- Enterasys Security Router User's Guide

Overview
6-4 Configuring the Border Gateway Protocol
AS Path
The AS_PATH attribute, as shown in Figure 6-2, is the sequence of AS numbers a route has
traversed to reach a destination. The AS that originates the route adds its own AS number when
sending the route to its EBGP peers. Subsequently, each AS that receives the route and passes it on
to other BGP peers will prepend its own AS number to the list. When the route is passed to a BGP
speaker within the same AS (IBGP peer), the AS_PATH data remains intact. The final list
represents all the AS numbers that a route has traversed with the AS number of the AS that
originated the route all the way at the end of the list.
BGP uses the AS_PATH attribute in its routing updates to ensure a loop-free topology on the
Internet. If the route is advertised to the AS that originated it, that AS will see itself as part of the
AS_PATH attribute list and will not accept the route. The attribute can be manipulated with the
ip
as-path access-list
command. The match as-path command associates a route map’s as-
path with a particular ACL. The
set as-path command increases the length of the AS-path
attribute for updates that meet the match conditions specified within a route map.
Figure 6-2 AS Path List
AS_PATH data is one of the attributes BGP considers to determine the best route to a destination.
When comparing two or more different routes, if all other attributes are identical, a shorter path is
always preferred. In case of a tie in AS_PATH length, other attributes are used in the decision.
Origin
The ORIGIN attribute indicates the origin of the routing update as follows:
• IGP: The prefix is internal to the originating AS.
• EGP: The prefix was learned via some EGP.
• INCOMPLETE: The prefix was learned by some other means, usually via redistribution or
aggregation.