Last Updated on August 7, 2021 by Admin 3
Which of the following regular expression characters should be placed at the end of a BGP AS path filter to indicate the originating AS? (Select the best answer.)
- $
- ^
- *
- ]
- .
- _
Explanation:
The dollar sign ($) regular expression character should be placed at the end of a Border Gateway Protocol (BGP) autonomous system (AS) path filter to indicate the originating AS. Regular expressions are used to locate character strings that match a particular pattern. AS path filters are used to permit or deny routes that match the regular expression.
The $ character indicates that the preceding characters should match the end of the string. The originating router will insert its AS number into the AS path, and subsequent routers will prepend their AS numbers to the beginning of the AS path string. The last AS number in the AS path is the originating AS. For example, the ip as-path access-list 1 permit ^111_999$ command permits paths that originate from AS 999.
The caret (^) character should be placed at the beginning of a BGP AS path filter to indicate the AS from which the path was learned. The ^ character indicates that the subsequent characters should match the start of the string. The first number in an AS path indicates the AS from which the path was learned. For example, the ip aspath accesslist 1 permit ^111_999$ command permits paths that are learned from AS 111.
The underscore (_) character is used to indicate a comma, a brace, the start or end of an input string, or a space. When used between two AS path numbers, the _ character indicates that the ASes are directly connected. For example, the ip aspath accesslist 1 permit ^111_999$ command indicates that AS 111 and AS 999 should be directly connected.
The period (.) character is used to represent any single character. For example, the ip aspath accesslist 1 permit ^…_999$ command permits paths that originate from AS 999 and are learned from any threedigit AS.
The bracket (]) character is used to indicate a set of characters or a range of characters. For example, the ip aspath accesslist 1 permit ^[09]_999$ command permits paths that originate from AS 999 and are learned from any AS numbered from 0 through 9, and the ip aspath accesslist 1 permit ^[123]_999$ command permits paths that originate from AS 999 and are learned from AS 1, AS 2, or AS 3.
The asterisk (*) character indicates zero or more sequences of the previous expression. For example, the expression [09]* indicates a string of zero or more digits. Therefore, the ip aspath accesslist 1 permit ^111_ [09]*$ command permits paths that are learned from AS 111 and originate from any AS.
The dollar sign ($) regular expression character should be placed at the end of a Border Gateway Protocol (BGP) autonomous system (AS) path filter to indicate the originating AS. Regular expressions are used to locate character strings that match a particular pattern. AS path filters are used to permit or deny routes that match the regular expression.
The $ character indicates that the preceding characters should match the end of the string. The originating router will insert its AS number into the AS path, and subsequent routers will prepend their AS numbers to the beginning of the AS path string. The last AS number in the AS path is the originating AS. For example, the ip as-path access-list 1 permit ^111_999$ command permits paths that originate from AS 999.
The caret (^) character should be placed at the beginning of a BGP AS path filter to indicate the AS from which the path was learned. The ^ character indicates that the subsequent characters should match the start of the string. The first number in an AS path indicates the AS from which the path was learned. For example, the ip aspath accesslist 1 permit ^111_999$ command permits paths that are learned from AS 111.
The underscore (_) character is used to indicate a comma, a brace, the start or end of an input string, or a space. When used between two AS path numbers, the _ character indicates that the ASes are directly connected. For example, the ip aspath accesslist 1 permit ^111_999$ command indicates that AS 111 and AS 999 should be directly connected.
The period (.) character is used to represent any single character. For example, the ip aspath accesslist 1 permit ^…_999$ command permits paths that originate from AS 999 and are learned from any threedigit AS.
The bracket (]) character is used to indicate a set of characters or a range of characters. For example, the ip aspath accesslist 1 permit ^[09]_999$ command permits paths that originate from AS 999 and are learned from any AS numbered from 0 through 9, and the ip aspath accesslist 1 permit ^[123]_999$ command permits paths that originate from AS 999 and are learned from AS 1, AS 2, or AS 3.
The asterisk (*) character indicates zero or more sequences of the previous expression. For example, the expression [09]* indicates a string of zero or more digits. Therefore, the ip aspath accesslist 1 permit ^111_ [09]*$ command permits paths that are learned from AS 111 and originate from any AS.
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