Refer to the exhibit. Router R1 is expected to receive routes that originate from AS 65547 only. However, R1 is receiving routes from AS 65547 and several other ASs that are directly attached to it…

Question

Exhibit

Options

• AChange the regular expression to ^$.
• BChange the regular expression to ^65547$.
• CChange the regular expression to .*.
• DChange the regular expression to 65547.

• A
  3% (1)
• B
  82% (28)
• C
  6% (2)
• D
  9% (3)

Explanation

BGP AS path access lists use regular expressions to match against the AS_PATH attribute of received routes. The goal is to accept only routes that originated from AS 65547, meaning the AS path should contain only that single AS number with nothing else. The regex ^65547$ accomplishes this precisely: ^ anchors the match to the start of the string, 65547 matches that exact AS number, and $ anchors to the end - so only a path containing solely AS 65547 matches. Option A (^$) matches an empty AS path, which represents locally originated routes. Option C (.*) matches any AS path, which permits all routes regardless of origin. Option D (_65547_) uses word-boundary delimiters and matches any path where 65547 appears anywhere, including routes that merely transited AS 65547 but originated elsewhere. Only ^65547$ restricts acceptance to routes whose entire AS path is exactly AS 65547.

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