Border Gateway Protocol | 215
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.
Figure 9-27 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.
See Table 9-4 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.
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