Cisco 3845 Software Guide - Page 504

Understanding QoS Chapter 27 Configuring QoS Figure 27-7 WTD and Queue Operation CoS 6-7 100% 1000 CoS 4-5 60% 600 CoS 0-3 40% 400 86692 0 In this example, CoS values 6 and 7 have a greater importance than the other CoS values, and they are assigned to the 100-percent drop threshold (queue-full state). CoS values 4 and 5 are assigned to the 60-percent threshold, and CoS values 0 to 3 are assigned to the 40-percent threshold. If the queue is already filled with 600 microseconds worth of data, and a new frame arrives containing CoS values 4 or 5, the frame is subjected to the 60-percent threshold. When this frame is added to the queue, the threshold would be exceeded, so the switch drops it. You configure WTD by using the queue-limit policy-map class command to adjust the queue size (buffer size) associated with a particular class of traffic. Note Queue-limit is supported only in leaf-level (per-hop behavior) classes. You specify the maximum threshold in bytes or in microseconds. You can specify different queue sizes for different classes of traffic (CoS, DSCP, MPLS EXP, precedence, discard-class, or QoS group) in the same queue. Setting a queue limit establishes a drop threshold for the associated traffic when congestion occurs. Note You cannot configure queue size by using the queue-limit policy map class command without first configuring a scheduling action (bandwidth, shape average, or priority). The only exception to this is when you configure queue-limit for the class-default of an output policy map. The switch supports up to three unique queue-limit configurations (including the default) across all output policy maps. Within an output policy map, four or eight queues (classes) are allowed, including the class default. Each queue can have three defined thresholds. Only three unique threshold value configurations are allowed per class. Multiple policy maps can share the same queue-limits. Each class-map in a policy-map can either share the same threshold or can have its own unique values. You can use these same queue-limit values in multiple output policy maps on the switch. However, changing one of the queue-limit values in a class creates a new, unique queue-limit configuration. At any one time, you can attach only three unique queue-limit configurations in output policy maps to targets. If you attempt to attach an output policy map with a fourth unique queue-limit configuration, you see this error message: QoS: Configuration failed. Maximum number of allowable unique queue-limit configurations exceeded. By default, queues have unique thresholds based on the speed of the interface. You can decrease the queue size for latency-sensitive traffic or increase the queue size for bursty traffic. Queue bandwidth and queue size (queue limit) are configured separately and are not interdependent. You should consider the type of traffic being sent when you configure bandwidth and queue-limit: • A large buffer (queue limit) can better accommodate bursty traffic without packet loss, but at the cost of increased latency. 27-16 Cisco ME 3800X and 3600X Switch Software Configuration Guide OL-23400-01