Table of Contents

Quality of Service Feature Summary

This chapter describes the Quality of Service (QoS) features built into your switch router, and how to map QoS scheduling at both the system and interface levels. This chapter includes the following topics:

• Overview of Layer 3 Switching Quality of Service (QoS)
  
  
• About Scheduling and Weighted Round-Robin
  
  
• Configuring Precedence to WRR Scheduling
  
  

Overview of Layer 3 Switching Quality of Service (QoS)

Extensive core Quality of Service (QoS) mechanisms are built into the switch router architecture to ensure policy enforcement and queuing of the ingress port, as well as weighted round-robin (WRR) scheduling at the egress port.

Layer 3 switching QoS is based on IP precedence for partitioning traffic into multiple classes of service. IP precedence uses the three type-of-service precedence bits in the IP header to specify class of service assignment for each packet.

IP precedence can be mapped into adjacent technologies (for example, tag switching or ATM) to deliver end-to-end network QoS policies. This enables service classes to be established with no changes to existing applications and with no complicated network signaling requirements.

The system gathers IP precedence information from the IP header type-of-service field. For an incoming IP packet, the first two (most significant) bits of the service type field determine the delay priority. Layer 3 switching recognizes four QoS classes, Q-0 to Q-3, as summarized in Table 9-1.

Your switch router can read the precedence field and switch the packet accordingly, but it cannot reclassify traffic. The edge router or switch is expected to set the precedence field according to its local policy.

The switch router queues packets based on the delay priority and the target next-hop interface.

About Scheduling and Weighted Round-Robin

Frame scheduling becomes increasingly important when an outgoing interface is congested. To handle this situation, network administrators can assign weights to each of the different queues. This provides bandwidth to higher priority applications (using IP precedence), yet still fairly grants access to lower priority queues. The frame schedule affords each queue the bandwidth allotted to it by the network administrator. This mapping is configurable both at the system and interface levels (as described later in this chapter).

The four queues between any pair of interfaces are configured to be part of the same service class. Bandwidth is not explicitly reserved for these four queues. Each of them is assigned a different WRR-scheduling weight, which determines the way they share the interface bandwidth. The WRR weight is user configurable; you can assign a different WRR weight for each queue.

Tip  The higher the WRR weight, the higher the effective bandwidth for that particular queue.

You can find the effective bandwidth (in Mbps) for a particular queue with the following formula:

  (W/S) x B = n Mbps,

  where

For example, if W is 4, S is 15, and B is 100, the formula would be (4/15) x 100 = 26 Mbps, and the effective bandwidth for the specified queue in this example is 26 Mbps.

Configuring Precedence to WRR Scheduling

This section describes the Cisco IOS commands necessary to configure QoS mapping at the system and interface levels. The commands described in this section are unique to the Layer 3 switching software.

Layer 3 switching software enables QoS-based forwarding by default. If disabled, issue the following command to enable QoS switching:

# [no] qos switching

The [no] version of this command disables QoS switching on the entire system.

Table 9-2 shows an example of configuring QoS scheduling at the system level.

The syntax used in Table 9-2 is described below:

To set the precedence back to the default setting for the switch router, use the no version of the qos mapping precedence command.

Table 9-3 shows the default WRR weights for IP Precedence.

Mapping QoS Scheduling at the Interface Level

Configuring the QoS mapping at the interface level overrides the system-level mapping. Using the qos mapping precedence wrr-weight command, the network administrator can assign different WRR-scheduling weights for a particular precedence traffic between a pair of interfaces.

Table 9-4 shows and example of configuring QoS scheduling at the interface level.

The QoS commands are applicable to both Gigabit Ethernet and Fast Ethernet interfaces.

To set the precedence back to the system-level default setting for the switch router, use the no version of the qos mapping precedence wrr-weight command.

Both the source and destination interface parameters are optional. When both are not specified, the system-level QoS mapping is configured. Otherwise, you can specify the source interface, or the destination interface, or both to configure the WRR weight for the traffic streams listed below.

The configuration takes precedence in the following order:

Monitoring the QoS Configuration

The Cisco IOS show commands for QoS are shown in Table 9-5.