North American Network Operators Group

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Re: AS Path Loops in practice ?

  • From: Joe Provo
  • Date: Mon Dec 08 16:56:40 2003

On Mon, Dec 08, 2003 at 03:50:21PM -0500, Robert E. Seastrom wrote:
> Jaideep Chandrashekar <[email protected]> writes:
> > 11608 2914 1239 12064 22773 12064 11836
> > 1221 4637 1239 12064 22773 12064 11836
> In many (most?) these "loops" are intentional, and a result of playing
> prepending games...  There's no restriction on what you can prepend to
> an AS path (and it can be handy to put stuff in there to keep other
> providers from picking up your routes as we'll see momentarily), but
> your peer generally wants to see the most recent as in the path as
> your AS.
> So if I (AS3066) wanted to send routes to Sprint that were not to be
> picked up by UUnet, I could do as follows:
>    route-map to-as1239-nothanks-uu permit 10
>     set as-path prepend 3066 701
> and apply it outbound in the advertisement to Sprint.
> Then what you would see in the route reflector would be:
>    1239 3066 701 3066
While this is an explataion of the behavior, it should not be
an endorsement.  Prepending someone else's AS is a bad practive.
Not only does it munge 'pure' research data, but fowls some 
levels of peer evaluation [in the example, and as-701 connected 
entity seeing your path from 1239 would have to determine why 
they weren't getting your paths; or a casual glance would indicate 
you were'nt peer-worthy because you were behind a peer].  Worse, 
forging AS-paths obfuscates the operational chain of responsibility.  
Of course that is the goal of some of theses actrivities. 
Obviously-bogus AS paths are a strong indicator of suspicious 

Many providers publish specific BGP communities for customers to 
use to handle the redistribution at the provider's edge; some are 
coarse-grained and some provide real control. Many provide something 
but you have to ask for the information. If your provider doesn't 
give you this service/feature, demand it.

In RS's example, a trip to 
would tell you to just tag with community 65000:701
    route-map to-as1239-nothanks-uu permit 10
     set community 65000:701

Attempting action at a distance generally fails at the far-end of 
your service contract; any implementation that *does* work *should*
only spew data the same distance.

             RSUC / GweepNet / Spunk / FnB / Usenix / SAGE