AS Path

The AS path is one of the BGP attributes, it’s a well-known mandatory attribute which means that it’s included with all prefixes that are advertised through BGP. When a BGP router advertises a prefix, it will include its own AS number to the left of the AS path attribute. The AS path allows us to see through which autonomous systems we have to travel to get to a certain destination and is also used in BGP for loop prevention. When a router sees its own AS number in the AS path, it will not accept the prefix.