An AS path is a path to a destination in the Border Gateway Protocol (BGP). The path is represented as a sequence of AS numbers, which are the numbers uniquely identifying BGP AS. Each AS number represents an AS (which may be comprised of multiple networks) that a packet traverses if it takes the associated route to the destination.
For a packet to reach a destination using this route, it traverses the listed AS from the leftmost AS number to the rightmost, where the rightmost is the AS immediately preceding its destination.
Using policies, match conditions can be defined based on all or portions of the AS path. To do this, you can either specify the AS path directly in a policy command using a regular expression in the as-path attribute, or create a named AS path regular expression using the as-path-list attribute and including the name in a policy command.