You issue the ip as-path access-list 1 permit ^7_23$ command on a BGP router.
Which of the following paths are allowed by the AS path filter? (Select the best answer.)

A. paths that originate from AS 7 or AS 23
B. paths that pass through AS 7 or AS 23
C. paths that originate from AS 7 and are learned from AS 23

D. paths that are learned from AS 7 and originate from AS 23

Explanation:

Paths that are learned from Border Gateway Protocol (BGP) autonomous system (AS) 7 and originate from AS 23 are allowed by the AS path filter. Regular expressions are used to locate character strings that match a particular pattern.
The caret (^) character indicates that the subsequent characters should match the start of the string. Each router in the path prepends its AS number to the beginning of the AS path; therefore, the first AS number in the AS path is the AS from which the path islearned. Therefore, the ip aspath accesslist 1 permit ^7_23$ command allows paths that are learned from AS 7.

The dollar sign ($) character indicates that the preceding characters should match the end of the string. The originating router will insert itsAS number into the AS path, and subsequent routers will prepend their AS numbers to the beginning of the AS path string. The last AS number in the AS path is the originating AS; therefore, the ip aspath accesslist 1 permit ^7_23$ command allows paths that originate from AS 23.

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. Therefore, the ip as path accesslist 1 permit ^7_23$ command indicates that AS 7 is directly connected to AS 23.

The ip aspath accesslist 1 permit ^7_23$ command does not permit paths that originate from AS 7 and are learned from AS 23. To configure an AS path filter that permits paths that originate from AS 7 and are learned from AS 23, you could issue the ip as path accesslist 1 permit ^23_7$ command.

The ip aspath accesslist 1 permit ^7_23$ command does not permit paths that originate from AS 7 or AS 23; it onlypermits paths that originate from AS 23. To configure an AS path filter that permits paths that originate from AS 7 or AS 23, you could issue the following command set:

ip aspath accesslist 1 permit _7$
ip aspath accesslist 1 permit _23$

The ip aspathaccesslist 1 permit ^7_23$ command does not permit paths that pass through AS 7 or AS 23. To configure an AS path filter that permits paths that pass through AS 7 or AS 23, you could issue the following command set:

ip aspath accesslist 1 permit _7_
ipaspath accesslist 1 permit _23_

Reference:
Cisco: Using Regular Expressions in BGP
Cisco: Regular Expressions
Cisco: BGP Regular Expression AS Path Filter