In search of perfection (was Re: Getting a "portable" /19 or /20)

[Sean Donelan <sean () donelan com>]

On Tue, 10 April 2001, Sean M. Doran wrote:
> Perfectly aggregated networks are star-shaped.  
>
> Any more complicated topology cannot be perfectly aggregated.
>
> In real networks, aggregation at best follows a "reasonable"
> trade-off between optimizing and stabilizing route selection.
> Not everyone will agree on what is a "reasonable" balance.  

Oh great, I'm having flash backs to my Data Structures class.

A balanced tree is an example of something which can be perfectly
aggregated.  It also has a nasty habit of generating extremely
long paths.  A full mesh is an example of something which has
no aggregation potential, but has the shortest possible paths
for every link.

When folks add a cost component, you generally end up with a
set of interconnected hubs-and spokes.

The current CIDR allocation method assumes aggregation (and the
hubs and spokes) will occur along network providers and their
network topologies.  The other alternative, used by the telephone
system, aggregates hubs and spokes along geographic boundaries.
Which requires multiple providers within a geographic region
exchange more data, but requires less network information visible
on the global level.