Local Preference; Multi-Exit Discriminators (Meds) - Dell C9000 Series Networking Configuration Manual

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 22. 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.
Border Gateway Protocol IPv4 (BGPv4) 199