CONCEPT Cited by 3 sources

Border Gateway Protocol (BGP)¶

BGP is the path-vector routing protocol that glues the Internet together. Each Autonomous System (AS) speaks BGP with its neighbors to exchange reachability information — "I can reach the following prefixes via the following AS path." The global Internet routing table is the emergent, per-router outcome of all these pairwise sessions and their locally-applied policies.

Core semantics¶

Path-vector: advertisements carry the full sequence of ASes the route traverses (the AS path). Loops are prevented by rejecting any route whose AS path already contains the receiving AS.
Policy, not shortest-path. BGP routers pick a best path per prefix via a deterministic tiebreaker ladder (local preference → AS-path length → MED → origin → eBGP vs iBGP → router ID). Local preference usually wins, which means business policy (prefer customer over peer over provider) routinely overrides shorter path.
Trust-based. BGP has historically no cryptographic enforcement of who may originate a prefix or who may appear on a path. RPKI / ROV added origin validation in the 2010s; ASPA + OTC are adding path validation in the late 2020s.

Relationships shape advertisements¶

Pairwise relationships drive export policy:

customer→ provider: the customer pays the provider for Internet transit. The customer advertises only its own routes + its own customers' routes upstream. The provider advertises the full Internet downstream.
peer↔peer (settlement- free): each peer advertises only its own routes + its own customers' routes to the other.

The emergent well-formedness property is valley-free routing — a path should never go customer → provider → customer.

Why it breaks¶

Route leaks — an AS advertises routes beyond their intended scope, e.g. takes routes from one provider and redistributes them to another (Type 1 hairpin).
Route hijacks — an AS originates a prefix it doesn't own. Fixed structurally by ROV.
Route withdrawals — unadvertising a prefix makes it globally unreachable at Internet speed; load-bearing failure primitive for anycast services.

Seen in¶

sources/2026-01-08-cloudflare-a-closer-look-at-a-bgp-anomaly-in-venezuela — the post teaches BGP semantics as the setup for its route- leak forensic argument. Intended scope, customer-provider vs peer-peer, valley-free rule, Type 1 hairpin, AS-path prepending, BGP community tags, IRR-derived prefix lists, origin vs path validation are all introduced or exercised.
sources/2025-07-16-cloudflare-1111-incident-on-july-14-2025 — BGP as the global-withdrawal primitive; a misconfigured service topology → a single global withdrawal → a 62-minute anycast outage.

First AS enforcement¶

First AS enforcement (RFC 4271 §6.3) verifies that the leftmost AS in a received AS_PATH matches the configured peer ASN. This simple check prevents forged-origin and path-shortening hijacks that bypass both ROV and ASPA. Cloudflare's 2026 measurement found half of Tier 1 networks fail to enforce it, primarily due to vendor defaults. (Source: sources/2026-06-03-cloudflare-enforcing-the-first-as-in-bgp-as-paths)