QoS trust question

Unanswered Question
Feb 6th, 2011

Hello,

On a cisco switch, if you do not have the commands:


mls qos trust cos, or mls qos trust dscp

what will the switch do with frames that arrive with dscp or cos markings?

I have this problem too.
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ranraju Sun, 02/06/2011 - 17:29

Switch will forward the frames without tampering the dscp or cos values - only when the QoS is not enabled on the switch.

If QoS is enabled on the switch (on the global config) then - these values could possibly change.

Hope this answers your query...

Regards,

Ranganath.

dwilliams1979 Sun, 02/06/2011 - 18:52

This link may provide some additional details.  How the switch reacts to the situation you present does depend upon whether or not you have the mls qos command configured globally as was previously mentioned.  Hope this helps.

http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_m2.html

ranraju Sun, 02/06/2011 - 19:04

Also according to this link:

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml#concept11

"...While QoS is disabled, all frames/packets are passed-through the switch unaltered. For example, if a frame with CoS 5 and the packet inside the frame with DSCP EF enters the switch, the CoS and DSCP labels are not changed. The traffic leaves with the same CoS and DSCP values as it enters. All the traffic, which includes voice, are delivered on the best effort basis. "

Hope this helps...

Regards,

Ranganath.

dwilliams1979 Sun, 02/06/2011 - 19:20

That is an excellent document!  Definitely one to keep handy. 

LucaSalvatore_2 Sun, 02/06/2011 - 19:30

Thanks, according to yet another link

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_35_se/configuration/guide/swqos.html#wp1021252

"When QoS is enabled with the mls qos global configuration command and all other QoS settings are at their  defaults, traffic is classified as best effort (the DSCP and CoS value  is set to 0) without any policing"

Does this mean that if traffic enters the switch marked with EF it will be re classified with DSCP 0?

ranraju Sun, 02/06/2011 - 19:38

Yes thats correct, traffic entering the switch marked with EF, it will get a DSCP value of 0 by the switch.

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