the AS path

The AS path is the heart of a BGP route advertisement: the ordered list of autonomous systems that a packet would cross to reach a given block of addresses. If you think of an AS path as a travel itinerary, [C, B, A] means "you'll pass through network C, then B, then arrive at A, which owns the destination." Reading right to left tells you where the prefix lives and which networks volunteer to carry traffic to it.

Each AS path is built one hop at a time. The network that owns a prefix originates the route with a path containing just itself, say [A]. Every AS that re-advertises the route to a neighbour prepends its own AS number to the front, so the list grows leftward: [A] becomes [B, A] becomes [C, B, A]. Because the AS number on the left is always the most recent advertiser, a router can read off exactly who handed it the route and the full chain back to the origin. This is what makes BGP a path-vector protocol.

The AS path does three jobs at once. It prevents loops (an AS rejects any path that already lists its own number). It is a tie-breaker in route selection: when policy doesn't decide, BGP usually prefers the route with the fewer ASes — a rough, hop-count-like proxy for closeness. And it gives operators a lever: a network can deliberately repeat its own number, called AS path prepending (e.g. [A, A, A] instead of [A]), to make a route look longer and therefore less attractive, steering traffic toward a different entry point.