6500 Series QoS activation

[Sam Preston]

Hi all,

I have a Cisco 6509 switch with PFC-3BXL onboard and currently no QoS activated on the PFC.
My question is : if I activate QoS on the PFC by typing the following command on the switch :

mls qos

... what really happens and what will change (if I only type this command and nothing else) ?

By default the trust state for all ports is "untrust" which means DSCP and CoS fields will be rewritten to 0 by the switch. So this is a change which I have already in mind. But will the switch start behaving differently regarding output queuing of the frames ? Are for example the buffers and queues on the ingress and egress of the switch going to be changed and could this induce frames drop which I wouldn't have had if I don't type the command "mls qos" ?

Thanks for clarifying this !

Cheers,
Sam.

[Reza Sharifi]

Hi Sam,

That command only enables qos globally.

f you enable QoS globally, QoS is enabled on all interfaces with the exception of the interfaces where you disabled QoS. If you disable QoS globally, all traffic is passed in QoS pass-through mode.

In port-queueing mode, Policy Feature Card (PFC) QoS (marking and policing) is disabled, and packet type of service (ToS) and class of service (CoS) are not changed by the PFC. All queueing on rcv and xmt is based on a QoS tag in the incoming packet, which is based on the incoming CoS.

link:

http://www.cisco.com/c/en/us/td/docs/ios/qos/command/reference/qos_book/qos_m2.html#wp1040901

HTH

[Sam Preston]

Thanks Reza for your reply.

Indeed what I intend to do is configure CoPP which requires "mls qos" to be configured in order to permit CoPP to be applied at the hardware level (on the PFC). So the "mls qos port-queueing" cannot be used in such context because I want QoS at the PFC.

What I would like to do is enable qos just to permit hardware CoPP but not change anything to how frames are queued on the ports (in/out) currently and not change the DSCP/CoS field value within the frames transiting through the switch.

What is the best approach for this please ?

Thanks !

[Reza Sharifi]

I think you can enable "mls qos" to configure COPP, and as long as you have not configured any COS or DSCP parameter than frames are untouched.

HTH