Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 200

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 23. Multi-Exit Discriminators Origin The origin indicates the origin of the prefix, or how the prefix came into BGP. There are three origin codes: IGP, EGP, INCOMPLETE. Origin Type IGP Description Indicates the prefix originated from information learned through an interior gateway protocol. EGP INCOMPLETE Indicates the prefix originated from information learned from an EGP protocol, which NGP replaced. 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) 200