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Errors announcing 32-bit ASNs in BGP routes

We present evidence of a phenomenon where there is large scale misconfigurations of BGP routers starting in 2007 and continuing to the present date. This has to do with the change from 16-bit to 32-bit autonomous system (AS) numbering and the employment of the fictitious AS, AS23456. This error strongly biases estimations of the number of ASes performing a type of inter-domain routing called multi-exit routing. In data from 2010, over half of apparent cases are in fact false. We show how to detect this error and obtain a truer picture of the extent of multi-exit routing.