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5.1.2. AS_PATH

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 it learned from another BGP speaker's UPDATE message, it modifies the route's AS_PATH attribute based on the location of the BGP speaker to which the route will be sent:

a) When a given BGP speaker advertises the route to an internal peer, the advertising speaker SHALL NOT modify the AS_PATH attribute associated with the route.

b) When a given BGP speaker advertises the route to an external peer, the advertising speaker updates the AS_PATH attribute as follows:

  1. if the first path segment of the AS_PATH is of type AS_SEQUENCE, the local system prepends its own AS number as the last element of the sequence (put it in the leftmost position with respect to the position of octets in the protocol message). If the act of prepending will cause an overflow in the AS_PATH segment (i.e., more than 255 ASes), it SHOULD prepend a new segment of type AS_SEQUENCE and prepend its own AS number to this new segment.

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

  3. if the AS_PATH is empty, the local system creates a path segment of type AS_SEQUENCE, places its own AS into that segment, and places that segment into the AS_PATH.

When a BGP speaker originates a route then:

a) the originating speaker includes its own AS number in a path segment, of type AS_SEQUENCE, in the AS_PATH attribute of all UPDATE messages sent to an external peer. In this case, the AS number of the originating speaker's autonomous system will be the only entry the path segment, and this path segment will be the only segment in the AS_PATH attribute.

b) the originating speaker includes an empty AS_PATH attribute in all UPDATE messages sent to internal peers. (An empty AS_PATH attribute is one whose length field contains the value zero).

Whenever the modification of the AS_PATH attribute calls for including or prepending the AS number of the local system, the local system MAY include/prepend more than one instance of its own AS number in the AS_PATH attribute. This is controlled via local configuration.

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