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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. For further information about the commands used in this chapter, refer to "Command Reference."
This chapter includes the following sections:
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.
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 8-1.
| 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 |
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.
 | 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:
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.
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 8-2 shows an example of configuring QoS scheduling at the system level.
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config)# qos mapping precedence value | From global configuration mode, set the mapping between IP precedence and the WRR weight. See the following syntax description. | ||
| Router(config-if)# Ctrl-Z | Return to privileged EXEC mode. |
To set the precedence back to the default setting for the switch router, use the no version of the qos mapping precedence command.
Table 8-3 shows the default WRR weights for IP precedence.
| IP Precedence | WRR Weight |
|---|---|
0 | 1 |
1 | 2 |
2 | 4 |
3 | 8 |
Table 8-4 shows an example of configuring QoS scheduling at the interface level.
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config)# qos mapping [source source-interface] [destination dest-interface] precedence value wrr-weight weight | From global configuration mode, assign different WRR-scheduling weights for a particular precedence traffic between a pair of interfaces. | ||
| Router(config-if)# Ctrl-Z | Return to privileged EXEC mode. |
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:
1. Traffic streams with a certain precedence, from a particular source interface to a particular destination interface
2. Traffic streams with a certain precedence to a particular destination interface
3. Traffic streams with a certain precedence from a particular source interface
The Cisco IOS show commands for QoS are shown in Table 8-5.
| Command | Displays |
|---|---|
show qos switching | |
show qos mapping [source source-interface] [destination dest-interface] | Effective mapping at either the system level or interface-pair level |
Posted: Wed Dec 22 14:12:29 PST 1999
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