Reference Guide
216 | Border Gateway Protocol IPv4 (BGPv4)
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0x2ffe884 0 1 18508 701 3561 9116 21350 i
0x2ff7284 0 99 18508 701 1239 577 855 ?
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--More--
AS-PATH ACLs use regular expressions to search AS_PATH values. AS-PATH ACLs have an “implicit
deny.” This means that routes that do not meet a deny or match filter are dropped.
Use these commands in the following sequence, starting in the CONFIGURATION mode to configure an
AS-PATH ACL to filter a specific AS_PATH value.
Regular Expressions as filters
Regular expressions are used to filter AS paths or community lists. A regular expression is a special
character used to define a pattern that is then compared with an input string.
For an AS-path access list as shown in the commands above, if the AS path matches the regular expression
in the access list, then the route matches the access list.
The example below 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.
Step Command Syntax Command Mode Purpose
1
ip as-path access-list
as-path-name
CONFIGURATION Assign a name to a AS-PATH ACL and enter AS-PATH
ACL mode.
2
{deny | permit} filter
parameter
CONFIG-AS-PATH Enter the parameter to match BGP AS-PATH for
filtering. This is the filter that will be used to match the
AS-path. The entries can be any format, letters,
numbers, or regular expressions.
This command can be entered multiple times if multiple
filters are desired.
Refer to Table 9-4, "Regular Expressions," in Border
Gateway Protocol IPv4 (BGPv4) for accepted
expressions.
3
exit
AS-PATH ACL Return to CONFIGURATION mode
4
router bgp as-number CONFIGURATION Enter ROUTER BGP mode.
5 neighbor {ip-address |
peer-group-name}
filter-list as-path-name {in
|
out}
CONFIG-ROUTER-B
GP
Use a configured AS-PATH ACL for route filtering and
manipulation.
If you assign an non-existent or empty AS-PATH ACL,
the software allows all routes.