User`s guide

210 XSR User’s Guide
Features Chapter 10
Configuring Quality of Service
Features
The XSR’s support of QoS module allows you to:
 Classify traffic in different traffic flows using user-defined filters
based on packet headers and payloads
 Meter and police traffic flows based on traffic policy
 Prioritize time-critical traffic flows and ensure that packets from these
flows are serviced with bounded delay
 Share output bandwidth in a fair manner between the number of
best-effort traffic flows
 Manage queues using two queue management strategies: tail-drop or
Random Early Detection (RED)
 Mark packets from a specific flow with DSCP or IP precedence values
QoS service on the XSR is proscribed by the following limits:
 Traffic policy can be applied to output only
 The maximum number of classes allowed is 64
 The traffic policer cannot be configured for traffic flows assigned to
priority queues. Each priority queue is metered and policed by
default to guarantee it conforms to the scheduled traffic pattern

Priority and bandwidth commands are mutually exclusive; a traffic
flow is assigned to either queue, not both
 Tail-drop (
queue-limit) and RED (random-detect) are mutually
exclusive; a queue is managed by either mechanism, not both
Mechanisms to Provide QoS
This following section describes the general mechanisms the XSR employs to
support Quality of Service.
Traffic Classification
Before the XSR can apply QoS to traffic, it must differentiate between types of
traffic. The process is called Traffic Classification.