512k day

512k day is the unofficial title of an event that started on August 12, 2014. Multiple Internet routers, manufactured by Cisco and other vendors, encountered a default software limit of 512K (512,000 - 524,288) IPv4 BGP routing table entries, causing assorted outages at various data centers. Various IT professionals reported the issue on Internet forums, sometimes as just "512k", and under a Twitter hashtag of #512k.
Technical details
With IPv4 address exhaustion occurring, and the lack of an expected switchover to IPv6 by network operators, aggressive subnetting of IPv4 has taken place to extend the life of that protocol. A number of Cisco routers commonly in use have TCAM, a form of high-speed content-addressable memory, for storing BGP advertised routes. On impacted routers, the TCAM is default allocated to 512k entries for IPv4 routes, and 512k entries for IPv6 routes. While the reported number of IPv6 advertised routes was only about 20k, the number of advertised IPv4 routes reached the default limit, causing a spillover effect as routers attempted to compensate for the issue by using slow software routing (as opposed to fast hardware routing via TCAM). The main method for dealing with this issue involves operators changing the TCAM allocation to allow more IPv4 entries, by reallocating some of the TCAM reserved for IPv6 routes. This requires a reboot on most routers. The 512k problem was predicted in advance by a number of IT professionals.
The actual allocations which pushed the number of routes above 512k was the announcement of about 15,000 new routes in short order, starting at 07:48 UTC. Almost all of these routes were to Verizon Autonomous Systems 701 and 705, created as a result of deaggregation of larger blocks, introducing thousands of new /24 routes, and making the routing table reach 515,000 entries. The new routes appear to have been reaggregated within 5 minutes, but instability across the Internet apparently continued for a number of hours. Even if Verizon had not caused the routing table to exceed 512k entries in the short spike, it would have happened soon anyway through natural growth.

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