BGP AS Path Length Calculator

BGP AS Path Length & Inter-AS Routing

The Border Gateway Protocol (BGP-4) is what actually glues the Internet together at the inter-domain level, carrying reachability information between Autonomous Systems (ASes). Every AS gets a 16 or 32-bit number handed out by an RIR: LACNIC in Latin America, ARIN in North America, RIPE in Europe. Whenever an AS advertises a route to a neighbor, it prepends its own ASN to the AS_PATH attribute, and what you end up with is a list of the networks the route has crossed.

Picking the best path in BGP runs through a fixed list of tie-breakers, checked in order: highest WEIGHT (a Cisco-local value), highest LOCAL_PREF, then locally originated routes, then the shortest AS_PATH, lowest ORIGIN, lowest MED, eBGP ahead of iBGP, lowest IGP cost, and lowest router-ID as the final tiebreak. So AS path length only gets a vote once local preference has had its say, yet it remains the knob you most often expose to neighbors. Specification: RFC 4271.

Applications

Operators reach for AS prepending, advertising the same prefix with a few extra copies of the local ASN, as a blunt traffic-engineering lever to push traffic away from a path on multi-homed sites. Internet Exchanges like PTT.br in Brazil let hundreds of ASes peer publicly, which shortens AS paths and cuts transit bills. Within a large AS, route reflectors spare you the n×(n-1)/2 iBGP full mesh and still keep the AS path semantics intact.

FAQ

Is shortest AS path always the chosen route? No. It only comes into play after WEIGHT, LOCAL_PREF and origin-self have been checked in the BGP decision process, and an operator can override length entirely with local preference policies.

Does AS prepending always work? Not really. An upstream peer running explicit LOCAL_PREF policies can simply ignore your prepends, and even where it does land, the effect tails off badly past 3-5 prepends.

What is a 32-bit ASN? Since 2007, under RFC 6793, ASNs can run to 32 bits, which took the pool from around 65k all the way past 4 billion. Modern BGP speakers negotiate the four-octet ASN capability on their own, without any manual setup.