Dell Force10 E300, Configuration Manual

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Border Gateway Protocol IPv4 (BGPv4) | 215
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 10-6 , 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.
Figure 10-6. MED Route Example
Origin
The Origin indicates the origin of the prefix, or how the prefix came into BGP. There are three Origin
codes: IGP, EGP, INCOMPLETE.
• IGP indicated the prefix originated from information learned through an interior gateway protocol.
• EGP indicated the prefix originated from information learned from an EGP protocol, which NGP
replaced.
• INCOMPLETE indicates that the prefix originated from an unknown source.
Note: With FTOS Release 8.3.1.0, 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.
Router A
Router B
AS 100
Router C
T1 Link
OC3 Link
Set MED to 50
Set MED to 100
AS 200
Router D
Router E