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:
+------------------------------------------------------+ | CHAR | USAGE |
+------------------------------------------------------| | ^ | Start of string |
|------|-----------------------------------------------| | $ | 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 |
+------------------------------------------------------+ Some commonly used regular expressions include:
+-------------+---------------------------+
|Expression | Meaning |
|-------------+---------------------------|
|.* | Anything |
|-------------+---------------------------|
|^$ | Locally originated routes |
|-------------+---------------------------|
|^100_ | Learned from AS 100 |
|-------------+---------------------------|
|_100$ | Originated in AS 100 |
|-------------+---------------------------|
|_100_ | Any instance of AS 100 |
|-------------+---------------------------|
|^[0-9]+$ | Directly connected ASes |
+-------------+---------------------------+
Reference. http://blog.ine.com/2008/01/06/understanding-bgp-regular-expressions/