[AusNOG] OT: AS_PATH prepend (was: Re: Largest IP Backbone)

John Edwards john at vocus.com.au
Sat May 2 15:10:14 EST 2009

On 01/05/2009, at 3:39 PM, Dale Shaw wrote:
> WHY? (surely they haven't done this to tip the balance somewhere many
> AS hops away?)

Think a little more laterally - there's no technical reason why you  
can't do it. The problems are that it may uncover bugs in vendor  
handling of long strings, be rejected by another provider's policy, or  
cause people on mailing lists to berate you for it.

You could use excessive prepending like a poor-man's transitive  
community string. Say that you (A) and another network (E) had a  
choice of several transit networks (B,C,D)  between you, each of those  
networks localpref'd customers and you wanted to signal which transit  
provider to use for a given route.

Say that traffic normally goes via transit provider B. 'Tag' the route  
to D with 5 prepends, and have the other provider (E) match on regex  
([0-9]+_)\1\1\1\1 to increase the preference to that transit provider,  
and then you have a method for A to control routing without manual  
intervention from E. The rest of the Internet will see the additional  
prepends to D, and continue sending traffic via B . E might even have  
a reason encourage other networks to use this feature. I hasten to add  
that there are almost certainly better ways to do this :)

Hypothetically, if you were some kind of blackhat who specialised in  
breaking into network infrastructure, adding a specific number of  
prepends to all announcements might serve as a way to boast publicly  
about your work to your community, without breaking the compromised  
network. Then the operator of that network will also suffer berating  
on mailing lists :)

John Edwards

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