Cisco 3845 Software Guide - Page 505

Chapter 27 Configuring QoS Understanding QoS • A small buffer reduces latency but is more appropriate for steady traffic flows than for bursty traffic. • Very small buffers are typically used to optimize priority queuing. For traffic that is priority queued, the buffer size usually needs to accommodate only a few packets; large buffer sizes that increase latency are not usually necessary. For high-priority latency-sensitive packets, configure a relatively large bandwidth and relatively small queue size. WTD thresholds: • You cannot use the queue-limit command to configure more than two threshold values for WTD qualifiers (cos, dscp, precedence, qos-group, discard-class, or mpls experimental). However, there is no limit to the number of qualifiers that you can map to these thresholds. • You can configure a third threshold value to set the maximum queue by using the queue-limit command with no qualifiers. • You can configure many more WTD thresholds, provided their threshold values are equal to the maximum threshold of the queue provided by the queue-limit command. See the "Configuring Weighted Tail Drop" section on page 27-50. Congestion Management and Scheduling MQC provides several related mechanisms in output policy maps to control outgoing traffic flow. The scheduling stage holds packets until the appropriate time to send them to one of the four or eight traffic queues. Queuing assigns a packet to a particular queue based on the packet class and is enhanced by the WTD algorithm for congestion avoidance. You can use different scheduling mechanisms to provide a guaranteed bandwidth to a particular class of traffic while also serving other traffic in a fair way. You can limit the maximum bandwidth that can be consumed by a particular class of traffic and ensure that delay-sensitive traffic in a low-latency queue is sent before traffic in other queues. The switch supports these scheduling mechanisms: • Traffic shaping You use the shape average policy map class configuration command to specify that a class of traffic should have a maximum permitted average rate. You specify the maximum rate in bits per second. • Class-based-weighted-fair-queuing (CBWFQ) You can use the bandwidth policy-map class configuration command to control the bandwidth allocated to a specific class. Minimum bandwidth can be specified as a bit rate, or as a percentage of total bandwidth, or as remaining bandwidth. • Priority queuing or class-based low-latency scheduling You use the priority policy-map class configuration command to specify the that a class of traffic has low latency requirements with respect to other classes. When you configure this command in a class, you cannot configure bandwidth in any other child class associated with the same parent. These sections contain additional information about scheduling: • Traffic Shaping, page 27-18 • Class-Based Weighted Fair Queuing, page 27-19 • Priority Queuing, page 27-20 OL-23400-01 Cisco ME 3800X and 3600X Switch Software Configuration Guide 27-17