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Re: Order of ASes in the BGP Path

  • From: Robert Bonomi
  • Date: Mon Aug 29 13:50:18 2005

> Date: Mon, 29 Aug 2005 22:15:26 +0530
> From: Abhishek Verma <>
> Subject: Order of ASes in the BGP Path
> 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.

AS_PATH is used for various administrative 'policy' matters, as well.

It is not used in the actual routing of data.  that functionality is
accomplished using only the destination netblock and 'next hop' info.

> If this is the case then the order should not really matter much.

Except for standards compliance.  <evil grin>

Per the RFCs on the subject, if you _receive_ an unordered set from
a downstream, you can propogate that unordered set, but you must
prepend your AS in the 'ordered' fashion.

And you must use the ordered path tagging for any new stuff you generate.

> 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} 

As I understand the specs, that is -not- allowed.  an unordered set
can appear only as the _last_ element of the AS path list.
> Would somebody mind if this was represented as {1 2 5} [3 4] ?

Did the topology change?  Or did "somebody" just decide to re-write the
AS_PATH elements ?

your first illustration would 'apparently' describe a topology on the order of;

            /  |  \
        1--2   |   5
            \  |  /

where the latter describes something on the order of:

     1--2--5-+ |

Anybody  that is 'mapping' interconnections is going to be "unhappy" if
the 'representation', and the 'reality' are not consistent.  :)

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