Cisco 3845 Software Guide - Page 503

Chapter 27 Configuring QoS Understanding QoS You specify and mark traffic by using the set commands in a policy map for all supported QoS markings (CoS, IP DSCP, IP precedence, and QoS groups). A set command unconditionally marks the packets that match a specific class. You then attach the policy map to an interface or service instance as an input policy map. You can simultaneously configure actions to modify DSCP, precedence, and COS markings in the packet for the same service along with QoS group marking actions. You can use the QoS group number defined in the marking action for egress classification. Figure 27-6 shows the steps for marking traffic. Figure 27-6 Receive Marking of Classified Traffic Classify Unconditionally mark traffic for reclassification Queuing, scheduling, and shaping 157193 This example uses a policy map to remark a packet. The first marking (the set command) applies to the QoS default class map that matches all traffic not matched by class AF31-AF33 and sets all traffic to an IP DSCP value of 1. The second marking sets the traffic in classes AF31 to AF33 to an IP DSCP of 3. Switch(config)# policy-map Example Switch(config-pmap)# class class-default Switch(config-pmap-c)# set ip dscp 1 Switch(config-pmap-c)# exit Switch(config-pmap)# class AF31-AF33 Switch(config-pmap-c)# set ip dscp 3 Switch(config-pmap-c)# exit Switch(config-pmap)# exit Switch(config)# interface gigabitethernet0/1 Switch(config-if)# service-policy input Example Switch(config-if)# exit Congestion Avoidance and Queuing Congestion avoidance uses algorithms such as tail drop to control the number of packets entering the queuing and scheduling stage to avoid congestion and network bottlenecks. The switch uses WTD to manage the queue sizes and provide a drop precedence for traffic classifications. You set the queue size limits depending on the markings of the packets in the queue. You can assign each packet that travels through the switch to a specific queue and threshold. For example, you can map specific DSCP or CoS values to a specific egress queue and threshold. If the total destination queue size is greater than the threshold of any classified traffic, the next frame of that traffic is dropped. Figure 27-7 shows an example of WTD operating on a queue with 1000 microseconds worth of data. Three drop percentages are configured: 40 percent (400 microseconds), 60 percent (600 microseconds), and 100 percent (1000 microseconds). These percentages mean that traffic classified to the 40-percent threshold is dropped when the queue depth exceeds 400 microseconds, traffic classified to 60 percent is dropped when the queue depth exceeds 600 microseconds. Traffic up to 400 microseconds can be queued at the 40-percent threshold, up to 600 microseconds at the 60-percent threshold, and up to 1000 microseconds at the 100-percent threshold. OL-23400-01 Cisco ME 3800X and 3600X Switch Software Configuration Guide 27-15