Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 261)

Brocade Virtual ADX Switch and Router Guide 247
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Filtering
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AS-path filters or AS-path ACLs can be referred to by the filter list number of a BGP4 neighbor as
well as by match clauses in a route map.
Defining an AS-path ACL
To configure an AS-path list that uses “acl 1”, enter a command such as the following.
Virtual ADX(config)# ip as-path access-list acl1 permit 100
Virtual ADX(config)# router bgp
Virtual ADX(config-bgp)# neighbor 10.10.10.1 filter-list acl1 in
Syntax: [no] ip as-path access-list string [seq seq-value] deny | permit regular-expression
The ip as-path command configures an AS-path ACL that permits routes containing AS number 100
in their AS paths. The neighbor command then applies the AS-path ACL to advertisements and
updates received from neighbor 10.10.10.1. In this example, the only routes the device permits
from neighbor 10.10.10.1 are those whose AS-paths contain AS-path number 100.
The string parameter specifies the ACL name. (If you enter a number, the CLI interprets the number
as a text string.)
The seq seq-value parameter is optional and specifies the sequence number for the AS-path list. If
you do not specify a sequence number, the software numbers in increments of 5, beginning with
number 5. The software interprets the entries in an AS-path list in numerical order, beginning with
the lowest sequence number.
The deny | permit parameter specifies the action the software takes if the AS-path list for a route
matches a match clause in this ACL. To configure the AS-path match clauses in a route map, use
the match as-path command. Refer to
“Matching based on AS-path ACL” on page 255.
The regular-expression parameter specifies the AS path information you want to permit or deny to
routes that match any of the match clauses within the ACL. You can enter a specific AS number or
use a regular expression.
The neighbor command uses the filter-list parameter to apply the AS-path ACL to the neighbor.
Refer to
“Configuring BGP4 neighbors” on page 228.
Using regular expressions
Use a regular expression for the as-path parameter to specify a single character or multiple
characters as a filter pattern. If the AS-path matches the pattern specified in the regular
expression, the filter evaluation is true; otherwise, the evaluation is false.
You can also include special characters that influence the way the software matches the AS-path
against the filter value.
To filter on a specific single-character value, enter the character for the as-path parameter. For
example, to filter on AS-paths that contain the letter “z”, enter the following command:
Virtual ADX(config-bgp)# ip as-path access-list acl1 permit z
To filter on a string of multiple characters, enter the characters in brackets. For example, to filter on
AS-paths that contain “x”, “y”, or “z”, enter the following command.
Virtual ADX(config-bgp)# ip as-path access-list acl1 permit [xyz]