DSCP or COS QoS

[akin_lopez]

Hello,

can someone tell me how to tell a catalyst swicth (maybe 3750) to use only DCSP or COS for queueing.

sometimes when i use DSCP, i notice some COS only marked packets goes into queue based on default COS mapping.

is there a way to tell the switch only to use DSCP or COS so that i need to only configure mapping for one of them. or do I have to configure mapping for both?

[Peter Paluch]

Hello,

The trust is decided based on the mls qos trust { cos | dscp } per-interface command. Is this what you are asking about?

sometimes when i use DSCP, i notice some COS only marked packets goes into queue based on default COS mapping.

Can you explain this in more detail please? How do you know that such packets exist and how they are enqueued?

Best regards,

Peter

[akin_lopez]

many thanks. but i am not talking about trust port. i am talking about output queues.

when i configured output queue mapping based on DSCP, i see packet in queues i have not configured anything for.

and i know this queue is matching COS values mapping.

but generally how do u know whether to mapping output queues using DSCP or COS?

[Peter Paluch]

Akin,

when i configured output queue mapping based on DSCP, i see packet in queues i have not configured anything for. 

I see. Can it be caused by the switches themselves originating control traffic (PVST, UDLD, LACP/PAgP, CDP, VTP, DTP ...) that is enqueued into these queues? Or perhaps is there any non-IP traffic in your network? I would suspect this as the primary cause.

and i know this queue is matching COS values mapping. 

I do not quite understand this statement. A queue always matches both DSCP and CoS values mappings. Just by seeing packets in a particular queue, you can not be entirely sure which value - DSCP or CoS - decided that the packet would end up in that queue.

but generally how do u know whether to mapping output queues using DSCP or COS?

To my best understanding, the ingress trust - DSCP or CoS - decides which value will be used for the entire journey of the frame through the switch. So once I trust DSCP on a port, only the DSCP value will accompany the frame inside the switch, including the queueing processes.

You may actually be interested in reading the following document:

https://supportforums.cisco.com/docs/DOC-8093

Best regards,

Peter

[smehrnia]

Hi Akin,

Look, there is this new feature supported on 10Gig interfaces ONLY. Command: mls qos queue-mode mode-dscp which forces the use of DSCP for queueing on a per interfaces basis. for the older versions and other interfaces i believe if you dont wanna get surprised by default mappings, you may decide to write your own manual DSCP-CoS mapping table.

plz Rate if it helped.

Soroush