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CCNP Self-Study: Understanding and Implementing Quality of Service in Cisco Multilayer Switched Networks

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: May 13, 2004.


  1. The Need for QoS
  2. QoS Service Models
  3. Catalyst QoS Fundamentals
  4. WAN QoS
  5. QoS in the Multilayer Switched Network
  6. Summary
  7. Configuration Exercise: Configuring QoS on Cisco IOSBased Catalyst Switches
  8. Review Questions

Chapter Description

This chapter describes how QoS is an integral part of any multilayer switched network deployment. With QoS, you can build a network of predictable behavior for latency, jitter, and packet loss. In addition, QoS mitigates anomalous network behavior and provides for differentiation of traffic flows.

Configuration Exercise: Configuring QoS on Cisco IOS–Based Catalyst Switches

Complete this configuration exercise to familiarize yourself with basic QoS configuration on Cisco IOS–based Catalyst switches as discussed in this chapter.

Required Resources

The resources and equipment required to complete this exercise are as follows (the last two items are optional):

  • Catalyst 3550

  • Terminal server or workstation connected directly to the console port of the Catalyst 3550 or out-of-band access to the Catalyst 3550

  • Cisco IP Phones infrastructure supporting voice calls (this resource verifies the configuration and is not mandatory)

  • Traffic generator (this resource verifies the configuration and is not mandatory)

Exercise Objective

The purpose of this exercise is to configure a Cisco IOS–based Catalyst switch for the following QoS features:

  • Classification

  • Marking

  • Policing

  • Congestion management

  • Congestion avoidance

The exercise exposes topics such as VLANs and spanning tree found in others chapters of this book. Nevertheless, the main purpose of this exercise is to demonstrate a QoS configuration. In this configuration exercise, your goal is to configure a Catalyst 3550 for the following QoS features:

  • Trust ingress DSCP values for interfaces FastEthernet0/1 through 0/10 when a Cisco IP Phone is attached

  • Reclassify frames on interface FastEthernet0/11 for a CoS value of 4

  • Mark ingress TCP Port 30000 frames on interface FastEthernet0/12 with a DSCP value of 16

  • Apply strict-priority queuing for VoIP traffic

  • Configure high-priority queues to have a 2-to-1 (2:1) priority over low-priority queues

Network Diagram

Figure 10-18 shows the network layout for this configuration exercise.

Figure 18Figure 10-18 Network Diagram for Lab Exercise

Command List

In this configuration exercise, you will use the commands listed in Table 10-7, which are in alphabetical order so that you can easily locate the information you need. Refer to this list if you need configuration command assistance during the configuration exercise. The table includes only the specific parameters used in the example and not all the available options for the command.

Table 10-7 Command List for Configuration Exercise



access-list access-list-number ?.

Access list configuration command


Enters the class-map configuration submode

configure terminal

EXEC command to enter the configuration mode

copy running-config startup-config

Copies the running configuration to NVRAM


EXEC command to enter privileged mode


Configuration EXEC command to end the configuration mode


EXEC command to exit a configuration mode to its antecedent mode

hostname hostname

Configures switch with a descriptive name

interface FastEthernet | GigabitEthernet interface

Configuration command to enter an interface configuration mode

interface range FastEthernet | GigabitEthernet interfaces

Configuration command to configure multiple interfaces simultaneously

interface vlan vlan-id

Configuration command to enter the VLAN configuration interface mode

match access-group access-list-number

Configures class-map matching clauses

mls qos

Globally enables QoS

mls qos trust device cisco-phone

Interface configuration command for trusting when a Cisco Phone is learned via CDP on the respective interface; works in conjunction with the mls qos trust dscp and mls qos trust cos commands

mls qos trust dscp

Interface configuration command for trusting DSCP values for ingress frames

no shutdown

Configures an interface in the Administrative UP state

policy-map policy_map_name

Enters the policy-map configuration submode

priority-queue out

Configures queue 4 on the Catalyst 3550 family of switches as a priority queue

service-policy input | output policy-map-name

Maps a policy map to an interface for ingress or egress traffic

set ip dscp dscp_value

Policy-map class action for marking DSCP

show mls qos interface FastEthernet | GigabitEthernet interface

Displays the trusting configuration of an interface

spanning-tree portfast

Configures an interface for the spanning-tree PortFast feature


Configures an interface for Layer 2 operation

switchport access vlan vlan-id

Configures an interface for a specific VLAN-ID

vlan vlan-id

Adds or removes a VLAN-ID in the VLAN database

wrr-queue bandwidth weight1 weight2 weight3 weight4

For weight1 weight2 weight3 weight4, enter the ratio that determines the frequency in which the WRR scheduler dequeues packets; separate each value with a space (the range is 1 to 65536)

wrr-queue cos-map queue-id cos1 ... cos8

Configures CoS value to egress queue mapping

Task 1: Globally Enable QoS

Step 1 Connect the Catalyst switch to a terminal server or directly to the workstation's serial port for in-band connectivity.

Step 2 Globally enable QoS features on the switch.

Switch#configure terminal
Switch(config)#mls qos

Step 3 Verify that QoS is globally enabled.

Switch(config)#do show mls qos
QoS is enabled


The Cisco IOS do command is a recent addition to Cisco IOS to allow execution of privileged mode commands within configuration mode. This command saves the time and annoyance of exiting out and re-entering configuration mode. do is only found in the most recent Cisco IOS version, so it may not be supported in your version. If not, exit configuration mode and type the command (minus the keyword do) in privileged mode.

Recall that for the Catalyst 4000 and 4500 families of switches running Cisco IOS, qos commands are not prefixed with the keyword mls.

Task 2: Configure the Switch to Trust DSCP on Interfaces FastEthernet0/1 Through 0/10 If a Cisco IP Phone Is Attached

Step 1 Enter the range command to configure multiple interfaces simultaneously.

Switch(config)#interface range FastEthernet 0/1 -10

Step 2 Specify an access VLAN for IP Phones (voice VLANs are not used in this exercise).

Switch(config-if-range)#switchport access vlan 500

Step 3 Configure the switch to trust DSCP for incoming frames only if Cisco IP Phones are attached to the interface.

Switch(config-if-range)#mls qos trust dscp
Switch(config-if-range)#mls qos trust device cisco-phone

Step 4 Configure the interfaces for spanning-tree PortFast.

Switch(config-if-range)#spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a single
 host. Connecting hubs, concentrators, switches, bridges, etc... to this
 interface when portfast is enabled, can cause temporary bridging loops.
 Use with CAUTION
%Portfast will be configured in 10 interfaces due to the range command
 but will only have effect when the interfaces are in a non-trunking mode.

Step 5 Enable the interfaces.

Switch(config-if-range)#no shutdown

Step 6 Verify the QoS configuration.

Switch#show mls qos interface FastEthernet 0/1
trust state: not trusted
trust mode: trust dscp
COS override: dis
default COS: 0
DSCP Mutation Map: Default DSCP Mutation Map
trust device: cisco-phone

Task 3: Configure the Switch to Classify All Incoming Frames on Interface FastEthernet 0/11 with a CoS Value of 4 for Untagged Frames

Step 1 Enter the interface configuration mode for FastEthernet0/11.

Switch(config)#interface FastEthernet 0/11

Step 2 Configure the interface to classify all ingress frames with a CoS value of 4.

Switch(config-if)#mls qos cos 4

Step 3 Verify the QoS configuration.

Switch#(config-if)#do show mls qos interface FastEthernet 0/11
trust state: not trusted
trust mode: not trusted
COS override: dis
default COS: 4
DSCP Mutation Map: Default DSCP Mutation Map
trust device: none

Task 4: Configure a Policy Map, Class Map, and the Interface Such That All Ingress TCP Port 30000 Packets on FastEthernet0/11 Have Their DSCP Set to 16

Step 1 Configure an access list to match packets on TCP port 30000.

Switch(config)#access-list 100 permit tcp any any eq 30000

Step 2 Configure a traffic profile using a class map.

Switch(config)#class-map TCP-PORT-30k
Switch(config-cmap)#match access-group 100

Step 3 Configure a policy map to apply the class map in Step 2 to the class action of setting the DSCP to 16.

Switch(config)#policy-map BCMSN
Switch(config-pmap)#class TCP-PORT-30k
Switch(config-pmap-c)#set ip dscp 16

Step 4 Apply the policy-map ingress on interface FastEthernet0/11.

Switch(config)#interface FastEthernet 0/11
Switch(config-if)#service-policy input BCMSN

Step 5 Verify the policy-map configuration.

Switch#show policy-map interface FastEthernet 0/11


 service-policy input: BCMSN

  class-map: TCP-PORT-30k (match-all)
   0 packets, 0 bytes
   5 minute offered rate 0 bps, drop rate 0 bps
   match: access-group 100

  class-map: class-default (match-any)
   0 packets, 0 bytes
   5 minute offered rate 0 bps, drop rate 0 bps
   match: any
    0 packets, 0 bytes
    5 minute rate 0 bps

Task 5: Configure All Egress Queues Such That CoS Values 4, 6, and 7 Use Queue 3 and a CoS Value of 5 Uses Queue 4

Step 1 Enter the range command to configure multiple interfaces simultaneously.

Switch(config)#interface range FastEthernet 0/1 -24

Step 2 Configure interfaces for appropriate CoS mapping.

Switch(config-if)#wrr-queue cos-map 4 5
Switch(config-if)#wrr-queue cos-map 3 4 6 7

Task 6: Configure Queue 4 as a Strict-Priority Queue

Step 1 Enter the range command to configure multiple interfaces simultaneously.

Switch(config)#interface range FastEthernet 0/1 -24

Step 2 Configure queue 4 as a strict-priority queue.

Switch(config-if)#priority-queue out

Task 7: Configure WRR Weights Such That Queue 3 Receives Twice as Much Service as Any Other Single Queue

Step 1 Enter the range command to configure multiple interfaces simultaneously.

Switch(config)#interface range FastEthernet 0/1 -24

Step 2 Configure queue 3 with twice the service level as that of any other queue.

Switch(config-if)#wrr-queue bandwidth 20 20 40 20

Step 3 Verify the WRR configuration.

Switch#show mls qos interface FastEthernet 0/1 queueing
Egress expedite queue: ena
wrr bandwidth weights:
 1 - 20
 2 - 20
 3 - 40
 4 - 20  when expedite queue is disabled
Cos-queue map:
 0 - 1
 1 - 1
 2 - 2
 3 - 2
 4 - 3
 5 - 4
 6 - 3
 7 - 3

Task 8: Verify All Configurations by Viewing Interface Statistics

Switch#show mls qos interface FastEthernet 0/1 statistics
 dscp: incoming  no_change classified policed  dropped (in bytes)
Others: 97663325  87828650  9834675  0     0
 dscp: incoming  no_change classified policed  dropped (in bytes)
Others: 30540345   n/a    n/a   0     0
8. Review Questions | Next Section Previous Section

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