cogent update suppression, and routing loops

Fri Oct 3 16:09:18 UTC 2014

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 at gmail.com>
Sent: Thursday, October 02, 2014 12:07 PM
To: "North American Network Operators Group" <nanog at 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