The Internet Outgrows Itself on August 12


The Internet outgrows itself as routers run out of room for new routes

The latest arbitrary limit event arrived on August 12 with a mysterious Internet slowdown predicted by experts but anticipated by few Internet users. The so-called 512K Day disruption received much less advance attention than the Y2K phenomenon 14 years ago, but the basic cause was oddly similar – an arbitrary limit exceeded.
In their default state, several Cisco router models (and possibly some routing devices from other vendors) can hold a total of 512,000 IPv4 routes in high-performance TCAM memory. The devices actually allow for a total of 512,000 IPv6 routes also, but because of the slow adoption of IPv6, most are operating well below the IPv6 limit.

To stave off the end of the IPv4 address era, many Internet providers are in the process of re-subnetting their networks to make more efficient use of IPv4 addresses. Verizon reportedly tripped the event by deaggregating a large block of addresses and introducing thousands of new /24 routes. The sudden addition of nearly 15,000 new routes put many Internet routers over the 512,000 limit, although, according to experts, the routers would have eventually run out of capacity even if Verizon's August 12 deaggregation didn't occur.

The event caused a disruption to the ultra-high-speed TCAM memory lookups, causing the routers to revert to the much slower, software-based routing, which brought the Internet to a crawl in some areas. The fix was relatively simple once the problem was discovered. Admins manually allocated some of the unused IPv6 TCAM memory over to IPv4; however, Internet traffic remained unstable for several hours.


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