Hello,
the length of the AS path attribute is one of the metrics in BGP to select the best path for a prefix. A shorter AS path is considered to be better. This can be utilized to prolong the AS path by inserting your own AS number several times. An example:
Assume you nnounce your prefix 192.168.0.0/16 to ISP A and ISP B. For some reason you want to send and RECEIVE your traffic through ISP A.
Outgoing IP routing decisions can be achieved f.e. by utilizing Local Preference. What can be done about the return traffic? You can prepend your own AS number a couple of times, when announcing to ISP B. This will indicate to the internet, that the path through ISP A is better and therefore to use this path for sending return traffic to your AS.
Hope this helps! Please rate all posts.
Regards, Martin
