AS-Path
Also known as: AS path, autonomous system path
The ordered list of ASNs a BGP route has traversed, used for loop prevention and path selection.
Every BGP announcement carries an AS-path: the sequence of autonomous systems the route passed through, most recent first, ending at the origin ASN. If a router sees its own ASN already in the path, it rejects the route - that is how BGP prevents loops.
The AS-path is also the main tiebreaker in route selection (shorter paths are generally preferred) and the raw material for detecting route leaks and analyzing peering relationships. AS-path manipulation, such as prepending, is a common traffic-engineering technique.
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