Border Gateway Protocol IPv4 (BGPv4) | 215One AS assigns the MED a value and the other AS uses that value to decide the preferred path. For thisexample, assume the MED is the only attribute applied. In Figure 10-6, AS100 and AS200 connect in twoplaces. Each connection is a BGP session. AS200 sets the MED for its T1 exit point to 100 and the MEDfor its OC3 exit point to 50. This sets up a path preference through the OC3 link. The MEDs are advertisedto 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 toAS300 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 connectivityto AS300 and AS400.Figure 10-6. MED Route ExampleOriginThe Origin indicates the origin of the prefix, or how the prefix came into BGP. There are three Origincodes: 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 NGPreplaced.• 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-mapadvertises the IGP cost as MED to outbound EBGP peers when redistributing routes. The configured setmetric value overwrites the default IGP cost.Router ARouter BAS 100Router CT1 LinkOC3 LinkSet MED to 50Set MED to 100AS 200Router DRouter E
