5.1.2 AS_PATH

AS_PATH is a well-known mandatory attribute. This attribute identifies the autonomous systems through which routing information carried in this UPDATE message has passed. The components of this list can be AS_SETs or AS_SEQUENCEs.

When a BGP speaker propagates a route which it has learned from another BGP speaker's UPDATE message, it shall modify the route's AS_PATH attribute based on the location of the BGP speaker to which the route will be sent:

1. When a given BGP speaker advertises the route to another BGP speaker located in its own autonomous system, the advertising speaker shall not modify the AS_PATH attribute associated with the route.

2. When a given BGP speaker advertises the route to a BGP speaker located in a neighboring autonomous system, then the advertising speaker shall update the AS_PATH attribute as follows:
   1. if the first path segment of the AS_PATH is of type AS_SEQUENCE, the local system shall prepend its own AS number as the last element of the sequence (put it in the leftmost position).

   2. if the first path segment of the AS_PATH is of type AS_SET, the local system shall prepend a new path segment of type AS_SEQUENCE to the AS_PATH, including its own AS number in that segment.

When a BGP speaker originates a route then:

1. the originating speaker shall include its own AS number in the AS_PATH attribute of all UPDATE messages sent to BGP speakers located in neighboring autonomous systems. (In this case, the AS number of the originating speaker's autonomous system will be the only entry in the AS_PATH attribute).

2. the originating speaker shall include an empty AS_PATH attribute in all UPDATE messages sent to BGP speakers located in its own autonomous system. (An empty AS_PATH attribute is one whose length field contains the value zero).