North American Network Operators Group

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Re: Order of ASes in the BGP Path

  • From: Stephen J. Wilcox
  • Date: Mon Aug 29 16:21:22 2005

On Mon, 29 Aug 2005, Abhishek Verma wrote:

> Hi,
> Is the order of AS numbers (except for perhaps the first one which
> denotes the AS the route was originated from) in the AS_PATH in BGP
> important? In fact, does anybody even care for the first AS number
> that appears in the Path?
> AFAIK, AS numbers in the BGP serves two purposes. It helps in loop
> detection and it helps us count the AS Path length.
> If this is the case then the order should not really matter much.
> My question is that whether the operators care if the order, for some
> reason changes?
> Eg.
> Legend: {} denotes the sequence, while [] denotes the set
> Path {1 2} [3 4] {5} 
> Would somebody mind if this was represented as {1 2 5} [3 4] ?

I'd mind, I value the predictability and ability to understand how a bgp path 
arrives into my network. Fiddling with this kind of thing is quite similar to 
spoofing in some ways, particularly that I can see fabricating the as-path 
could be used to confuse folks tracing announcements, perhaps I'm missing the 
positive use for this.

As no one has asked yet, allow me.. what are you trying to do?