Cabletron Systems 9032578-05 manual Using the AS Path Prepend Feature

Chapter 12: BGP Configuration Guide

AS-Path Regular Expression Examples

To import MCI routes with a preference of 165:

ip-router policy create bgp-import-source mciRoutes aspath-regular- expression "(.* 3561 .*)" origin any sequence-number 10

ip-router policy import source mciRoutes network all preference 165

To import all routes (.* matches all AS paths) with the default preference:

ip-router policy create bgp-import-source allOthers aspath-regular- expression "(.*)" origin any sequence-number 20

ip-router policy import source allOthers network all

To export all active routes from 284 or 813 or 814 or 815 or 816 or 3369 or 3561 to autonomous system 64800.

ip-router policy create bgp-export-destination to-64800 autonomous- system 64800

ip-router policy create aspath-export-source allRoutes aspath-regular- expression "(.*(28481381481581633693561) .*)" origin any protocol all

ip-router policy export destination to-64800 source allRoutes network all

Using the AS Path Prepend Feature

When BGP compares two advertisements of the same prefix that have differing AS paths, the default action is to prefer the path with the lowest number of transit AS hops; in other words, the preference is for the shorter AS path length. The AS path prepend feature is a way to manipulate AS path attributes to influence downstream route selection. AS path prepend involves inserting the originating AS into the beginning of the AS prior to announcing the route to the exterior neighbor.

Lengthening the AS path makes the path less desirable than would otherwise be the case. However, this method of influencing downstream path selection is feasible only when comparing prefixes of the same length because an instance of a more specific prefix always is preferable.

On the SSR, the number of instances of an AS that are put controlled by the as-countoption of the bgp set peer-host

in the route advertisement is command.

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