Multi-Exit Discriminators (Meds) - Dell Force10 S4810P Configuration Manual

Figure 9-5. LOCAL_PREF Example
AS 100
Set Local Preference to 200
Router E
AS 300

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
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
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
OC3 Link
Router F
Set Local Preference to 100
T1 Link
Router C
Router D
Figure
Figure 9-6, AS100 and AS200 connect in two
AS 200
Router E
9-4.
Border Gateway Protocol | 183