Cisco – Globally disable QoS on Cisco 3750 in production

You definitely should enable MLS QoS. It is also prerequisite for CoPP, which you should add in your TODO list to implement.

Enabling 'mls qos' without other config is extremely bad idea in low-end cats, like 3560/3750, due to unexpected default scheduling and due to heavily reduced buffers causing more microbursting.

On 7600/6500 it's comparatively safe to enable, but of course you can crash your router with 'show run'. I would feel comfortable enough to enable it during production hours.

You should monitor 'show queueing interface X' after enabling to see that you're not dropping anything. Long term, you should design QoS policy and implement it, I recommend using as few queues as possible (and assign 100% of buffer to those few queues). Example of possible QoS policy in WS-X6704-10GE:

interface TenGigabitEthernet4/1
 wrr-queue bandwidth percent 30 40 30 0 0 0 0  ## allocate 3 queues (+implied strictpriority) share buffer 30% 40% 30% on thos three queues
 wrr-queue cos-map 1 1 0 7 # map cos values to queues
 wrr-queue cos-map 2 2 3 
 wrr-queue cos-map 3 1 4 
 wrr-queue cos-map 3 2 6 

For advanced config, you may want to supplement that with 'wrr-queue random-detect' with RED curve per queue+threshold. Be sure to mark your traffic correctly when it enters your network.

You can disable this feature using:

no setup express

To quote Cisco:

The primary purpose of the no setup express command is to prevent someone from deleting the switch configuration by pressing the Mode button for 10 seconds.