Dell EMC SmartFabric OS10 User Guide Release 10.5.1

If two autonomous systems connect in more than one place, use a multiexit discriminator (MED) to assign a preference to a preferred path. MED is one of the criteria used to determine best path—other criteria may also impact selection.

One AS assigns the MED a value. Other AS uses that value to decide the preferred path. Assume that the MED is the only attribute applied and there are two connections between AS 100 and AS 200. Each connection is a BGP session. AS 200 sets the MED for its Link 1 exit point to 100 and the MED for its Link 2 exit point to 50. This sets up a path preference through Link 2. The MEDs advertise to AS 100 routers so they know which is the preferred path.

MEDs are nontransitive attributes. If AS 100 sends the MED to AS 200, AS 200 does not pass it on to AS 300 or AS 400. The MED is a locally relevant attribute to the two participating AS — AS 100 and AS 200. The MEDs advertise across both links—if a link goes down, AS 100 has connectivity to AS 300 and AS 400.