Information About Configuring Congestion Management; Congestion Management Overview; Modified Deficit Round Robin - Cisco ASR 9000 Series Configuration Manual

Configuring Modular QoS Congestion Management
• You must be familiar with Cisco IOS XR QoS configuration tasks and concepts.

Information About Configuring Congestion Management

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 queuing 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 queuing 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.
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 queuing mechanism, to hold the traffic for
transmission at a later time.
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.

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 queuing algorithm.
When MDRR is configured in the queuing 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 compensate for
the excess data that was served previously. As a result, the average amount of data dequeued per queue is
close to the configured value. In addition, MDRR allows for a strict priority queue for delay-sensitive traffic.
Each queue within MDRR is defined by two variables:
• Quantum value—Average number of bytes served in each round.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide, Release 6.1.x
Information About Configuring Congestion Management
53