23456 is AS_TRANS. Either your router does not support 4 byte AS or there is a bug at AS 12956 or AS 12956 is intentionally prepending 23456. Thanks, Jakob.

Date: Tue, 20 Jun 2017 23:12:45 +0000 From: James Braunegg <james.braunegg@micron21.com> To: "nanog@nanog.org" <nanog@nanog.org> Subject: Long AS Path Message-ID: <e679487be750411a874b7376a7037aa9@EX-01.m21.local> Content-Type: text/plain; charset="us-ascii"

Dear All

Just wondering if anyone else saw this yesterday afternoon ?

Jun 20 16:57:29:E:BGP: From Peer 38.X.X.X received Long AS_PATH= AS_SEQ(2) 174 12956 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 ... attribute length (567) More than configured MAXAS-LIMIT

Jun 20 16:15:26:E:BGP: From Peer 78.X.X.X received Long AS_PATH= AS_SEQ(2) 5580 3257 12956 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 ... attribute length (568) More than configured MAXAS-LIMIT

Someone is having fun, creating weird and wonderful long AS paths based around AS 23456, we saw the same pattern of data from numerous upstream providers.

Kindest Regards,

James Braunegg