MED

Stands for: Multi-Exit Discriminator (MULTI_EXIT_DISC)

An optional BGP attribute by which a neighbouring AS suggests which of several links into it to use; lower wins.

MULTI_EXIT_DISC, type code 4 in RFC 4271, is meant for external links: it discriminates among multiple exit or entry points to the same neighbouring AS. The reader meets it in the Metric column of a BGP table and at step c of the best-path decision, where only routes from the same neighbouring AS are compared.

The attribute is optional and non-transitive. All other factors being equal, the exit point with the lower value should be preferred, and a MED received from a neighbouring AS must not be propagated to other neighbouring ASes. RFC 4271 also requires a locally configurable mechanism to remove it from a route, so the receiver decides whether to honour it at all.

In the decision process it comes third among the tie-breaks, after AS_PATH length and ORIGIN, and it is comparable only between routes learned from the same neighbouring AS, which is read from the AS_PATH. A route without a MED counts as having the lowest possible value. Two links into one AS carrying MED 50 and MED 10 resolve to the link with 10; the same two values from two different ASes do not compete on MED at all, and the choice falls to the later steps.

It is easy to overrate. It is a hint from the neighbour, weaker than the receiving AS's own degree of preference, so a LOCAL_PREF set by import policy overrides it. FRRouting prints it in the Metric column of its BGP table, which invites confusion with an interior metric; the interior cost to the NEXT_HOP is a separate, later step.

Also known as: multi_exit_disc, multi-exit discriminator, multi exit discriminator, bgp med, bgp metric

Sources

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