FTOS Configuration Guide for the C-Series - Force10 Networks

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.
The 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 Figure 338. For this example, assume that
LOCAL_PREF is the only attribute applied. In Figure 339, AS100 has two possible paths to AS 200.
Although the path through the 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 339 LOCAL_PREF Example
AS 100
Set Local Preference to 200
AS 300
Router E
Router B
Router F
Router A
OC3 Link
T1 Link
Multi-Exit Discriminators (MEDs)
Set Local Preference to 100
Router D
Router C
AS 200
Router E
If two Autonomous Systems (AS) connect in more than one place, a Multi-Exit Discriminator (MED) can
be used to assign a preference to a preferred path. The 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 Figure 338.
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 Figure 340, AS100 and AS200 connect in two
places. Each connection is a BGP session. AS200 sets the MED for its T1 exit point to 100 and the MED
for its OC3 exit point to 50. This sets up a path preference through the OC3 link. The MEDs are advertised
to AS100 routers so they know which is the preferred path.
468 Border Gateway Protocol (BGP)