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What leads to suboptimal routing when MED values differ across providers?

Asked on Nov 21, 2025

Answer

Suboptimal routing can occur when Multi-Exit Discriminator (MED) values differ across providers because BGP uses MED to influence the preferred path for incoming traffic, but MED is not always compared across different autonomous systems. This can lead to situations where a less optimal path is chosen if providers do not consistently interpret or propagate MED values.

Example Concept: The MED attribute in BGP is used to convey to adjacent autonomous systems which path they should prefer when multiple entry points exist. However, because MED is non-transitive and not compared between different AS paths, if two providers assign different MED values, the decision process may not yield the most efficient route. This can result in traffic taking a longer or less efficient path, potentially increasing latency and reducing network performance.

Additional Comment:

MED is an optional, non-transitive BGP attribute, meaning it is not passed beyond the neighboring AS.
Providers may have different policies for interpreting MED, leading to inconsistencies.
Consider using other attributes like Local Preference for more consistent routing decisions across AS boundaries.
Ensure that all providers involved have a clear agreement on how MED values should be interpreted.

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What leads to suboptimal routing when MED values differ across providers?

Asked on Nov 21, 2025

Answer

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