Re: How to view default cos-dscp mappings in MQC QoS

graham robinson · ‎04-11-2018

Hi,

We have 3850 switches at the access layer with L2 trunk uplinks to 6500 switches with Sup2T at the Distribution/Core layer. We have auto-qos enabled at both layers but I am unable to find the cos-dscp and dscp-cos mappings being used by these switches.

In the old 3750's you could see the cos-dscp mappings using the commands "show mls qos maps cos-dscp" and "show mls qos dscp-cos" but I have not been able to find an equivalent command for mqc qos that shows me these mappings.

The issue is that we are marking some traffic with a dscp value of 12 at the access layer, but when it reaches the core layer this has been changed to a dscp value of 8. I am assuming that when it goes over the trunk uplink it is being given a cos value of 1, and when it reaches the 6500 this is taking the cos value of 1 and applying a dscp value of 8, because that's what the standard cos-dscp and dscp-cos maps would have done in mls qos. But I cant find any way of viewing the cos-dscp and dscp-cos maps in mqc qos so I cant actually confirm this.

Could someone point me in the right direction please?

Reza Sharifi · ‎04-11-2018

Hi,

Try:

sh platform qos dscp counter <interfacex/x/x>

Also, have a look at this document for the old show commands vs the new commands:

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3850-series-switches/118629-technote-qos-00.html#anc10

HTH

graham robinson · ‎04-11-2018

Hi Reza

Thanks for your reply, but this command doesn't show what I need. To confirm, I want to see the cos-dscp and dscp-cos mappings being used by the 3850's and 6500's.

In a 3750 this would look like this:

c3750#sh mls qos maps cos-dscp
Cos-dscp map:
cos: 0 1 2 3 4 5 6 7
--------------------------------
dscp: 0 8 16 26 32 46 48 56

c3750#sh mls qos maps dscp-cos
   Dscp-cos map:
     d1 : d2 0 1 2 3 4 5 6 7 8 9
     ---------------------------------------
      0 :    00 00 00 00 00 00 00 00 01 01
      1 :    01 01 01 01 01 01 02 02 02 02
      2 :    02 02 02 02 03 03 03 03 03 03
      3 :    03 03 04 04 04 04 04 04 04 04
      4 :    05 05 05 05 05 05 05 05 06 06
      5 :    06 06 06 06 06 06 07 07 07 07
      6 :    07 07 07 07

So these outputs tell me that [for example] if a packet has a dscp value of 12 it will be given a cos value of 1 when it is sent over a l2 trunk, and if a frame is received with a cos value of 1 it will be given a dscp value of 8 [if the interface is set to trust cos values].

What I need is the equivalent commands for the 3850 and 6500/sup2t so i can see the default cos-dscp and dscp-cos mappings on these devices.

I have checked the document and there doesn't appear to be any equivalent command?

chrisbell1 · ‎09-27-2020

Gidday,

I also encountered this problem.

It seems that these switches have default mappings which you cannot change. You can only disable the behaviour.

I have used the command "no qos rewrite ip dscp" to prevent the switch from doing it in the cos > dscp direction. I was then able to fluff around with table maps to achieve my goal.

Cheers,