Which of the following regarding the atomic aggregate attribute is TRUE?

Question

Options

• AIt is an optional transitive attribute.
• BIt is an optional non-transitive attribute.
• CThe atomic aggregate flag is always set if the aggregator attribute is set.
• DThe atomic aggregate flag is always set if the AS Path information is lost due to aggregation.

• A
  5% (2)
• B
  3% (1)
• C
  13% (5)
• D
  79% (31)

Explanation

Option D is correct because the ATOMIC_AGGREGATE attribute exists precisely to signal that AS_PATH information was lost during route aggregation. Per RFC 4271, when a BGP speaker aggregates routes and cannot fully represent all contributing AS paths (e.g., it drops an AS_SET to simplify the path), it must set ATOMIC_AGGREGATE to warn downstream routers that the routing information is incomplete and the more-specific routes should not be de-aggregated.

Why the distractors are wrong:

• A & B are both wrong on the same grounds - ATOMIC_AGGREGATE is a well-known discretionary attribute, not optional. "Optional" attributes in BGP (transitive or non-transitive) are a separate category for attributes that BGP implementations are not required to recognize.
• C reverses the relationship. ATOMIC_AGGREGATE and AGGREGATOR are independent attributes; AGGREGATOR identifies which router performed the aggregation, but it can be absent while ATOMIC_AGGREGATE is set (and vice versa). One being set does not guarantee the other.

Memory tip: Think of "atomic" as a warning label - "I compacted this route and you can't split it back apart." Whenever path detail is lost (AS_PATH compressed/dropped), the atomic flag goes on to signal the loss. If no information was lost, no flag is needed.

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