What kind of BGP attribute is the Multi_Exit_Disc?

Question

Options

• AOptional transitive attribute.
• BOptional non-transitive attribute.
• CWell-known mandatory attribute.
• DWell-known discretionary attribute.

• A
  3% (1)
• B
  88% (29)
• C
  3% (1)
• D
  6% (2)

Explanation

MED (Multi_Exit_Disc) is classified as optional because BGP implementations are not required to support or send it, and non-transitive because it is not forwarded to downstream BGP peers - it is stripped when an update exits the receiving AS. This makes B the correct answer.

Why the distractors are wrong:

• A (Optional transitive): Transitive attributes must be passed along even if unrecognized; MED is explicitly dropped at AS boundaries, making it non-transitive.
• C (Well-known mandatory): Well-known means all BGP speakers must recognize it, and mandatory means it must appear in every UPDATE - MED satisfies neither requirement.
• D (Well-known discretionary): Again, MED is optional (not universally recognized/required), so it cannot be "well-known" regardless of discretionary vs. mandatory.

Memory tip: Think of MED as a "local suggestion" - your neighbor uses it to pick which entrance into your AS to prefer, but it never travels beyond that neighbor (non-transitive = not exported downstream).

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