BGP AS-Path Visualizer
Paste an AS path. See how long BGP considers it, where prepending has been used, and whether anything in it is private, repeated or invalid.
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repeated (prepend) private invalid
How the path is read
The AS path records which autonomous systems a route has crossed, most recent first. The rightmost ASN is where the route originated.
Length is what BGP compares, and it is not simply the number of ASNs written. A sequence contributes one per ASN. An AS_SET, written in braces, counts as one however many it contains — it records an unordered set produced by aggregation rather than a series of hops. Counting its members instead changes which route wins, which is why the distinction is worth knowing.
Prepending is an operator repeating their own ASN to make a path look longer, so other networks prefer a different route. It is the standard way to make one link less attractive without withdrawing it. Repetition is therefore normal and not a fault.
A genuine loop is an ASN appearing more than once with something else in between. BGP drops a route whose path already contains its own AS, which is the loop prevention mechanism; a path showing one in the middle usually means aggregation or a configuration doing something unusual.
Private ASNs — 64512–65534 and 4200000000–4294967294 — should not appear in paths on the public internet. Seeing one usually means a provider forgot to strip it.
Related tools
Best-Path Simulator uses this length to choose a route. Community Builder parses the other common attribute.