North American Network Operators Group

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Re: Order of ASes in the BGP Path

  • From: Richard A Steenbergen
  • Date: Mon Aug 29 13:04:45 2005

On Mon, Aug 29, 2005 at 10:15:26PM +0530, 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?

Just that pesky little thing called sanity, aka having a hope in hell of 
being able to figure out which network is connected to which and in what 
order. While it is technially possible to run everything with an unordered 
AS-PATH set, it is so rare that "most" people looking at AS-PATHs either 
don't know it is possible, or don't take its possibility into account 

Besides being likely to confuse the hell out of a lot of people, you're 
even more likely to break someone's BGP querying perl scripts. I wouldn't 
underestimate the amount of that stuff out there either, especially in 
areas like abuse tracking/reporting. Basically you'd be making a general 
pain in the ass out of yourself, so hopefully you have a damn good reason 
for it. :)

Richard A Steenbergen <[email protected]>
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)