An administrator needs to create an AS Path filter (ipas-path-filter) to deny the route that contains 65001 in the AS Path, so which of the following configurations is correct?

Question

Options

• Aip as-path-filter 1 deny _65001^ip as-path-filter 1 permit.
• Bip as-path-filter 1 deny 65001_ ip as-path-filter 1 permit S
• Cip as-path-filter 1 deny^65001_ ip as-path-filter 1 permit
• Dip as-path-filter 1 deny _65001_ip as-path-filter 1 permit .*

• A
  18% (7)
• B
  8% (3)
• C
  3% (1)
• D
  73% (29)

Explanation

Option D is correct because _65001_ uses underscores on both sides, which in BGP AS-path regex act as word-boundary delimiters (matching a space, start-of-string, end-of-string, or comma) - this ensures AS 65001 is matched as a complete, standalone AS number anywhere in the path, and the second line permit .* (match any AS path) allows all other routes through.

Option A (_65001^) is wrong because ^ is the start-of-string anchor and is misplaced after the AS number, producing invalid/unintended regex.

Option B (65001_) is wrong because it lacks a leading underscore, so it would also match AS numbers that merely end in the digits "65001" (e.g., AS 165001), causing over-filtering.

Option C (^65001_) is wrong because ^ anchors the match to the beginning of the AS path, meaning it only denies routes where 65001 is the first AS - routes where 65001 appears in the middle or end of the path would slip through.

Memory tip: Think of _ as "whitespace/boundary" - wrapping an AS number in underscores (_65001_) is the BGP regex equivalent of a "whole word match," just like \b65001\b in standard regex; if you want to catch 65001 anywhere in the path, put a fence on both sides.

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