ip as-path access-list

Defines an access list based on autonomous system path information.
Syntax: [no] ip as-path access-list list-name {permit|deny} as-regular-expression
. list-name
Unique alpha-numeric name that identifies the regular expression access list. List names can be up to 255 characters in length and contain the following characters: A-Z, a-z, 0-9, _, and -. Use the show ip as-path-access-list to display the names of all defined as-path access lists.
permit
Permits access for matching conditions.
deny
Denies access for matching conditions.
as-regular-expression
Autonomous system in the access list that uses a regular expression.


Description: Access lists are filters that enable you to restrict the routing information a router learns or advertises to and from a neighbor.
Multiple BGP peers or route maps can reference a single access list. You can apply access lists to both inbound route updates and outbound route updates. Each route update is passed through the access-list. BGP applies each rule in the access list in the order it appears in the list. When a route matches any rule, the decision to permit the route through the filter or denyis made, and no further rules are processed
Access lists based on autonomous system path enable you to control routing updates based on BGP autonomous paths information. If you want to control updates for all the routes to or from an autonomous system, filtering based on autonomous system path is more efficient than listing each route individually.
A regular expression is a pattern used to match against an input string. In BGP, you can build a regular expression to match information about an autonomous system path. Regular expressions can include:

Characters and Components of Regular Expressions
Component Character Description Ranges
A sequence of characters within square brackets: [1234]
Atoms
A single character with special meaning. Atoms include:
.
Matches any single character.
^
Matches the beginning of an input string.
$
Matches the end of an input string.
_ (underscore)
Matches a comma, left brace, right brace, the beginning of an input string, the end of an input string, or a space.
Pieces
Atoms followed by a special character listed below:
Branches
0 or more concatenated pieces.

The following table provides sample regular expressions:


Regular Expression Description ^nnn$
Indicates the route originated in autonomous system nnn. The ^ means the expression must start with the next character. The $ means the expression must end with the preceding character.
^100$ matches: 100
^aaa_bbb$
The autonomous system list must contain both aaa and bbb with aaa appearing before the bbb. For example:
^123_456$ matches: 123 456
                                    868 123 999 456
^nnn.*nnn$
The expression must begin with a specific autonomous system and end with a specific autonomous system, but can have anything in between. For example:

• 123 456
  
  
• 123 123 456
  
  
• ^123.*456$ (matches:  123 123 123 456)
  
  
• 123 868 999 456
  
  
• 1234 1456

_nnn_
Indicates the route was via autonomous system nnn.
^$
Indicates the route originated in this autonomous system.

NOTE Regular expressions are matched against the autonomous system path as if it is a string - not a sequence of numbers. Enclose autonomous system numbers within underscores to match a specific autonomous system number.
Permit and denyapply only when there is an exact match between the regular expression and the autonomous system path.