The local preference is typically used to direct all outbound AS traffic to a certain peer. When you configure a local preference, all routes that are advertised through that peer are assigned the preference value. The preference is a numeric value, and higher values are preferred during BGP route selection. Figure 58 illustrates how to use local preference to determine BGP route selection.
Figure 58: Local Preference
The network in Figure 58 shows two possible routes to the prefixes accessible through Host E. The first route, through Router A, uses an OC3 link to Router C and is then forwarded to Host E. The second route, through Router B, uses an OC48 link to Router D and is then forwarded to Host E. Although the number of hops to Host E is identical regardless of the route selected, the route through Router B is more desirable because of the increased bandwidth. To force traffic through Router B, you can set the local preference on Router A to 100 and the local preference on Router B to 300. During BGP route selection, the route with the higher local preference is selected.
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Note: In contrast to almost every other metric associated with dynamic routing protocols, the local preference gives higher precedence to the larger value. |