Dell S4048T, Configuration Manual

Page: 216 / 1277

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 .
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 the following illustration, 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.
MEDs are non-transitive attributes. 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 ASs (AS100 and AS200).
NOTE: The MEDs are advertised across both links, so if a link goes down, AS 1 still has connectivity to
AS300 and AS400.
Figure 27. Multi-Exit Discriminators
NOTE: Configuring the
set metric-type internal
command in a route-map advertises the IGP cost
as MED to outbound EBGP peers when redistributing routes. The configured
set metric
value
overwrites the default IGP cost. If the outbound route-map uses MED, it overwrites IGP MED.
Border Gateway Protocol IPv4 (BGPv4) 216