Refer to the exhibit. While provisioning a new BGP session between the PE and CE router, you issue the as-override command. Which statement describes modification of the prefix before being sent to…

Question

Exhibit

Options

• AThe fourth AS changes, but no other autonomous systems change.
• BThe first and fourth autonomous systems change.
• CThe second and third autonomous systems change.
• DThe first AS changes, but no other autonomous systems change.

• A
  3% (1)
• B
  3% (1)
• C
  11% (4)
• D
  84% (32)

Explanation

Option D is correct because as-override instructs the PE router to replace occurrences of the CE's AS number in the AS_PATH with its own AS number before advertising the prefix. In the exhibit's AS_PATH, the CE's AS number (10.10.10.1's AS) appears only in the first (leftmost) position - the most recently prepended AS - so only that position is overwritten by the PE's AS. This is necessary in MPLS L3VPN scenarios where two sites share the same AS number; without it, the CE would drop the route due to its own AS appearing in the path (loop prevention).

Why the distractors are wrong:

• A is wrong because the CE's AS is at position 1 (leftmost), not position 4 (rightmost); as-override targets the CE's AS wherever it appears.
• B is wrong because the CE's AS only occupies one position in the path shown in the exhibit, not two.
• C is wrong because positions 2 and 3 belong to transit AS numbers that are unrelated to the CE's AS and are never touched by as-override.

Memory tip: Think of as-override as "stamp over the neighbor's AS." BGP prepends AS numbers leftward, so the remote site's AS sits at the front (first/leftmost). The command only replaces the neighbor's AS - whatever position(s) it occupies - leaving all other AS numbers untouched.

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