Configuring Modular QoS Congestion Management on Cisco ASR 9000 Series Routers
Information About Configuring QoS Congestion Management on Cisco ASR 9000 Series Routers
QC-45
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
OL-23108-02
Information About Configuring QoS Congestion Management on
Cisco
ASR 9000 Series Routers
To implement QoS congestion management features in this document, you must understand the
following concepts:
• Congestion Management Overview, page 45
• Modified Deficit Round Robin, page 46
• Low-Latency Queueing with Strict Priority Queueing, page 46
• Traffic Shaping, page 47
• Traffic-Shaping Mechanism Regulates Traffic, page 47
• Traffic Policing, page 48
• Regulation of Traffic with the Policing Mechanism, page 49
• Traffic Shaping Versus Traffic Policing, page 50
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
Congestion Management on Cisco
ASR 9000 Series Routers” module in this guide 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
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.
To Next Page
Loading...
Need help?
General
