why not AS number based prefixes aggregation

Nathan Ward nanog at daork.net
Mon Sep 8 08:36:36 CDT 2008

On 9/09/2008, at 1:20 AM, yangyang. wang wrote:

> Hi, everyone:
>     For routing scalability issues, I have a question: why not  
> deploy AS
> number based routing scheme?  BGP is path vector protocol and the  
> shortest
> paths are calculated based on traversed AS numbers. The prefixes in  
> the same
> AS almost have the same AS_PATH associated, and aggregating prefixes
> according to AS will shrink BGP routing table significantly. I don't  
> know
> what comments the ISPs make on this kind of routing scheme.

I expect you'll see something similar to this in IPv6 long term - most  
ASes will likely not need to originate more than their RIR 'block' (I  
can never figure out whether to call it an assignment or an  
allocation, and whenever I do I seem to get it wrong).

Note that many ASes are expected to intentionally de-aggregate their  
prefixes. We've so far expected to see this for traffic engineering  
purposes (ie. to control sharing of load across several different  
links), however we might see intentional de-aggregation by people  
attempting to minimise BGP prefix hijacking? Who knows. That's a  
pretty scary though when considering IPv6..

There was a Cisco slide somewhere (I think originally by Tony Hain?)  
that had info about that, ie. actual numbers.
It's slide 24 (and several slides leading up to it) in the following  
slide pack by Vince Fuller, but it was old in Feb 2007 when this was  
put together, so it's even older now - anyone have anything more  
recent. Anyway, enjoy:

Nathan Ward

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