BGP AS-PATH filter question

[pauljackson059]

Hi, Could some one please help me with understanding what would the below AS-PATH access-list accomplish?

Router01#sh run | in as-path
ip as-path access-list 100 permit ^64512(_64512)*$
ip as-path access-list 100 permit ^64512(_64512)*(_64513)*$

This is then configured under BGP  with the following command - 'neighbor x.x.x.x filter-list 100 in'

I understand that ^ means that the router will only accept prefixes that have originated from AS 64512. However, I'm unable to understand how the parentheses and the other expressions (*,_,$) applies in this configuration.

Any help would be appreciated. Thanks!

(I have changed the AS numbers to private AS numbers for security reasons)

Regards,

Paul

[Rolf Fischer]

Hi Paul,

you can find some general information about Regular Expressions in IOS HERE.

Another useful link is the AS Regular Expressions section of the BGP Case Studies.

_64512 means one delimiter (Space/Blank, Tab, etc.) followed by 64512, e.g. " 64512".

(_64512) is the same thing, the parentheses group the expression _64512 so you can combine it with a quantifier.

* is a quantifier. It means 0 or more occurrences of the group. So (_64512)* matches an empty string (0), " 64512" (1), " 64512 64512" (2), " 64512 64512 64512" (3) and so on.

^ and $ match the beginning and the end of a string.

So the first line says the as-path has to begin with 64512, optionally followed by any number of repetitions of that AS, e.g. "64512 64512 64512" (Origin: 64512). The second line additionally allows optionally any number of occurrences of AS 64513 at the end of as-path, e.g. "64512 64512 64513 64513" (Origin: 64512 OR 64513). The first line seems to be somewhat redundant.

In a nutshell:
• minimum accepted AS_PATH: "65512"
• accepted AS_PATH expressions 1 and 2: "65512 65512 65512 (...) 65512"
• accepted AS_PATH expression 2: "65512 65513"
• accepted AS_PATH expression 2: "65512 65513 65513 (...) 65513"
• accepted AS_PATH expression 2: "65512 65512 65512 (...) 65512 65513 65513 (...) 65513"

It's not easy to explain ...

HTH
Rolf