Refer to the exhibit. Company A established BGP sessions with several ISPs. A network engineer at the company must filter out all traffic except for routes that transit AS 152. The engineer configured

Question

Exhibit

Options

• AAdd a second filtering policy in the format ip access-list 1 permit ^152_([0-9]+).
• BAdd a second filtering policy in the format ip prefix-list 1 permit ^152^.
• CChange the filtering policy to ip explicit-path 1 permit $152^.
• DChange the filtering policy to ip as-path access-list 1 permit 152.

• A
  5% (2)
• B
  10% (4)
• C
  3% (1)
• D
  82% (32)

Explanation

The original regex 'permit 152$([0-9])' is malformed and does not correctly match AS 152 as a transit AS in the BGP AS path. In Cisco BGP AS-path filtering, the underscore '' is a special metacharacter that matches any AS path delimiter (space, comma, brace, start of string, or end of string). The correct regular expression '152' matches AS number 152 appearing anywhere in the AS path (surrounded by delimiters), which identifies it as a transit AS. The proper command syntax is 'ip as-path access-list 1 permit 152'. This correctly identifies and permits only routes whose AS path contains AS 152 as a transit entry, while an implicit deny blocks everything else.

No community discussion yet for this question.