AS Path Prepending

Autonomous System (AS) path prepending is a tool to manipulate the BGP AS_PATH attribute through prepending one or more Autonomous System Numbers (ASNs). AS path prepending is used to deprioritize a route in the presence of a route with a shorter AS_PATH. By prepending a local ASN multiple times, ASes can make advertised AS paths appear artificially longer. However, excessive AS path prepending has caused routing issues in the Internet. This document provides guidance for the use of AS path prepending, including alternative solutions, in order to avoid negatively affecting the Internet.