BGP Path Filtering

Leo Bicknell bicknell at
Fri May 16 13:44:28 UTC 2003

In a message written on Thu, May 15, 2003 at 10:29:18PM -0400, 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 strongly recomend you prefix list filter your customers, rather
than AS path filter them.  While AS path filters to prevent some
kinds of abuse and accidental mistake, they still allow your customer
to hijack any address space in your network (and possibly beyond)
at any time.

> ip as-path access-list 3 permit ^12345$
> but I think this breaks if AS12345 prepends their advertisement.

Probably you want something more like:

ip as-path access-list 3 permit ^(12345_)+$
ip as-path access-list 3 permit ^(12345_)+(6789_)+$

Giving both the customer, and customer with a customer case.  That is
both specific, and allows for prepends.  Your example has a couple of

> ip as-path access-list 3 permit ^12345_[0-9]$*

First, it's not a valid regex ($* need to be *$), second, it allows any
(single) AS behind 12345, so it's hardly a useful filter.

       Leo Bicknell - bicknell at - CCIE 3440
        PGP keys at
Read TMBG List - tmbg-list-request at,
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