What kind of attribute is the aggregator?

Question

Options

• AWell-known mandatory
• BWell-known discretionary
• COptional transitive
• DOptional non-transitive

• A
  8% (4)
• B
  4% (2)
• C
  87% (46)
• D
  2% (1)

Explanation

AGGREGATOR is an Optional Transitive BGP path attribute because it is not required to be recognized by all BGP routers (making it optional), yet must be passed along to downstream peers even when a router doesn't understand it (making it transitive). It identifies the AS number and router IP that performed route aggregation, so preserving this information as it propagates through the network is essential for traceability.

Why the distractors are wrong:

• A (Well-known mandatory): Well-known mandatory attributes like AS_PATH, NEXT_HOP, and ORIGIN must appear in every UPDATE message. AGGREGATOR only appears when aggregation actually occurs, so it cannot be mandatory.
• B (Well-known discretionary): Well-known means all BGP implementations must recognize it. AGGREGATOR support is not universally required, so "well-known" doesn't apply regardless of discretionary vs. mandatory.
• D (Optional non-transitive): Non-transitive attributes (like MED) are silently dropped when unrecognized. AGGREGATOR must be forwarded so downstream routers retain knowledge of where aggregation happened - dropping it would lose that audit trail.

Memory tip: Think "AGGREGATOR = Aggregate + Travel." Aggregated routes need to carry the AGGREGATOR attribute with them as they travel (transitive) across the internet, but not every router is required to understand it (optional). Optional + Travel = Optional Transitive.

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