Filter
Top Products
Configuring Modular Quality of Service Congestion Management on Cisco IOS XR Software
Information About Configuring QoS Congestion Management on Cisco IOS XR Software
QC-31
Cisco IOS XR Modular Quality of Service Configuration Guide
• 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