LOCAL_PREF is a positive integer represented by 32 bits, with a default value of 100. The attribute must be recognizable by all BGP speakers, but it is not mandatory to be included in a BGP update.Īny BGP router that receives a LOCAL_PREF attribute from an eBGP peer must ignore it (except in the case of BGP confederations).

LOCAL_PREF is an attribute shared between iBGP peers within an AS.

The LOCAL_PREF attribute is used to determine the preferred outbound path from the local AS. The following table summarizes some of the most important characteristics of the LOCAL_PREF attribute. Weight, in the strictest sense, is not really a BGP attribute since it is only locally significant to the router in which it has been configured, and it is never included in any BGP updates. Some additional vendors have adopted the use of Weight, but it is not an official part of the BGP spec in RFC 4271. Note that in Cisco’s implementation of BGP, the first parameter that is examined even before LOCAL_PREF is Weight. The higher the local preference of a particular prefix, the more preferred it is. It is used to determine the most preferred outbound path from the local AS to reach a particular prefix in another AS. This attribute is exchanged between internal BGP (iBGP) peers within a single AS.

The very first attribute that is examined is called Local Preference, often written as LOCAL_PREF. These BGP attributes provide you with much more granular control over how you forward your traffic and to which autonomous systems (ASes) you forward that traffic, instead of just going for the shortest path based on a metric. If there is a tie for any particular attribute, the next attribute in order is compared until one path wins out over the other. For a particular destination, if there are two or more paths to that destination, each attribute for each path is compared to determine which route is best. These attributes are examined in a specific order to determine the next hop.īGP is designed to determine the “preferred path” by going through a series of attributes in a specific order.

Unlike internal gateway protocols (IGPs) such as OSPF or EIGRP, which use metrics to determine the best path, BGP selects the best path to a particular destination using a series of attributes. BGP is a routing protocol that is unique among its peers.

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