Connected: An Internet Encyclopedia
5.1.2 AS_PATH
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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 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:
- 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.
- 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:
- 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).
- 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:
- 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).
- 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).
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Connected: An Internet Encyclopedia
5.1.2 AS_PATH