MultiExitDiscriminator

Multi-Exit Discriminator (MED) is a Border Gateway Protocol (BGP) attribute used to convey to neighboring autonomous systems a preferred entry point into an autonomous system that has multiple ingress points. It is an optional, non-transitive attribute carried in BGP updates and does not travel beyond the immediate neighbor that advertised it. The MED value is a 32-bit integer, and lower values are intended to be preferred by the receiving router when comparing routes to the same destination learned from the same neighboring AS.

MED is primarily used for inbound traffic engineering: by setting a smaller MED for a route learned from one external neighbor, an advertising AS signals a preferred ingress point into its network for that destination. In practice, its influence is strongest when multiple routes to the same prefix are received from the same neighboring AS; the lower MED value is chosen for the best path among those options.

However, MED has limitations. It is not globally comparable across different neighboring ASes, and not all networks honor or propagate MED consistently. Because it is non-transitive, MED is generally not forwarded to other ASes and its visibility and impact depend on the peer network’s policy and configuration. Some operators rely on MED in combination with other mechanisms, such as AS path prepending or local preference, to shape inbound traffic, while others may ignore MED altogether.

In summary, MED is a signaling mechanism for inbound traffic engineering within BGP, offering a way to hint preferred entry points into an AS with multiple egress points, but its effectiveness depends on the policies of external neighbors and network-specific configurations.