Reference Guide

Border Gateway Protocol | 165
Figure 8-5. LOCAL_PREF Example
Multi-Exit Discriminators (MEDs)
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 8-4.
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 8-6, 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.
An MED is a non-transitive attribute. If AS100 sends an MED to AS200, AS200 does not pass it on to
AS300 or AS400. The MED is a locally relevant attribute to the two participating Autonomous Systems
(AS100 and AS200).
Note that the MEDs are advertised across both links, so that if a link goes down AS 1 still has connectivity
to AS300 and AS400.
Router A
Router B
AS 100
Router C
T1 Link
OC3 Link
AS 200
Router D
Router E
AS 300
Router F
Router E
Set Local Preference to 200
Set Local Preference to 100