What kind of BGP attribute is the Local Preference?

Question

Options

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

• A
  2% (1)
• B
  4% (2)
• C
  2% (1)
• D
  92% (45)

Explanation

Local Preference (LOCAL_PREF) is well-known discretionary because all BGP implementations must recognize it, but it is not required to be present in every BGP UPDATE message - it's only included when needed, typically within an iBGP domain to influence outbound path selection.

Why the distractors are wrong:

• A & B (Optional): "Optional" means some routers may not recognize the attribute. LOCAL_PREF is well-known - every compliant BGP speaker must understand it, so it cannot be optional.
• C (Well-known mandatory): Mandatory attributes must appear in every UPDATE message. LOCAL_PREF is absent from eBGP UPDATE messages entirely (it only travels between iBGP peers), so it clearly isn't mandatory.

Memory tip: Think "Discretionary = Depends on whether you need it." LOCAL_PREF is the iBGP attribute for choosing the best exit from your AS - you only include it when it's relevant, making it discretionary. Pair it with ATOMIC_AGGREGATE as the two well-known discretionary attributes to remember for exams.

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