BGP routing policy can be specified by using

• Weights: provides local routing policy (within a router)

• Local preference: provides AS-wide routing policy BGP weights are specified per neighbor.

• Default weight

• AS-path-based weight

• Complex criteria with route-maps

The weight attribute is local to a single router only. The weight value is never propagated by the BGP protocol. It constitutes a routing policy local to the router.

Local preference is assigned to a route as an attribute. It is carried with the route on all internal BGP sessions. This situation means that all other BGP-speaking routers within the autonomous system (AS) receive the same information. Normally, a router assigns a local preference to a route when it is received on an external BGP session, before it is accepted and entered in the BGP table of the border router. Routers propagate the local preference attribute on internal BGP sessions only. This policy constitutes a routing policy for the entire AS.

The router can assign the weight attribute to a route in two ways:

■ All routes that are received from a specific neighbor can be assigned a default weight value. This weight value indicates that the neighbor is more preferred than the other neighbors.

■ A route-map that is applied on incoming routes from a neighbor can be used to select some routes and assign them weight values. Remember that a route-map also acts as a filter and will silently drop those routes that are not permitted by any statement in the route-map.

If configured, the default weight assignment on routes that are received from a neighbor is applied first. All routes that are received from the neighbor are assigned a weight value as defined by the default weight.

4-6 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

Secondly, a route-map is applied, if it is configured on the router. The route-map can be arbitrarily complex and select routes based on various selection criteria, such as a network number or AS path. The selected routes can have some attributes altered. The route-map can set the weight values of permitted routes. Selection can be done in several route-map statements, giving the opportunity to assign a certain weight value to some routes and another weight value to others. A route-map can also completely filter out routes.

Copyright © 2004, Cisco Systems, Inc. Route Selection Using Attributes 4-7

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

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