From [bgp.potaroo.net <http://bgp.potaroo.net>], the number of all ASs seen in all the route-views routing tables is around 21,000.
Plenty of space to recover, even though some of those might be in private use (and might or might not be able to use private ASNs).
There just doesn't seem to be the political will to do so (e.g., by starting charging some amount of money per year, so dead/unpaid ones would be turned up). Or folks may consider that too big an effort compared to just upgrading to 4B as numbers now.
Seems a bit irresponsible to me. Personally I'd rather focus on cleaning up the AS number mess a bit rather than throwing more technology at the problem.
Its true that we see only some 20,700 AS's in the BGP table today, and its also true that there are some 12,500 unadvertised AS's that have been assigned by the RIR system (and its predecessors), and some 6,600 AS numbers held in the RIRs that they are currently allocating from ( http://www.potaroo.net/tools/asns) The AS story is about the trends in recent times, which see a best fit of an exponential growth model to the past 3 years of AS number allocations by the RIRs, and if we assume no change in AS number policies, and no change in the trend of ageing out 'old' AS numbers at a rate of some 5% per year into the unadvertised pool, then the 2byte field will exhaust sometime in October 2010. At that time there will be some 40,000 advertised AS numbers and some 20,000 unadvertised AS numbers. The discussion of whether there are 'natural' limits to AS number consumption is an interesting one - it seems that more smaller sites are multi-homing now, and the pressure for more AS numbers in the routing system appears to be based strongly in the area of distinct routing policies in an ever-richer inter-AS connectivity mesh ( http://www.potaroo.net/ispcol/2005-08/as.html contains one view of this). I'm of the view that this consumption trend reflects a behavioural trend in routing that is going to prove difficult to stop, and what we will see is a steady growth in the number of stub AS numbers that perform no transit at all. Is AS reclaimation an option? We don't know how many 'dark' (unadvertised) AS numbers are used as VPN IDs in 2547 contexts. We don't know whether this use of AS numbers will continue to grow. The recent data suggests a steady growth in the unadvertised AS count, but not at the same rate as advertised AS numbers. How much time would aggressive reclaimation buy us? Current figures indicate that it would work for 3 years if we were able to reclaim ALL unadvertised AS numbers and recycle them. How much effort would it take to get this additional 3 years? Is it worth it when ytou consider that the only AS domains trhat actually need to run the NEW BGP code are thos routing domains that use AS numbers with a non-zero high-order two bytes. So, as I see it, the requirement for 4-Byte AS numbers is, at the present, very much a 'when' not 'if' question. regards, Geoff