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Re: Order of ASes in the BGP Path

  • From: Steven M. Bellovin
  • Date: Mon Aug 29 15:27:10 2005

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






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