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Multiple Exit Discriminator MED or Metric is a routing attribute mainly used in BGP to influence received routes between adjacent autonomous systems.
Usually, when a BGP neighbor receives routes from different neighbors for the same subnet with different MED values, the receiving neighbor will choose a route to discard, and it is usually the highest received BGP path with the highest metric value.
In the case of allowing the additional-path feature on the BGP config, the behaviour might not be consistent, and the receiving BGP neighbor may not discard the neighbor with the highest metric; instead, it will randomly select a route from one or more neighbors to be discarded.
This issue can happen due to MED is not being used to compare routes. This can happen when the Metric is passed to the receiving BGP neighbor from a different autonomous system AS.
Solution:
The following commands resolve this problem by forcing FortiOS to use MED for route comparison:
config router bgp
set always-compare-med enable
set deterministic-med enable
end
Explanation of each command/feature:
- always-compare-med: This is a feature that ensures MED is not ignored or bypassed when comparing between routes from adjacent BGP.
- deterministic-med: This is an enhancement feature to allow different BGP neighbors to use the same path selection approach.
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