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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. This chapter includes the following topics:
Extensive core quality of service (QoS) mechanisms are built into the switch router architecture to ensure policy enforcement and queuing, 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 9-1.
IP Precedence Bits | Delay Priority | Queue Selected |
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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 to the appropriate queue based on the delay priority.
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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:
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For example, if W is 4, S is 15, and B is 100, the formula would be (4/15) x 100 = 26 Mbps. 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.
To configure QoS scheduling at the system level, use the following steps beginning in global configuration mode:
| Command | Purpose | |
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Step 1 | Router(config)# qos mapping precedence value wrr-weight weight | Sets the mapping between IP precedence and the WRR weight. The command syntax is as follows:
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Step 2 | Router(config)# end Router# | Returns 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 9-2 shows the default WRR weights for IP precedence.
IP Precedence | WRR Weight |
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To configure QoS scheduling at the interface level, use the following steps beginning in global configuration mode:
| Command | Purpose | |
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Step 1 | Router(config)# qos mapping [destination dest-interface] precedence value wrr-weight weight | Assigns different WRR-scheduling weights for a particular precedence traffic destined to an interface. |
Step 2 | Router(config)# end Router# | Returns 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.
The destination interface parameter is optional. When the destination interface parameter is not specified, the system-level QoS mapping is configured. If interface specific QoS mapping is configured for a certain precedence, it takes precedence over the system-level QoS mapping for that precedence.
Use the following Cisco IOS show commands to verify the QoS configuration:
Command | Purpose |
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For more information on QoS, refer to the Cisco IOS Quality of Service Solutions Configuration Guide.
Posted: Thu Sep 7 09:20:45 PDT 2000
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