Regular
Expression
Definition
* (asterisk) Matches 0 or more sequences of the immediately previous character or pattern.
+ (plus) Matches 1 or more sequences of the immediately previous character or pattern.
? (question) Matches 0 or 1 sequence of the immediately previous character or pattern.
( ) (parenthesis) Specifies patterns for multiple use when one of the multiplier metacharacters follows:
asterisk *, plus sign +, or question mark ?
[ ] (brackets) Matches any enclosed character and specifies a range of single characters.
- (hyphen) Used within brackets to specify a range of AS or community numbers.
_ (underscore) Matches a ^, a $, a comma, a space, or a {, or a }. Placed on either side of a string to
specify a literal and disallow substring matching. You can precede or follow numerals
enclosed by underscores by any of the characters listed.
| (pipe) Matches characters on either side of the metacharacter; logical OR.
As seen in the following example, the expressions are displayed when using the show commands. To view the
AS-PATH ACL configuration, use the show config command in CONFIGURATION AS-PATH ACL mode and
the show ip as-path-access-list command in EXEC Privilege mode.
For more information about this command and route filtering, refer to Filtering BGP Routes.
The following example applies access list Eagle to routes inbound from BGP peer 10.5.5.2. Access list Eagle
uses a regular expression to deny routes originating in AS 32. The first lines shown in bold create the access
list and filter. The second lines shown in bold are the regular expression shown as part of the access list filter.
Example of Using Regular Expression to Filter AS Paths
Dell(config)#router bgp 99
Dell(conf-router_bgp)#neigh AAA peer-group
Dell(conf-router_bgp)#neigh AAA no shut
Dell(conf-router_bgp)#show conf
!
router bgp 99
neighbor AAA peer-group
neighbor AAA no shutdown
neighbor 10.155.15.2 remote-as 32
neighbor 10.155.15.2 shutdown
Dell(conf-router_bgp)#neigh 10.155.15.2 filter-list 1 in
Dell(conf-router_bgp)#ex
Dell(conf)#ip as-path access-list Eagle
Dell(config-as-path)#deny 32$
Dell(config-as-path)#ex
Dell(conf)#router bgp 99
Dell(conf-router_bgp)#neighbor AAA filter-list Eagle in
Dell(conf-router_bgp)#show conf
!
router bgp 99
neighbor AAA peer-group
neighbor AAA filter-list Eaglein
neighbor AAA no shutdown
neighbor 10.155.15.2 remote-as 32
neighbor 10.155.15.2 filter-list 1 in
neighbor 10.155.15.2 shutdown
Border Gateway Protocol IPv4 (BGPv4) 243
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