Table of Contents

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

Mapping QoS Scheduling at the Interface Level
Monitoring the QoS Configuration

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.

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.

Table 9-1: QoS Delay Priorities and Queues

IP Precedence Bits	Delay Priority	Queue Selected
0 0 0	0 0	Q-0
0 0 1	0 0	Q-0
0 1 0	0 1	Q-1
0 1 1	0 1	Q-1
1 0 0	1 0	Q-2
1 0 1	1 0	Q-2
1 1 0	1 1	Q-3
1 1 1	1 1	Q-3

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.

Tips 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

W	is the WRR weight of the specified queue
S	is the sum of the weight of all active queues on the outgoing interface
B	is the available bandwidth in Mbps
n	is the effective bandwidth in Mbps

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.

# [no] qos switching

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

Table 9-2: Mapping QoS Scheduling at the System Level

Step	Command	Description
1 .	Router(config)# qos mapping precedence value wrr-weight weight	From global configuration mode, set the mapping between IP precedence and the WRR weight. See the following syntax description.
2 .	Router(config-if)# Ctrl-Z Router#	Return to privileged EXEC mode.

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

value	The precedence value (0 to 3) is derived from the IP precedence field. The higher 2-bits of the IP precedence field is used. When a precedence value x is specified, it also implicitly assigns the same WRR-weight to precedence x + 1.
weight	The WRR-scheduling weight (1 to 15). This parameter specifies the weight assigned to traffic with the given precedence.

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.

Table 9-3: IP Precedence and Default WRR Weights

IP Precedence	WRR Weight
0	1
1	2
2	4
3	8

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.

Table 9-4: Mapping QoS Scheduling at the Interface Level

Step	Command	Description
1 .	Router(config)# qos mapping [source gi x/0/z] [destination gi a/0/c] precedence value wrr-weight weight	From global configuration mode, assign different WRR-scheduling weights for a particular precedence traffic between a pair of interfaces.
2 .	Router(config-if)# Ctrl-Z Router#	Return to privileged EXEC mode.

Monitoring the QoS Configuration

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

Table 9-5: Monitoring QoS Operation

Command	Display
show qos switching	Whether QoS-based switching is enabled
show qos mapping [source gi x/0/z destination gi a/0/c]	Effective mapping at either the system level or interface-pair level

Posted: Wed Aug 2 15:41:50 PDT 2000
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