why not AS number based prefixes aggregation

Mon Sep 8 13:46:27 UTC 2008

Excerpts from yangyang. wang on Mon, Sep 08, 2008 09:20:38PM +0800:
> 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.
> 
> 
> -yang

It might be the right level of granularity for policy but is too
coarse for routing.  You want to be able to route on prefixes (even if
not everyone does it) for flexibility/TE.  Also, ASNs are not
aggregatable so we can't use them to represent a large number of
independently routed networks.