AS 65100 is advertising 192.168.1.0/27 into BGP. Which of the following AS Paths would the prefix 192.168.1.0/27 contain when viewed on router R8?

Question

Options

• A65200 (65206 65202) 65100
• B(65206 65202)65100
• C65250 (65206 65202) 65100
• D65200 65100

• A
  17% (7)
• B
  5% (2)
• C
  7% (3)
• D
  71% (30)

Explanation

Option D is correct because R8 is an external BGP peer outside the confederation, and BGP confederations strip internal sub-AS numbers from the AS Path before advertising routes externally - so R8 only sees the confederation's public-facing AS number (65200) prepended to the origin AS (65100), yielding 65200 65100.

Why the distractors fail:

• A (65200 (65206 65202) 65100) - Confederation sub-AS numbers (shown in parentheses) are visible within the confederation, but are always removed before the route leaves it; an external peer like R8 never sees them.
• B ((65206 65202) 65100) - Same confederation-visibility error as A, and it also omits the confederation's external AS number (65200) entirely, which would appear in the path.
• C (65250 (65206 65202) 65100) - Uses the wrong external AS number (65250 instead of 65200) and still incorrectly exposes the internal sub-AS hops.

Memory tip: Think "What happens inside the confederation, stays inside the confederation." Sub-AS numbers (the parenthetical hops) are scrubbed at the confederation border - external neighbors only ever see the single confederation identifier, not the internal topology.

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