Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring QoS (Rev. 03)
Overview
© 2008 Foundry Networks, Inc.
Page 3 of 62
Overview
Today’s networks transmit data streams for various applications using many different protocols.
Different types of traffic sharing a data path through the network can interact in ways that affect
their application performance. Traffic prioritization becomes especially important when delay-
sensitive, interactive applications are supported across the network. In many cases a guaranteed
level of throughput is part of contractual obligations between the network operator and customers
or third-party service providers.
Policy-based Quality of Service (QoS) allows the user to specify different service levels for traffic
traversing the device. Policy-based QoS is an effective control mechanism for networks that have
heterogeneous traffic patterns. Using Policy-based QoS, the user can specify the service level for a
traffic type or host.
QoS controls congestion by determining the order in which packets are transmitted based on
priorities assigned to those packets. QoS queuing policies can protect bandwidth for important
categories of applications, or specifically limit the bandwidth associated with less critical traffic.
For example, if Voice over IP (VoIP) traffic requires a reserved amount of bandwidth to function
properly, QoS policies can reserve sufficient bandwidth for this type of application. Other
applications deemed less critical can be limited in their bandwidth usage.
During periods of light traffic, QoS policies have little effect, and packets are transmitted as soon
as they arrive. During periods of congestion, outbound packets accumulating at an interface are
sorted into eight queues. They are transmitted from the queues according to the queuing
mechanism configured for the interface.
When using QoS feature, each physical port sorts inbound and outbound traffic into eight queues
for the QoS processing.
The user controls Quality of Service behavior in two ways:
By configuring the criteria used to sort inbound and outbound packets into the eight queues
. By
default, values of the 802.1p priority field of the packet header are mapped to the eight QoS
queues. The user can also map destination MAC address priority values to the QoS queues.
By selecting the queuing mechanism to be applied to the outbound queues
. Two basic queuing
mechanisms are provided:
•
Weighted Round-Robin
queuing lets the user assigns a relative weight to each queue, which
determines the bandwidth assigned to each queue relative to the others.
•
Strict Priority
queuing sets the eight queues in a rigid order, and always transmits packets
from the highest-priority queue that has packets waiting.
In addition, six hybrid queuing schemes are available, which combine the Weighted Round Robin
and Strict Priority mechanisms.
Traffic Analysis for QoS Deployment
To effectively configure QoS, the user must analyze the types of traffic using the interface and
determine their relative bandwidth demands. The user should also evaluate the supported
applications’ sensitivity to latency, jitter, and packet loss.
General guidelines for each traffic type are given below. Consider them as general guidelines and
not strict recommendations. Once QoS parameters are set, the user can monitor the performance of
the application to determine if the actual behavior of the applications matches the expectations.
Voice
applications demand small amounts of bandwidth. However, the bandwidth must be constant
and predictable because voice applications are typically sensitive to latency (inter-packet delay)
and jitter (variation in inter-packet delay).