Nexus 7K QoS Marking, both DSCP & COS

[s-daly]

Hello:

I have a pair of Nexus 7K's running 5.1(3). I have a handful of edge devices that I need to mark ingress traffic, and need to mark both DSCP and CoS. Right now, I have a working config that marks DSCP appropriately:

    class-map type qos match-any SAN
      match access-group name SAN

    class-map type qos match-any NETWORK-CTL
      match access-group name NETWORK-CTL

    class-map type qos match-any MISSION-CRITICAL
      match access-group name MISSION-CRITICAL

  policy-map type qos SVR-EDGE-POLICY1

    class  NETWORK-CTL

      set dscp cs2

    class  MISSION-CRITICAL

      set dscp af21

    class  SAN

      set dscp 25

    class  class-default

      set dscp default

interface port-channel61

  switchport mode trunk

  spanning-tree port type edge trunk

  spanning-tree bpduguard enable

  spanning-tree bpdufilter enable

  service-policy type qos input SVR-EDGE-POLICY1

While that works dor DSCP, the MQC will not allow me to mark both DSCP and COS in the same class, and unlike IOS, it appears that Nexus does not have a default DSCP-to-COS mapping. My understanding is this can be solved using table maps, but I don't see how that can solve my problem in this specific scenario (it appears I can do marking or table-map mutation, but not both?). Can someone tell me how I can accomplish both? Thanks.

[Jerry Ye]

You can only mark it with DSCP. The system has an internal DSCP-COS map where it will look at the first 3 bit of TOS and treat it with its respective COS value:

CS7 = COS7

CS6 = COS6

EF, CS5 = COS 5

CS4 = COS4

CS3 = COS3

CS2 = COS2

CS1 = COS1

Default = COS0

HTH,

jerry

[s-daly]

Okay, so what if I use Assured Forwarding PHB, instead of CS markings, such as af21, in the above example? Or if I use a non-standard marking, such as DSCP 25? Is there a show command or documentation that shows the full default DSCP-COS map behavior?

Also, based on your description, it sounds like Nexus is actually not marking CoS in the dot1q header, just treating the DSCP as if it were a COS value? I actually need to mark CoS so downstream switches know how to treat the traffic.

Thanks!

[Jerry Ye · Accepted solution]

It will copy the first 3-bit from the TOS field:

AF21 = 18 = 010010 -> first 3-bit of the TOS = CS2

If you use the non-standard marking, it will behave the same:

CS25 = 25 = 011001 -> first 3-bit of the TOS = CS3

No, the internal table-map will do a copy from DSCP into COS inside the dot1q header. Your downstream will be able to match the COS value.

HTH,

jerry

[s-daly]

Thanks jeye.

I do have one other question: Is there a way to change this default behavior, say, for example, I want my DSCP 25 marking translated to the dot1q header as COS 4, instead of COS 3?

Also, do the Nexus 55XX switches behave this way as well?

[Jerry Ye · Accepted solution]

I don't believe you can change it with the current release in N7K.

If you are using the newer N55xx, you can set both DSCP and COS. DSCP will be set during inbound when you assign each traffic a qos-group. And COS will be set under network-qos.

Regards,

jerry