BGP Path Filtering

Stephen J. Wilcox steve at
Fri May 16 09:01:16 UTC 2003

On Thu, 15 May 2003, Mark Radabaugh wrote:

> I'm having a hard time finding best practices for filtering outbound bgp
> announcements when providing transit to bgp-speaking customers.  While we
> currently multi-home to several providers it appears we will soon need to
> provide transit for customers with their own AS's.
> I find lots of references (and understand) the basic
> ip as-path access-list 3 permit ^$
> and it would seem that should we wish to provide transit for a bgp customer
> AS12345 we would use:
> ip as-path access-list 3 permit ^12345$
> but I think this breaks if AS12345 prepends their advertisement.

yes it will
> Next up is:
> ip as-path access-list 3 permit ^12345_[0-9]$*
> Which seems correct to me.   Is this still best practice (or even correct)?

no, perhaps you mean ..[0-9]*$ but that still wont allow multiple prepends as it 
wont match the space only the numbers


ip as-path access-list 3 permit ^(_12345_)+$

which will allow one or more of their as's

ASN is pretty crude tho, consider using a prefix list to filter each prefix and 
include length


> Mark Radabaugh
> Amplex
> (419) 720-3635

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