Allied Telesis X8100, x908 manual BGP Configuring AS Path Filters, AS path lists

BGP: Configuring AS Path Filters

BGP: Configuring AS Path Filters

To configure path filters we need to first understand something about AS path lists and how to use them.

AS path lists

Path filters use a construct known as an AS path list. An AS path list has a name and consists of one or more (unnumbered) entries. Each entry specifies:

which AS paths to consider.

whether the AS paths in question should be included or excluded from the list.

The set of paths to consider is specified using regular expressions. The AlliedWare Plus OS supports the full set of syntax elements for simple regular expressions:

SYMBOL	CHARACTER	USED TO...
^	Caret	Match the beginning of the input string. When
		used at the beginning of a string of characters, it
		negates a pattern match.
$	Dollar sign	Match the end of the input string.
.	Period	Match a single character (white spaces included).
*	Asterix	Match zero or more sequences of a pattern.
+	Plus sign	Match one or more sequences of a pattern.
?	Question mark	Match zero or one occurrences of a pattern.
_	Underscore	Match spaces, commas, braces, parentheses, or the
		beginning and end of an input string.
[]	Brackets	Specify a range of single-characters.
-	Hyphen	Separate the end points of a range.

For example, you can specify things like:

any path that contains a certain set of AS numbers: e.g. 23334 45634 8988

any path that starts with a particular AS number: e.g. ^23334

any path that ends with a particular AS number: e.g. 8988$

a specific path: e.g. ^23334 45634 8988$

an empty path: ^$

Syntax To create an entry in an AS path list, use the command:

awplus(config)# ip as-path access-list <list-name> {deny permit} <regular-expression>

There is an implicit deny all entry at the end of each path list. Any AS path that does not match any earlier entry is excluded from the list.

Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes Page 17