Configure AS-PATH List
Configuration Condition
None
Configure an AS-PATH List
An AS-PATH list is a tool for filtration based on AS numbers. It is used for BGP route filtration. The AS path property of a BGP route records all ASs that the route passes. When BGP advertises a route to a network outside the local AS, it adds the local AS number to the AS path property to record the AS paths that the route passes.
An AS-PATH list contains multiple entries, and the entries are in the OR relation. When a route tries to match an AS-PATH list, it checks the entries following the sequence of configuration. Once the route matches an entry, it passes the filtration of the AS-PATH list.
Table 15-3 Configure an AS-PATH list
|
Step
|
Command
|
Description
|
|
Enter the global configuration mode.
|
configure terminal
|
-
|
|
Configure an AS-PATH list.
|
ip as-path access-list path-list-number { permit | deny } regular-expression
|
Mandatory.
By default, no AS-PATH list is configured.
|
An AS-PATH list uses a regular expression to specify a collection of AS properties that meet the requirement. A regular expression consists of some common characters and some metacharacters. Common characters include upper- and lower-case characters and numbers while metacharacters have special meanings, as shown in the following table.
Table 15-4 Meanings of metacharacters in a regular expression
|
Symbol
|
Meaning
|
|
.
|
Matches any single character.
|
|
*
|
Matches a sequence which consists of 0 or more bits in the mode.
|
|
+
|
Matches a sequence which consists of 1 or more bits in the mode.
|
|
?
|
Matches a sequence which consists of 0 or 1 bit in the mode.
|
|
^
|
Matches the start of the inputted character string.
|
|
$
|
Matches the end of the inputted character string.
|
|
_
|
Matches commas, brackets, start and end of the inputted character string, and blank spaces.
|
|
[]
|
Matches single characters in a certain range.
|
|
-
|
Separates the end point of a range.
|
Switch
.png){kind=icon}
.png){kind=logo}
