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Local Preference
Local preference (LOCAL_PREF) represents the degree of preference within the entire AS. The higher the
number, the greater the preference for the route.
Local preference (LOCAL_PREF) is one of the criteria used to determine the best path, so keep in mind
that other criteria may impact selection, as shown in the illustration in Best Path Selection Criteria. For
this example, assume that thelocal preference (LOCAL_PREF) is the only attribute applied. In the
following illustration, AS100 has two possible paths to AS 200. Although the path through Router A is
shorter (one hop instead of two), the LOCAL_PREF settings have the preferred path go through Router B
and AS300. This is advertised to all routers within AS100, causing all BGP speakers to prefer the path
through Router B.
Figure 21. BGP Local Preference
Multi-Exit Discriminators (MEDs)
If two ASs connect in more than one place, a multi-exit discriminator (MED) can be used to assign a
preference to a preferred path.
MED is one of the criteria used to determine the best path, so keep in mind that other criteria may impact
selection, as shown in the illustration in Best Path Selection Criteria.
One AS assigns the MED a value and the other AS uses that value to decide the preferred path. For this
example, assume the MED is the only attribute applied. In the following illustration, AS100 and AS200
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Border Gateway Protocol IPv4 (BGPv4)