BGP as-path Access-list

The_guroo_2 · ‎05-30-2008

guys if we use permit ^$ what does that mean ??? will it allow routes or not???? cheers

Jon Marshall · ‎05-30-2008

It will allow routes from within the same AS but not from any other AS so they could be received from an IGBP session but not an EBGP session.

Jon

guruprasadr · ‎05-30-2008

HI, [Pls Rate all Informative POST]

In addition to Jon POST, refer link below for cisco Document on, Using Regular Expression in BGP.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a92.shtml

Hope I am Informative.

Pls Rate all Informative POST

Best Regards,

Guru Prasad R

lamav · ‎05-30-2008

Hi:

That regexp (regular expression) is typically used to ensure that an AS that has connections to 2 separate Autonomous Systems (ISPs, typically) does not act as a transit AS and allow routes advertised by one ISP to be advertised over to the other.

So, AS 100 has a BGP connection to AS 200 and AS 300. As 300 advertises prefixes to AS 100, but AS 100 makes sure that it does not pass those routes, and instead advertises ONLY the local prefixes, as Jon pointed out.

HTH

Victor