AS_PATH is a well-known mandatory attribute that describes all the AS it has passed through. BGP router adds its AS number to the AS_PATH only when Update is sent to a neighbor in another AS. AS_PATH number is changed only when route is being advertised between EBGP peers.
The AS_PATH is displayed as a series of autonomous system (AS) numbers with the originator's AS number at the end of the path.
This attribute is also very good loop prevention. If a BGP router receive a route from an external peer whose AS_PATH include its own AS number, the router knows that the route has looped so is dropped.
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