Dell Force10 S4820T S4820T Configuration Guide - Page 189

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. 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. AS 100 Router A Router B Set MED to 100 T1 Link Router C OC3 Link AS 200 Router E Set MED to 50 Router D 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. 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. Generally, an IGP indicator means that the route was derived inside the originating AS. EGP generally means that a route was learned from an external gateway protocol. An INCOMPLETE origin code generally results from aggregation, redistribution or other indirect ways of installing routes into BGP. Border Gateway Protocol IPv4 (BGPv4) | 189