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w.kujadt
Beginner

cos-mapping or dscp-mapping preferred?

Hi all,

for output-scheduling on a LAN Switch which qos-mapping (cos-mapping or dscp-mapping) is preferred? or necessary?

If I had a misconfiguration, for example:

mls qos srr-queue output cos-map queue 1 threshold 1 0

mls qos srr-queue output dscp-map queue 3 threshold 3 0                  

Which one is chosen?

Thanks

Wolfram

2 REPLIES 2
Peter Paluch
Hall of Fame Cisco Employee

Hi Wolfram,

To my understanding, the usage of either cos-map or dscp-map is on a per-frame basis and depends on the trust mode of the ingress port where the frame was originally received. If mls qos trust cos was used on an input port then the internal QoS label is derived from the CoS marking and on the egress port, the cos-map will be used. If mls qos trust dscp was used on an input port then the internal QoS label is derived from the DSCP marking and on the egress port, the dscp-map will be used.

You may be interested in reading this extensive article:

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

Best regards,

Peter

Hi Peter,

thank you for your reply!

You are right, the QoS label depends on the trust mode (trust cos or trust dscp) of the ingress port:

"Depending upon how the switch and the ports are configured, the QOS label assigned to the incoming packet will vary. ... ... When ports are configured for “mls qos trust dscp” then the assigned QOS label is DSCP value + 2. When “mls qos trust cos” is configured then QOS Label is COS value + 66."

It can be seen with:

"switch#sh platform qos label

Label   : Rx queue-thr : Tx queue-thr : rewrite dscp-cos

--------------------------------------------------------------

...

30                  3 - 0         0 - 0                        2E   - 5        (trust dscp, DSCP ef for VoIP))

... 

47                  3 - 0         0 - 0                        28   - 5         (trust cos, Cos 5 for VoIP)"

But there is still the possibility of misconfiguration:

mls qos srr-queue output cos-map queue 1 threshold 1 5                =match QoS-Label 47?

mls qos srr-queue output dscp-map queue 3 threshold 3 46             =match QoS-Label 30?

"switch#sh mls qos maps

...

Dscp-outputq-threshold map:
     d1 :d2    0       1       2       3        4       5       6       7       8       9
     ------------------------------------------------------------
      0 :    03-03 03-01 03-01 03-01 03-01 03-01 03-01 03-01 04-03 04-01
      1 :    04-01 04-01 04-01 04-01 04-01 04-01 02-01 02-01 02-01 02-01
      2 :    02-01 02-01 02-01 02-01 02-02 02-01 02-02 02-01 02-01 02-01
      3 :    02-01 02-01 02-01 02-02 02-02 02-02 02-02 02-02 02-02 02-02
      4 :    01-01 01-01 01-01 01-01 01-01 01-01 03-03 01-01 02-03 02-03

...

Cos-outputq-threshold map:
              cos:         0    1    2   3   4     5    6    7 
              ------------------------------------
  queue-threshold: 3-3 4-3 2-1 2-2 2-1 1-1 2-3 2-3

..."

I think, the Dscp-outputq-threshold map is chosen.

But I don't know why .

Best regard,

Wolfram