Over Subscription Ratio

[Alexander1023]

Hi,

Going from Access to Distribution layer Cisco's recommendation is a 20:1 subscription ratio. Is it possible to dynamically throttle switch ports (prevent devices from connecting at 1000M) once that uplink threshold is met? 

Regards,

-Alex

[Georg Pauwen]

Hello,

some sort of QoS/traffic shaping should work. What platform/models are your switches ? 

[Joseph W. Doherty]

It might be possible using an EEM script, but realize the 20:1 ratio is a very. very rough rule-of-thumb, and also understand uplink bandwidth management might be better done using QoS.

[Scott Hodgdon]

Alexander,

Is it possible ? Yes. You could use scripting (depending on which switch / IOS you are using) and EEM to monitor total uplink bandwidth every so often and then execute a command to set all ports to 100-full if the bandwidth exceeds a certain amount. However, you would also then want to reset the ports to "auto" (to get back to 1000-full) if the bandwidth is lower than 20:1, right ? Resetting the port speed takes down the connection to the client so that negotiation can occur. What if the bandwidth is fluctuating to barely above and then barely below 20:1 ? The system would be resetting port speeds very frequently, causing a very negative user experience due to the ports flapping. So while possible, it would not be recommended at all. I think you would be better off to identify your power users that might actually need the 1000-full setting, leave their ports at "auto" and then set the rest of the ports to 100-full for the rest of your users. 

Cheers,
Scott Hodgdon

Senior Technical Marketing Engineer

Enterprise Networking Group