Cisco Systems QC-29 manual Congestion Management Overview, Modified Deficit Round Robin, QC-31

Configuring Modular Quality of Service Congestion Management on Cisco IOS XR Software

Information About Configuring QoS Congestion Management on Cisco IOS XR Software

•Low-Latency Queueing with Strict Priority Queueing, page QC-32

•Traffic Shaping, page QC-33

•Traffic Shaping Mechanism Regulates Traffic, page QC-33

•Traffic Policing, page QC-34

•Traffic Policing Mechanism Regulates Traffic, page QC-35

•Traffic Shaping Versus Traffic Policing, page QC-37

Congestion Management Overview

Congestion management features allow you to control congestion by determining the order in which a traffic flow (or packets) is sent out an interface based on priorities assigned to packets. Congestion management entails the creation of queues, assignment of packets to those queues based on the classification of the packet, and scheduling of the packets in a queue for transmission. The congestion management features in Cisco IOS XR software allow you to specify creation of a different number of queues, affording greater or lesser degree of differentiation of traffic, and to specify the order in which that traffic is sent.

During periods with light traffic flow, that is, when no congestion exists, packets are sent out the interface as soon as they arrive. During periods of transmit congestion at the outgoing interface, packets arrive faster than the interface can send them. If you use congestion management features, packets accumulating at an interface are queued until the interface is free to send them; they are then scheduled for transmission according to their assigned priority and the queueing method configured for the interface. The router determines the order of packet transmission by controlling which packets are placed in which queue and how queues are serviced with respect to each other.

In addition to queueing methods, QoS congestion management mechanisms, such as policers and shapers, are needed to ensure that a packet adheres to a contract and service. Both policing and shaping mechanisms use the traffic descriptor for a packet. See the “Configuring Modular Quality of Service Packet Classification on Cisco IOS XR Software” module for information about the traffic descriptor.

Policers and shapers usually identify traffic descriptor violations in an identical manner through the token bucket mechanism, but they differ in the way they respond to violations. A policer typically drops traffic flow; whereas, a shaper delays excess traffic flow using a buffer, or queueing mechanism, to hold Low-Latency Queueing feature that uses a strict priority queue with policing to shape the flow.)

Traffic shaping and policing can work in tandem. For example, a good traffic shaping scheme should make it easy for nodes inside the network to detect abnormal flows. This activity is sometimes called policing the traffic of the flow.

Modified Deficit Round Robin

MDRR is a class-based composite scheduling mechanism that allows for queueing of up to eight traffic classes. It operates in the same manner as class-based weighted fair queueing (CBWFQ) and allows definition of traffic classes based on customer match criteria (such as access lists); however, MDRR does not use the weighted fair queueing algorithm.

With MDRR configured in the queueing strategy, nonempty queues are served one after the other. Each time a queue is served, a fixed amount of data is dequeued. The algorithm then services the next queue. When a queue is served, MDDR keeps track of the number of bytes of data that were dequeued in excess of the configured value. In the next pass, when the queue is served again, less data is dequeued to

Cisco IOS XR Modular Quality of Service Configuration Guide

QC-31