[83865] in North American Network Operators' Group
Re: Order of ASes in the BGP Path
daemon@ATHENA.MIT.EDU (Steven M. Bellovin)
Mon Aug 29 15:26:21 2005
From: "Steven M. Bellovin" <smb@cs.columbia.edu>
To: Abhishek Verma <abhishekv.verma@gmail.com>
Cc: nanog@nanog.org
In-Reply-To: Your message of "Mon, 29 Aug 2005 22:15:26 +0530."
<ce8d9033050829094562f3dc2e@mail.gmail.com>
Date: Mon, 29 Aug 2005 15:25:52 -0400
Errors-To: owner-nanog@merit.edu
In message <ce8d9033050829094562f3dc2e@mail.gmail.com>, Abhishek Verma writes:
>
>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] ?
>
Any of the proposed BGP security schemes would have trouble with
reordering.
--Steven M. Bellovin, http://www.cs.columbia.edu/~smb
