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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: why not AS number based prefixes aggregation
On Mon, Sep 8, 2008 at 9:20 AM, yangyang. wang <[email protected]> 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. Yang, 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. Regards, Bill Herrin -- William D. Herrin ................ [email protected] [email protected] 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004
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