Users Guide

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 effect selection, as shown in the illustration in Best Path Selection Criteria. For this example,
assume that the local preference (LOCAL_PREF) is the only attribute applied. In the following illustration,
AS100 has two possible paths to AS200. 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 25. 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 effect
selection, as shown in the illustration in Best Path Selection Criteria.
Border Gateway Protocol IPv4 (BGPv4) 213