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Nexus 7K QoS – Part 1

2011/12/02 2 comments

I’m working on a project for a customer where QoS for the Nexus 7K is a requirement.  Anyone who has attempted to configure QoS on these boxes has probably questioned how different are these devices from, say, the Catalyst 6500s.  Well, they are quite different.  If you are familiar with Modular QoS CLI (MQC,) that is a huge advantage, as all QoS configuration on the N7K is based on MQC.

IMPORTANT NOTE:  DISCUSSIONS AND CONFIGURATIONS FOR THIS SERIES ARE BASED ON NX-OS 5.1(1).  PREVIOUS FIRMWARE VERSIONS DO NOT SUPPORT ALL OF THESE FEATURES!

Let me start by pointing out some key differences between the 6500 and the N7k.

6500 N7K
ENABLE QOS       mls qos Enabled by default
TRUST mls qos trust [cos|dscp|ip-precedence] DSCP (on M1 modules) and CoS (on F1 modules) trusted by default
INTERNAL QOS QoS Label is used internally CoS and/or DSCP passed through, though QoS-Groups can be used
COS TO DSCP MAPPING Default of CoS to 3 most significant bits of DSCP (CoS 1 to DSCP 8 ) Same
DSCP to COS MAPPING Default 3 most significant bits of DSCP to CoS (DSCP 10 to CoS 1) Same
CHANGE COS/DSCP MAPPING Modify cos-dscp or dscp-cos maps Create and apply qos policy-map(s) ingress and/or egress

So, it’s a different way of thinking about QoS when it comes to the Nexus 7Ks.  Why should things stay the same (rhetorical question…)  And, I haven’t even discussed ingress or egress queueing.

In addition to thinking in terms of class-maps and policy-maps, there are some other key pieces that need to be understood.  First, there are three class-map and policy-map object-types that can be created:

  1. Network qos: This is defined in the default VDC.  It defines CoS properties for the entire switch, including all VDCs.  These can be overriden per interface.
  2. QoS: They can be applied ingress and egress to interfaces.  They can be used to mark and police traffic.
  3. Queuing: They can be  ingress and egress to interfaces.  They can be used to mark, shape and (not surprisingly) queue traffic.
    • NOTE: “queuing” class-maps are pre-defined and CANNOT be changed.  These are defined per the input and output queuing options of the specific module.

Another aspect that makes N7K interesting is that different modules (the M1(-XL) and the F1) have different options for QoS.  In particular, the F1 queueing policies should match the network-qos policies.  Also, F1 modules don’t support mapping to QoS Groups.  The “Cisco Nexus 7000 Series NX-OS Quality of Service Configuration Guide, Release 5.X” has further information on the F1 and specific items for its configuration.

In working through an N7K QoS configuration, I came to the conclusion that it generally makes sense to do the following:

  1. Develop a QoS policy for inbound traffic.  Trusting is fine, but is module dependent (see above on the M1 and F1 differences.)  Matching and either trusting or changing DSCP values, in particular, was key to the proper development of the config.
  2. Develop a queuing policy for outbound traffic.  What CoS values should be used for which output queues (module dependent)?  Is priority queuing needed? What DWRR weights should be used for each CoS value?

So, again – inbound QoS and outbound queueing seems to make the most sense for building QoS configurations for most situations.  And, having that decided helps in better determining the actual configurations.

In part 2, I’ll go through a network-qos policy configuration, an ingress qos policy, and an ingress queueing policy to provide some more concrete examples.

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