know in a public route collector.