why not AS number based prefixes aggregation

William Herrin herrin-nanog at dirtside.com
Mon Sep 8 09:35:14 CDT 2008

On Mon, Sep 8, 2008 at 9:20 AM, yangyang. wang <wyystar at gmail.com> wrote:
>     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.


Two reasons:

1. The AS# alone is insufficiently granular to support traffic
engineering for inbound traffic.

2. It would overload a location function on the existing identity
function. The AS# presently identifies the entity announcing the
routes. Routes from the same AS# come from the same entity.  If we
routed by AS#, the AS# would also serve to specify the entity's
location in the network graph.

One of the implications of the research in the RRG is that this
identity + location functional overloading is the root cause of our
route table problems. Specifically, the IP address both provides a
node identity for use by the layer-4 protocols and a layer-3 location
within the network graph. Had the IP protocol separated the two
functions, it would be almost as trivial to expand the routing system
as it is to expand the DNS system because the location part
effectively aggregates by topology while the identity part has a nasty
habit of changing locations.

Unfortunately that realization doesn't appear to have left any
corrective actions which are both clean and compatible with the
existing system.

Bill Herrin

William D. Herrin ................ herrin at dirtside.com bill at herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

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