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Cisco Bug: CSCuo39282 - N3K: QOS Classification Requires Set-QoS Group Cmd to Remark DSCP

Last Modified

Feb 19, 2018

Products (7)

  • Cisco Nexus 3000 Series Switches
  • Cisco Nexus 3064 Switch
  • Cisco Nexus 3016 Switch
  • Cisco Nexus 3064-T Switch
  • Cisco Nexus 3048 Switch
  • Cisco Nexus 3132Q Switch
  • Cisco Nexus 3172 Switch

Known Affected Releases


Description (partial)

This is day-one Nexus 3x00 QoS behavior (not a bug).

A user-defined class-map cannot be configured with the "set qos-group 0" command.

A qos-group has to be set when classifying traffic on DSCP value and rewriting it to another value.  However, it should be noted that the traffic has to set to a qos-group that is not configured in the network-qos/ergress queuing policy to map it back to the default qos-group 0 when the classification policy matches specific DSCP values and preserves other DSCP values. 

In the following example, traffic matching DSCP 3 and 4 should be remarked to DSCP 0 and mapped to qos-group 0 (qos-group 7 is configured, but it isn't present in network-qos policy).  Traffic matching DSCP 2 is mapped to qos-group 2 and all other traffic with DSCP values not previously matched are honored (not modified) and mapped to qos-group 0 using the "class-default" class-map.

QoS Configuration:

class-map type qos match-all RDMA
  match dscp 4
class-map type qos match-all SCAVENGER
  match dscp 8
class-map type qos match-all LOSSLESSTCP
  match dscp 3

policy-map type qos UPLINK_REMARKING
    set dscp 0
    set qos-group 7
  class RDMA
    set qos-group 7
    set dscp 0
    set qos-group 2

QoS Verification:

N3K-2# show policy-map type qos UPLINK_REMARKING

  Type qos policy-maps

  policy-map type qos UPLINK_REMARKING 
    class type qos LOSSLESSTCP
      set dscp 0
      set qos-group 7
    class type qos RDMA
      set qos-group 7
      set dscp 0
    class type qos SCAVENGER
      set qos-group 2
    class type qos class-default
      set qos-group 0

This is only observed when classifying traffic using a DSCP value and setting it to another DSCP value.  the "set qos-group" command has to be configured to rewrite the DSCP value.
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