Refer to the exhibit. R1 is expected to receive routes originating from AS 65516 and from any ASs that are directly attached to it. However, R1 is receiving routes only from AS 65516. Which action cor

Question

Exhibit

Options

• AChange the regular expression in the AS-path permit filter to .*.
• BChange the regular expression in the AS-path permit filter to ^65516_[0-9]*$.
• CAdd the regular expression ^$. in the AS-path filter to permit the traffic from R2.
• DChange the regular expression in the AS-path permit filter to 65516.

• A
  11% (3)
• B
  59% (16)
• C
  22% (6)
• D
  7% (2)

Explanation

BGP AS-path regular expressions filter routes based on the AS-PATH attribute. R1 needs to accept routes that originated in AS 65516 OR in ASes directly attached to (one hop away from) AS 65516. From R1's perspective, routes from AS 65516 itself have an AS path of '65516', while routes from ASes directly attached to 65516 have an AS path like '65516 <other-AS>'. The regex ^65516_[0-9]$ matches: '^' (start), '65516' (AS number), '_' (separator), '[0-9]' (zero or more digit characters for an adjacent AS), '$' (end). The '[0-9]' with '' (zero or more) allows it to match 65516 alone (zero additional digits) as well as '65516 12345' (with adjacent AS). Option A (.*) is too broad and matches all routes. Option C (^$) matches only locally originated routes. Option D (65516) matches AS 65516 anywhere in the middle of the path, which is too permissive and misses edge cases.

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