Which regular expression will only allow prefixes that originated from AS 65000 and that are learned through AS 65001?
Correct Answer: D
Explanation/Reference:
Explanation:
The following table lists the regular expressions and their meanings:
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------------------------------------------------------+ | CHAR | USAGE |
------------------------------------------------------| | ^ | Start of string |
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------|-----------------------------------------------| | $ | End of string |
------|-----------------------------------------------| | [] | Range of characters |
------|-----------------------------------------------| | - | Used to specify range ( i.e. [0-9] ) |
------|-----------------------------------------------| | ( ) | Logical grouping |
------|-----------------------------------------------| | . | Any single character |
------|-----------------------------------------------| | * | Zero or more instances |
------|-----------------------------------------------| | + | One or more instance |
------|-----------------------------------------------| | ? | Zero or one instance |
------|-----------------------------------------------| | _ | Comma, open or close brace, open or close |
| parentheses, start or end of string, or space |
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Some commonly used regular expressions include:
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Expression | Meaning |
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.* | Anything |
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^$ | Locally originated routes |
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^100_ | Learned from AS 100 |
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_100$ | Originated in AS 100 |
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_100_ | Any instance of AS 100 |
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^[0-9]+$ | Directly connected ASes |
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Reference. http://blog.ine.com/2008/01/06/understanding-bgp-regular-expressions/