Cisco 3845 Software Guide - Page 490

Understanding QoS Chapter 27 Configuring QoS Understanding QoS When networks operate on a best-effort delivery basis, all traffic has equal priority and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an equal chance of being dropped. When you configure QoS, you can select specific network traffic, prioritize it according to its relative importance, and use traffic-management techniques to provide preferential treatment. Implementing QoS in your network makes network performance more predictable and bandwidth utilization more effective. Figure 27-1 shows the MQC model. Figure 27-1 Modular QoS CLI Model Classification Policing Marking Congestion Avoidance Queuing 141149 Policer Drops Congestion Drops Scheduling Basic QoS includes these actions. • Packet classification organizes traffic on the basis of whether or not the traffic matches a specific criteria. When a packet is received, the switch identifies all key packet fields: CoS, DSCP, IP precedence, or MPLS EXP. The switch classifies the packet based on this content or based on an ACL lookup. The class class-default is used in a policy map for any traffic that does not explicitly match any other class in the policy map. See the "Classification" section on page 27-5. • Packet policing determines whether a packet is in or out of profile by comparing the rate of the incoming traffic to the configured policer. You can configure a committed information rate (CIR) and peak information rate (PIR) and set actions to perform on packets that conform to the CIR and PIR (conform-action), packets that conform to the PIR, but not the CIR (exceed-action), and packets that exceed the PIR value (violate-action). See the "Policing" section on page 27-13. • All packets that belong to a classification can be remarked. When you configure a policer, packets that meet or exceed the permitted bandwidth requirements (bits per second) can be conditionally passed through, dropped, or marked. You use the police command to conditionally mark incoming packets based on the CIR and PIR. You use the set command to unconditionally mark packets. See the "Marking" section on page 27-14. • Congestion management uses queuing and scheduling algorithms to queue and sort traffic that is leaving a port. The switch supports these scheduling and traffic-limiting features: class-based weighted fair queuing (CBWFQ), class-based traffic shaping, port shaping, and class-based priority queuing. You can provide guaranteed bandwidth to a particular class of traffic while still servicing other traffic queues. See the "Congestion Management and Scheduling" section on page 27-17. • Queuing on the switch uses the WTD algorithm, a congestion-avoidance mechanism. WTD differentiates traffic classes and regulates the queue size based on the classification. See the "Congestion Avoidance and Queuing" section on page 27-15. This section includes information about these topics: • Modular QoS CLI Configuration, page 27-3 27-2 Cisco ME 3800X and 3600X Switch Software Configuration Guide OL-23400-01