A Border Gateway Protocol (BGP) AS path is a path to a destination. An AS path consists of AS numbers of networks that a packet traverses if it takes the associated route to a destination. The AS numbers are assembled in a sequence from left to right, for example, AS5, AS4, AS3, AS2, AS1. For a packet to reach a destination using this route, it first traverses AS5 and so on until it reaches AS1, which is the last AS before its destination.
You can define a match condition based on all or portions of the AS path. To do this, you create a named AS path regular expression and then include it in a routing policy.
The following sections discuss the following tasks for configuring AS path regular expressions and provides the following examples: