AS-Path BGP Attribute

AS-Path is used for loop prevention. It can also be used if you want to manipulate how traffic is sent back to you/inbound. You can prepend your AS on one of your points of presence, thus making the AS-Path longer to that router. This will cause the peer AS to use the shorter AS-Path to your network.

So it is used to manipulate both incoming and outgoing traffic ?

Well there is a lot to it. In general there are more effective ways to manipulate traffic than using AS-path prepending. You mentioned several above. The key takeaway is the foremost function is loop prevention.

You can read further on the subject here:

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-mt/irg-15-mt-book.pdf

Per this link https://supportforums.cisco.com/discussion/9532491/bgp-inbound-and-outbound-traffic
"When you are multi homed to two different providers it is sometimes required to prefer one provider's link for some incoming traffic"
it sounds like it is used to manipulate traffic entering your AS



but I guess it can also be used to manipulate the traffic leaving your AS

Thank you