An AS path is a list of autonomous_systems that routing information has passed through to get to this router, and an indicator of the origin of this information. This information can be used to prefer one path to a destination network over another. The primary method for doing this with GateD is to specify a list of patterns to be applied to AS paths when importing and exporting routes.
Each autonomous system through which a route passes prepends its AS number to the beginning of the AS path.
The origin information details the completeness of AS path information. An origin of igp indicates the route was learned from an Intra-Domain Routing Protocol and is most likely complete. An origin of egp indicates the route was learned from an Inter-Domain Routing Protocol that does not support AS paths (EGP for example) and the path is most likely not complete. When the path information is definitely not complete, an origin of incomplete is used.
AS path regular expressions are defined in RFC 1164 section 4.2.
An AS path is matched using the following syntax.
aspath aspath_regexp origin any | ( [ igp ] [egp ] [ incomplete ] )
This specifies that an AS matching the aspath_regexp with the specified origin is matched.
Technically, an AS path regular expression is a regular expression with the alphabet being the set of AS numbers. An AS path regular expression is composed of one or more AS-path expressions. An AS path expressions is composed of AS path terms and AS path operators.
An AS path term is one of the following three objects:
autonomous_system . ( aspath_regexp )
An AS path operator is one of the following:
aspath_term {m,n} aspath_term {m} aspath_term {m,} aspath_term * aspath_term + aspath_term ? aspath_term | aspath_term
Last updated April 26, 1997
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