AS 65550 owns the CIDR block 10.64.0.0/12. Router R4 advertises the aggregate 10.64.0.0/12 to AS65540. Which of the following is TRUE regarding the aggregate route advertisement?

Question

Options

• AThe aggregator attribute will not be present, and the atomic aggregate flag will not be set.
• BThe aggregator attribute will not be present, and the atomic aggregate flag will be set.
• CThe aggregator attribute will be present, and the atomic aggregate flag will not be set.
• DThe aggregator attribute will be present, and the atomic aggregate flag will be set.

• A
  3% (1)
• B
  9% (3)
• C
  83% (29)
• D
  6% (2)

Explanation

Option C is correct because when AS 65550 owns the aggregate prefix 10.64.0.0/12, all sub-prefixes within that block originated from the same AS. The AGGREGATOR attribute is always included when BGP aggregation is performed (it identifies the aggregating AS and router), but ATOMIC_AGGREGATE is only set when path information is lost - specifically when routes from different ASes are summarized, discarding their distinct AS_PATH data. Since every component route here originated from AS 65550 itself, no foreign path information is sacrificed, so the atomic aggregate flag is unnecessary.

Why the distractors fail:

• A is wrong because aggregation always attaches the AGGREGATOR attribute, regardless of whether ATOMIC_AGGREGATE is set.
• B is wrong for the same AGGREGATOR reason, and additionally wrong to set ATOMIC_AGGREGATE since ownership means no AS_PATH diversity was discarded.
• D is wrong because ATOMIC_AGGREGATE is reserved for cross-AS aggregation where path granularity is genuinely lost; aggregating your own block doesn't qualify.

Memory tip: Think "AGGREGATOR = who did it, ATOMIC_AGGREGATE = what was lost." If the AS owns the block, it aggregated its own routes - nobody else's path info was dropped, so only AGGREGATOR shows up, and the atomic flag stays silent.

No community discussion yet for this question.