Re: Order of ASes in the BGP Path

["Steven M. Bellovin" <smb () cs columbia 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