AS_PATH is the BGP attribute that keeps track of each AS a route advertisement has passed through. AS_PATH is used by confederations and by exterior BGP (EBGP) to help prevent routing loops. A router knows there is a loop if it receives an AS_PATH with that routers AS in it. The figure below shows the route between router A and router B. The AS_PATH from A to B would read 701,702,703 for each AS the route passes through.

As of the start of 2010, the industry upgraded from 2-byte to 4-byte AS_PATHs. This upgrade was due to the imminent exhaustion of 2-byte AS_PATH numbers. FortiOS supports 4-byte AS_PATHs in its BGP implementation.

The BGP commands related to AS_PATH include: