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Defining AS Path Regular Expressions

You can create a named AS path regular expression and then include it in a routing policy with the as-path match condition (described in Table 14). To create a named AS path regular expression, include the as-path statement:



as-path name regular-expression;

You can include this statement at the following hierarchy levels:

[edit policy-options]
[edit logical-systems logical-systemr-name policy-options]

To include the AS path regular expression in a routing policy, include the as-path match condition in the from statement:



as-path name regular-expression;

policy-statement policy-name {


term term-name {


from {

names; 
}


}


}

Additionally, you can create a named AS path group made up of AS path regular expressions and then include it in a routing policy with the as-path-group match condition. To create a named AS path group, include the as-path-group statement:



as-path-group group-name  {

name [ regular-expressions ]; 
}

You can include this statement at the following hierarchy levels:

[edit policy-options]
[edit logical-systems logical-system-name policy-options]

To include the AS path regular expressions within the AS path group in a routing policy, include the as-path-group match condition in the from statement:



as-path-group group-name {

name [ regular-expressions ]; 
}

policy-statement policy-name {


term term-name {


from {
as-path-group group-name; 
}


}


}

Note: You cannot have both as-path and as-path-group in the same policy term.

Note: You can include the names of multiple AS path regular expressions in the as-path match condition in the from statement. If you do this, only one AS path regular expression needs to match for a match to occur. The AS path regular expression matching is effectively a logical OR operation.

The AS path name identifies the regular expression. It can contain letters, numbers, and hyphens (-), and can be up to 255 characters. To include spaces in the name, enclose the entire name in quotation marks (“ ”).

The regular expression is used to match all or portions of the AS path. It consists of two components, which you specify in the following format:




term <operator>

term—Identifies an AS. You can specify it in one of the following ways:
- AS number—The entire AS number composes one term. You cannot reference individual characters within an AS number, which differs from regular expressions as defined in POSIX 1003.2.
- Wildcard character—Matches any single AS number. The wildcard character is a period (.). You can specify multiple wildcard characters.
- AS path—A single AS number or a group of AS numbers enclosed in parentheses. Grouping the regular expression in this way allows you to perform a common operation on the group as a whole and to give the group precedence. The grouped path can itself include operators.
  Beginning with JUNOS Release 9.1, the range for AS numbers has been extended to provide BGP support for 4-byte AS numbers as defined in
  RFC 4893, BGP Support for Four-octet AS Number Space. You can now configure a value that is from 1 through 4,294,967,295. The JUNOS software continues to support 2-byte AS numbers. For more information about configuring the AS number for the local router and the Border Gateway Protocol (BGP), see the JUNOS Routing Protocols Configuration Guide.
operator—(Optional) An operator specifying how the term must match. Most operators describe how many times the term must be found to be considered a match (for example, any number of occurrences, or zero, or one occurrence). Table 19 lists the regular expression operators supported for AS paths. You place operators immediately after term with no intervening space, except for the pipe ( | ) and dash (–) operators, which you place between two terms, and parentheses, with which you enclose terms.

You can specify one or more term–operator pairs in a single regular expression.

Table 20 shows examples of how to define regular expressions to match AS paths.

Table 19: AS Path Regular Expression Operators

Operator	Match Definition
{m,n}	At least m and at most n repetitions of term. Both m and n must be positive integers, and m must be smaller than n.
{m}	Exactly m repetitions of term. m must be a positive integer.
{m,}	m or more repetitions of term. m must be a positive integer.
*	Zero or more repetitions of term. This is equivalent to {0,}.
+	One or more repetitions of term. This is equivalent to {1,}.
?	Zero or one repetition of term. This is equivalent to {0,1}.
\|	One of two terms on either side of the pipe.
–	Between a starting and ending range, inclusive.
^	A character at the beginning of a community attribute regular expression. This character is added implicitly; therefore, the use of it is optional.
$	A character at the end of a community attribute regular expression. This character is added implicitly; therefore, the use of it is optional.
( )	A group of terms that are enclosed in the parentheses. If enclosed in quotation marks with no intervening space ("()" ), indicates a null. Intervening space between the parentheses and the terms is ignored.
[ ]	Set of characters. One character from the set can match. To specify the start and end of a range, use a hyphen (-).
!	The not operator.

Table 20: Examples of Defining AS Path Regular Expressions

AS Path to Match	Regular Expression	Sample Matches
AS path is 1234	1234	1234
Zero or more occurrences of AS number 1234	1234*	1234 1234 1234 1234 1234 1234 Null AS path
Zero or one occurrence of AS number 1234	1234? or 1234{0,1}	1234 Null AS path
One through four occurrences of AS number 1234	1234{1,4}	1234 1234 1234 1234 1234 1234 1234 1234 1234 1234
One through four occurrences of AS number 12, followed by one occurrence of AS number 34	12{1,4} 34	12 34 12 12 34 12 12 12 34 12 12 12 12 34
Range of AS numbers to match a single AS number	123–125 [123–125]*	123 or 124 or 125 Null AS path 123 124 124 125 125 125
Path whose second AS number must be 56 or 78	(. 56) \| (. 78) or . (56\|78)	1234 56 34 78
Path whose second AS number might be 56 or 78	. (56\|78)?	1234 56 34
Path whose first AS number is 123 and second AS number is either 56 or 78	123 (56\|78)	123 56 123 78
Path of any length, except nonexistent, whose second AS number can be anything, including nonexistent	.* or .{0,}	1234 1234 5678 1234 5 6 7 8
AS path is 1 2 3	1 2 3	1 2 3
One occurrence of the AS numbers 1 and 2, followed by one or more occurrences of the number 3	1 2 3+	1 2 3 1 2 3 3 1 2 3 3 3
One or more occurrences of AS number 1, followed by one or more occurrences of AS number 2, followed by one or more occurrences of AS number 3	1+ 2+ 3+	1 2 3 1 1 2 3 1 1 2 2 3 1 1 2 2 3 3
Path of any length that begins with AS numbers 4, 5, 6	4 5 6 .*	4 5 6 4 5 6 7 8 9
Path of any length that ends with AS numbers 4, 5, 6	.* 4 5 6	4 5 6 1 2 3 4 5 6
AS path 5, 12, or 18	5 \| 12 \|18	5 12 18

Null AS Path

You can use AS path regular expressions to create a null AS path that matches routes (prefixes) that have originated in your AS. These routes have not been advertised to your AS by any external peers. To create a null AS path, use the parentheses operator enclosed in quotation marks with no intervening spaces:



“()"

Example: Null AS Path

AS 1 and AS 3 are connected to AS 2, which you administrate. AS 3 advertises its routes to your AS, but you do not want to advertise AS 3 routes to AS 1 and thereby begin routing traffic from AS 1 to AS 3 through your AS. To prevent this situation from occurring, you can configure an export policy for AS 1 (1.2.2.6) that allows routes for your AS to be advertised to AS 1 but does not allow routes for AS 3 or routes for any other connected AS to be advertised to AS 1:



[edit policy-options] 
null-as "()";
policy-statement only-my-routes {


term just-my-as {


from {
protocol bgp;
as-path null-as;
}


then accept;
}




term nothing-else {
then reject;
}


}
protocol {


bgp {


neighbor 10.2.2.6 {
export only-my-routes;
}


}


}

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