Order of ASes in the BGP Path

Mon Aug 29 18:50:18 UTC 2005

On Mon, 29 Aug 2005, Abhishek Verma wrote:

> 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] ?

Yes, they are different paths. You are allowed to merge adjacent 
sequences, eg:

 	{1 2} {5} [3 4]

the two sequences can be merged, to give:

 	{1 2 5} [3 4]

You can *not* merge AS_SET's, as the current BGP specs imply an 
AS_SET has a fixed path-length, hence you should NOT merge the sets 
in:

 	{1 2} [3 4] [5 6]

into:

 	{1 2} [3 4 5 6]

as the former path has a length of 3, the latter a length of just 2 - 
merging sets could change their meaning. Note though that you're not 
at all likely to see such paths with BGP speakers implementing the 
RFC / draft-ietf-idr-bgp-26.txt draft.

regards,
-- 
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
Fortune:
The best way to make a fire with two sticks is to make sure one of them
is a match.
 		-- Will Rogers