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Configuring BGP4
NOTE: You cannot apply the AS path ACLs to a neighbor using the Web management interface. You must use
the CLI. The AS Path Filter List for Weight field in the BGP Neighbor panel of the Web management interface is
not used for AS path filtering, but is instead used for changing a route’s weight based on the AS path list.
Using Regular Expressions
You 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.
In addition, you can 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:
HP9300(config-bgp-router)# as-path-filter 1 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:
HP9300(config-bgp-router)# as-path-filter 1 permit [xyz]
Special Characters
When you enter as single-character expression or a list of characters, you also can use the following special
characters. Table 10.2 on page 10-49 lists the special characters. The description for each special character
includes an example. Notice that you place some special characters in front of the characters they control but you
place other special characters after the characters they control. In each case, the examples show where to place
the special character.
Table 10.2: BGP4 Special Characters for Regular Expressions
Character Operation
. The period matches on any single character, including a blank space. For
example, the following regular expression matches for “aa”, “ab”, “ac”, and
so on, but not just “a”.
a.
* The asterisk matches on zero or more sequences of a pattern. or
example, the following regular expression matches on an AS-path that
contains the string “1111” followed by any value:
1111*
+ The plus sign matches on one or more sequences of a pattern. For
example, the following regular expression matches on an AS-path that
contains a sequence of “g”s, such as “deg”, “degg”, “deggg”, and so on:
deg+
? The question mark matches on zero occurrences or one occurrence of a
pattern. ample, the following regular expression matches on an AS-
path that contains “dg” or “deg”:
de?g
F
For ex
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