I've had the exact same thing happen. Recently, I removed a bunch of /24's that were member of a larger /20. We use to advertise the /24's to certain carriers for traffic engineering. I saw the exact same issue you're describing. I chocked it up to Slow BGP in their core. For instance, I removed the route from my session with them in Orlando. It would get removed from Orlando, But still exist in Jacksonville. And basically route loop coming in from the north. It'd land in JAX, And get forwarded to MCO. Which would then forward it back to JAX. 20-30 seconds later, it would get removed from JAX, and I'd see the same behavior between JAX and ATL. It took a 4-5 minutes in my experience for the route to finally leave the Cogent network. I lessened the blow by making Cogent the first peer I'd remove the /24 from. Leaving the other /24's out there via the other carriers. So only those that chose cogent as the most direct route felt the unintentional blackhole. Other carriers removed almost instantly as expected. Nick Olsen Network Operations (855) FLSPEED x106 ---------------------------------------- From: "ryanL" <ryan.landry@gmail.com> Sent: Thursday, October 02, 2014 12:07 PM To: "North American Network Operators Group" <nanog@nanog.org> Subject: cogent update suppression, and routing loops hi. relatively new cogent customer. is what i've stated in my subject line kinda standard fare with them? i've discovered that when i advertise a /24 from inside a larger /22 to XO, (who peers with cogent), and then pull the /24 some time later, that cogent holds onto the /24 and then bounces packets around in their network a bunch of times for upwards of 8-10 minutes until they finally yank it. this effectively blackholes traffic to my /24 for anyone that is using a path thru cogent. example: http://ryry.foursquare.com/image/0e0K1K0t0W2M it's been a bit of a frustrating experience talking to their noc to demonstrate it, but i'm able to duplicate it on demand. even pushing routes using their communities to offload the circuit takes forever to propagate even on their own looking-glasses. thx ryan