Click the exhibit. AS 65550 owns CIDR block 10.64.0.0/12. Router R6 advertises the aggregate 10.64.0.0/12 to AS 65541 with "as-set" enabled. Which of the following is TRUE regarding the aggregate…

Question

Options

• AThe AS Path is set to 65540, and the Atomic Aggregate flag is set.
• BThe AS Path is set to 65540 65550, and the Atomic Aggregate flag is set.
• CThe AS Path is set to 65540 65550, and the Atomic Aggregate flag is not set.
• DThe AS Path is set to 65550, and the Atomic Aggregate flag is not set.

• A
  23% (7)
• B
  7% (2)
• C
  57% (17)
• D
  13% (4)

Explanation

Option C is correct because when as-set is configured on an aggregate, BGP preserves the AS path information of all contributing routes by including them in an AS-SET; since no path information is suppressed, the Atomic Aggregate flag is not set. R6 (in AS 65540) prepends its own AS number and appends the contributing AS 65550 in the AS-SET, so R7 sees the path as 65540 65550.

Why the distractors fail:

• A is wrong on both counts - the path omits AS 65550, and as-set specifically prevents the Atomic Aggregate flag from being set.
• B gets the AS Path right but incorrectly sets the Atomic Aggregate flag; that flag only appears when aggregation occurs without as-set, signaling that downstream routers cannot see the full contributing path.
• D drops AS 65540 from the path - the aggregating router always prepends its own AS number before advertising the aggregate onward.

Memory tip: Think of as-set as "path preserved = no flag needed." The Atomic Aggregate flag is a warning that path info was lost; as-set prevents that loss, so the warning never fires.

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