Cisco 3845 Software Guide - Page 506

Understanding QoS Chapter 27 Configuring QoS Traffic Shaping Traffic shaping or class-based peak rate scheduling is used to specify the maximum transmission rate for a traffic class. Unlike traffic policing, where nonconforming packets are dropped or marked right away, packets exceeding the rate specified in the shape command are usually buffered and can be sent later when bandwidth is available. While policing propagates traffic bursts, shaping smooths bursts by sending the packets later. Traffic policing is used in input policy maps, and traffic shaping occurs as traffic leaves an interface or service instance. Configuring a queue for traffic shaping sets the maximum bandwidth or PIR of the queue. You can set the PIR from 1 Kb/s to 10 Gb/s. You can also configure the PIR as a percentage of the PIR of a parent level. Note You cannot configure traffic shaping (shape average) and or priority queuing (priority) for the same class in an output policy map. However, you can configure CIR, PIR, and EIR bandwidth independently for a class and use the bandwidth, bandwidth remaining, and shape average commands at the same time within a class. Class-Based Shaping Class-based shaping uses the shape average policy-map class configuration command to limit the rate of data transmission in bits per second to be used for the committed information rate for a class of traffic. The switch supports separate queues for four or eight classes of traffic, including the default queue for class class-default, unclassified traffic. See the "Configuring Class-Based Shaping" section on page 27-47. Port Shaping Port shaping is applied to all traffic leaving a target. It uses a policy map with only class default when you specify the maximum bandwidth for the port by using the shape average command. You can attach a child policy to the class-default in a hierarchical policy map format to specify class-based and VLAN-based actions. See the "Configuring Port Shaping" section on page 27-48. Parent-Child Hierarchy The switch also supports parent policy levels and child policy levels for traffic shaping. The QoS parent-child structure is used for specific purposes when a child policy is referenced in a parent policy to provide additional control of a specific traffic type. Note The total of the minimum bandwidth guarantees (CIR) for each queue of the child policy cannot exceed the total port-shape rate. This is an example of a parent-child configuration: Switch(config)# policy-map parent Switch(config-pmap)# class class-default Switch(config-pmap-c)# shape average 50000000 Switch(config-pmap-c)# service-policy child Switch(config-pmap-c)# exit Switch(config-pmap)# exit Switch(config)# interface gigabitthernet0/1 Switch(config-if)# service-policy output parent Switch(config-if)# exit 27-18 Cisco ME 3800X and 3600X Switch Software Configuration Guide OL-23400-01