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AS Regular Expression

This section explains the creation of a regular expression.

A regular expression is a pattern to match against an input string. When you build a regular expression, you specify a string that input must match. In the case of BGP, you specify a string that consists of path information that an input must match.

In the example in the section Path Filtering, you specified the string ^200$. You wanted path information that comes inside updates to match the string in order to make a decision.

A regular expression comprises:

Range

A range is a sequence of characters within left and right square brackets. An example is [abcd].
Atom

An atom is a single character. Here are some examples:
```
.
```
- The . matches any single character.
```
^
```
- The ^ matches the start of the input string.
```
$
```
- The $ matches the end of the input string.
```
\
```
- The \ matches the character.
```
-
```
- The _ matches a comma (,), left brace ({), right brace (}), the start of the input string, the end of the input string, or a space.
Piece

A piece is one of these symbols, which follows an atom:
```
*
```
- The * matches 0 or more sequences of the atom.
```
+
```
- The + matches 1 or more sequences of the atom.
```
?
```
- The ? matches the atom or the null string.
Branch

A branch is 0 or more concatenated pieces.

Here are some examples of regular expressions:

a*

This expression indicates any occurrence of the letter “a”, which includes none.

a+

This expression indicates that at least one occurrence of the letter “a” must be present.

ab?a

This expression matches “aa” or “aba”.

_100_

This expression means via AS100.

_100$

This expression indicates an origin of AS100.

^100 .*

This expression indicates transmission from AS100.

^$

This expression indicates origination from this AS.

BGP path manipulation

Why laugh at me?

Method	Direction Applied	Direction Affected	Best Metric
Weight	Inbound	Outbound	Highest
Local Preference	Inbound	Outbound	Highest
AS Path	Outbound	Inbound	Shortest
MED (metric)	Outbound	Inbound	Lowest

no-prepend has importance for incoming
replace-as has importance for outgoing

See my blog for more info:

http://usaccie.wordpress.com/