AS Path Loops in practice ?
nanog-post at rsuc.gweep.net
Mon Dec 8 21:54:07 UTC 2003
On Mon, Dec 08, 2003 at 03:50:21PM -0500, Robert E. Seastrom wrote:
> Jaideep Chandrashekar <jaideepc at cs.umn.edu> 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 http://www.sprint.net/policy/bgp.html
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
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