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Re: AS Path Loops in practice ?
On Mon, Dec 08, 2003 at 03:50:21PM -0500, Robert E. Seastrom wrote: > Jaideep Chandrashekar <[email protected]> writes: [snip] > > 11608 2914 1239 12064 22773 12064 11836 > > 1221 4637 1239 12064 22773 12064 11836 [snip] > 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 activity. 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