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Routing & Switching · Intermediate

AS Path Prepending

Adding extra copies of an ASN to the AS_PATH of advertised routes to make a path less attractive to other ASes.

Definition

Prepending is the common way to influence inbound traffic across multiple ISPs because remote ASes prefer shorter AS paths. It is not guaranteed, since remote ASes can override with local preference.

Example

route-map ISP2-OUT permit 10 / set as-path prepend 65001 65001 65001

Exam tip

Prepending is applied outbound to influence inbound traffic; applying it inbound affects your own outbound choice.

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