User Manual

Enterasys X-Pedition User Reference Manual 203

Basic BGP Tasks

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

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

system 64800.

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 X-Pedition, the number of instances of an AS that are put in the route advertisement is

controlled by the as-count option of the bgp set peer-host command.

The following is an example:

Notes on Using the AS Path Prepend Feature

•Use the as-count option for external peer-hosts only.

ip-router policy create bgp-import-source allOthers aspath-regular-expression “(.*)” origin any sequence-

number 20

ip-router policy import source allOthers network all

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

ip-router policy create aspath-export-source allRoutes aspath-regular-expression

“(.*(284|813|814|815|816|3369|3561) .*)” origin any protocol all

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

# insert two instances of the AS when advertising the route to this peer

bgp set peer-host 194.178.244.33 group nlnet as-count 2

# insert three instances of the AS when advertising the route to this

# peer

bgp set peer-host 194.109.86.5 group webnet as-count 3