BGP Path Filtering

Jay Ford jay-ford at
Fri May 16 04:23:30 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 like using BGP communities:
   o  filter announcements you receive from downstreams based on your
      criteria of choice
   o  set a specific BGP community (yourAS:whatever) on announcements which
      are to be propagated to various places (upstream providers, etc.)
   o  do output filtering based on the communities you set
Make sure you clear yourAS:whatever in announcements from your BGP peers.

You can use this same technique for prefixes you originate, so that all your
outbound BGP filtering is based on communities.

Jay Ford, Network Engineering Group, Information Technology Services
University of Iowa, Iowa City, IA 52242
email: jay-ford at, phone: 319-335-5555, fax: 319-335-2951

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