Configuring Quality of Service

By using the quality of service (QoS) feature, you can give preferential service to selected packets. When QoS is not enabled on a device, the device offers best-effort service to each packet, regardless of the packet contents or size. When QoS is configured and enabled, you have much more control over network traffic. For example, you can:

QoS is always enabled on Catalyst 2950 switches. To enable QoS on Catalyst 3550 switches, choose Device > QoS > Enable/Disable and use the QoS Enable/Disable window.

Before you use QoS, you must perform these tasks:

  1. Decide which marker in incoming packets to trust. Incoming packets can contain any or none of these markers: a class of service (CoS) value, 0 to 7; an IP precedence value, 0 to 7; a Differentiated Services Code Point (DSCP) value, 0 to 63. You decide which of these marker types you want to trustthat is, which one you want to use to classify incoming packets on the interface. You also decide what default value (always a CoS value) to assign if a packet contains no marker. To perform this task, you choose Device > QoS > Trust Settings and use the QoS Trust Settings window.
    Restriction: For Catalyst 2950 switches, you can set interfaces to trust only CoS and DSCP values.
  2. Map markers to DSCP and to CoS. You might decide to trust CoS or IP Precedence on a given interface, but the device uses only DSCP values to associate packets with queue thresholds. Therefore, you must fill in tables that map CoS and IP precedence values to DSCP values. Also, the device uses only CoS values to assign packets to egress queues, so you must fill in another table that maps DSCP values to CoS values. To perform these tasks, you choose Device > QoS > Maps and use the QoS Maps window. This window also has features that let you: Restriction: QoS maps are supported on Catalyst 2950 switches only if they are running the enhanced software image. Even then, only two mappings are supported: CoS to DSCP and DSCP to CoS.
  3. Configure queues on device interfaces. Each device interface has four egress queues, and you make a number of decisions about their characteristics. For instance: To perform this task, you choose Device > QoS > Queues and use the QoS Queues window.
  4. Create policers. A policer is a specification that contains a maximum permissible rate of transmission, a maximum burst size for transmissions, and an action to take if either maximum is exceeded. You associate a policer with a QoS class. Ultimately, you will associate one or more classes with a policy and attach policies to interfaces. In this way, policers set transmission and burst limits, per class of packets, on an interface.
    You can create aggregate policers and nonaggregate policers. To create an aggregate policer, you choose Device > QoS > Aggregate Policers and click Create in the QoS Aggregate Policers window. You create a nonaggregate policer in the process of adding a class to a policy.
    Restriction: QoS policers are supported on Catalyst 2950 switches only if they are running the enhanced software image. Even then, only nonaggregate policers are supported.
  5. Create classes. A class identifies packets that have a certain DSCP or IP precedence value or that match a filtering criterion in a certain ACL. When a class is specified in a policy, it is paired with a policer. To create classes, you choose Device > QoS > Classes and click Create in the QoS Classes window.
    Restriction: QoS classes are supported on Catalyst 2950 switches only if they are running the enhanced software image. Even then, a class can be defined to match an ACL but not to match a DSCP or IP precedence value.
  6. Create policies. A policy is a set of one or more classes and their associated policers. When you create a policy, you can specify classes and policers that already exist (the results of tasks 5 and 6), or you can define classes and policers in the process of creating the policy. To create policies, you choose Device > QoS > Policies and click Create in the QoS Policies window.
    Restriction: QoS policies are supported on Catalyst 2950 switches only if they are running the enhanced software image.
  7. Attach policies to interfaces. You can use a policy to regulate input, output, or both, on an interface, and you can use the same or different policies with different interfaces. When you attach a policy to an interface, it is in effect, assuming QoS is enabled on the device. To attach policies to interfaces, you choose Device > QoS > Policies and click Attach/Detach in the QoS Policies window.

If a Catalyst 3550 switch is in the cluster, you can use the QoS Statistics window to specify the devices, interfaces, and packet types for which QoS statistics are kept. Your specifications determine the statistics that are available in the QoS Reports window and QoS Graphs window.


Related Web Links

"Configuring QoS," Catalyst 2950 Desktop Switch Software Configuration Guide
"Configuring QoS," Catalyst 3550 Multilayer Switch Software Configuration Guide