BGP as-path regular expressions

[rthakker]

Dose any one knows good link or reference material I can use to understand BGP as-path regular expressions for route filtering. I have basic understanding but just in case if I can understand this in depth I am looking for as many as regular expression examples with explanation

Thanks

Rits

[thisisshanky]

Rits,

These may help!

http://www.quagga.net/docs/docs-multi/AS-Path-Regular-Expression.html

http://www.cisco.com/warp/public/459/26.html

http://www.avici.com/documentation/HTMLDocs/02223-06_revBA/Routing_Pol7.html

HTH

Sankar.

[ekiriakos]

If you haven't got access to a test network, when you have studied how reg expressions work try...

http://www.netdigix.com/servers.html

It's a list of public route servers.You are allowed access to do sh ip bgp regexp so you can test your knowledge..-)

Rgds

E.

[sdoremus33]

This info might be helpful for your needs

Range

A range is a sequence of characters contained within left and right square brackets. For example: [abcd]

Atom

An atom is a single character, such as the following:

. (Matches any single character)

^ (Matches the beginning of the input string)

$ (Matches the end of the input string)

\ (Matches the character)

- (Matches a comma (,), left brace ({), right brace (}), the beginning of the input string, the end of the input string, or a space.

Piece

A piece is an atom followed by one of the following symbols:

* (Matches 0 or more sequences of the atom)

+ (Matches 1 or more sequences of the atom)

? (Matches the atom or the null string)

Branch

A branch is a 0 or more concatenated pieces.

Examples of regular expressions follow:

a* (Any occurrence of the letter "a", including none)

a+ ( At least one occurrence of the letter "a" should be present)

ab?a (This matches "aa" or "aba")

_100_ (Via AS100)

_100$ (Origin AS100)

^100 .* (Coming from AS100)

^$ (Originated from this AS)

Refer to Using Regular Expressions in BGP for sample configurations on regular expression filtering