Summary

  • BGP MED is an optional, non-transitive and deliberately weak metric; a lower value matters only within the receiving network’s candidate set and policy.
  • Seeing the intended value in an update or looking glass does not prove that every relevant router selected that ingress, installed it for forwarding or sent production flows there.
  • An ingress claim needs a joined receipt: advertisements, import policy, comparable candidates, best paths, forwarding state and traffic measurements over a defined window.

The lower number and the unchanged link

Imagine a maintenance window in which a network lowers MED on one of two interconnects. A route collector shows the new value. The change record is nearly closed: the control-plane attribute is visible, the lower number looks decisive and no route has withdrawn. Yet interface counters show that inbound traffic still arrives mostly at the other site.

Nothing in that sequence proves that BGP ignored MED. The receiving network may have assigned a different LOCAL_PREF, preferred a shorter AS path, normalised MED on import, compared it only with routes from the same neighbouring AS, or selected different paths on different routers. The observed update establishes one fact: a particular speaker exposed a particular attribute. It does not establish the entire remote decision or the resulting packet movement.

A preference input, not a command

RFC 4271 defines MULTI_EXIT_DISC as an optional non-transitive four-octet metric intended to discriminate among multiple entry or exit points to the same neighbouring AS. All other factors being equal, the lower metric should be preferred. Each phrase limits the claim. The attribute is optional. It is not carried onward to another neighbouring AS. It is normally compared only between routes learned from the same neighbouring AS. And “all other factors being equal” places it behind policy choices that may already have removed a route from consideration.

The receiving network owns that decision. RFC 4271 requires a local mechanism that can remove MED and permits local configuration to alter a received value before route selection. A peer may accept the value, replace it, ignore it in the intended comparison or compare it under a wider implementation-specific policy. A sender can express preference; it cannot confer decision authority on itself.

Missing MED also needs an explicit contract. The current base specification treats a route without the attribute as having the lowest possible value in the defined comparison. RFC 4451 records historical differences among implementations and specifications, including contrary missing-MED treatment. Many operators therefore reset MED at ingress to a fixed value. A dashboard that shows 50 against 100 is incomplete unless it also identifies what happened to the absent values and where normalisation occurred.

The candidate set is the real denominator

MED has meaning only against routes that survive to its comparison step. A route with a lower MED can lose earlier to local preference, path length or another policy outcome. A route learned from a different neighbouring AS is not in the same MED comparison under the base rule. Enabling comparison across autonomous systems changes that assumption and can mix unrelated metric spaces.

RFC 4451 warns that MED behaviour varies widely and that comparing values from different autonomous systems can create weighted, unintended decisions. It also describes how route reflection or confederation designs can expose the lack of a total ordering and contribute to persistent oscillation. The relevant evidence is therefore not a screenshot of one route. It is the full candidate set at the routers whose choices matter, including which attributes survived earlier stages and whether every router saw the same alternatives.

From selected route to moving traffic

A best-path record is stronger than an update, but it is still not a traffic receipt. The selected next hop must be installed in the forwarding plane. Equal-cost or per-flow behaviour may divide traffic. Demand may change while the policy change is being observed. A withdrawal, session reset or topology event can move traffic at the same time as the MED edit.

The chain of inference should remain visible. Advertisement proves what was sent. Import state proves what was accepted or rewritten. Decision state proves what one router selected. Forwarding state proves what it installed. Interface counters or sampled flows prove where packets moved during a stated interval. None of these facts should borrow the authority of the next.

RFC 4451 adds a temporal warning. MEDs derived from IGP metrics can turn an internal metric change into widespread BGP update churn. A session reset can change advertised MEDs and alter traffic patterns. An assurance captured before a reset or on one route reflector is not indefinitely portable.

Sources