🌐 BGP Route Propagation

Origin AS (holds the prefix)

Fail / restore an AS

Fail / restore a link

Routing policy

Real BGP policy

Shortest path only

Inspect an AS

Convergence—

Selected AS—

Best path (AS-path)—

AS-path length—

Path-exploration events0

Reachable ASes—

Tier-1 (core) Tier-2 (regional) Stub / edge Down

Each AS keeps the best route it has heard from every neighbor and re-announces only that one, prepending its own AS number. With real policy on, a customer-learned route always beats a peer route, which always beats a provider route (customers pay you, you pay providers) — AS-path length is only the tie-breaker. Customer routes are re-announced to everyone; peer/provider routes are re-announced only to customers, so traffic never transits for free. Click Withdraw or fail a node/link to watch a real re-convergence, including brief path exploration as ASes hunt for a new best route.

Amber links = customer→provider (pays upward) · blue links = settlement-free peering · bright highlight = selected AS's current best path to the origin