User`s guide

214 XSR User’s Guide
Mechanisms to Provide QoS Chapter 10
Configuring Quality of Service
default comprises whatever remains after all other classes are served. You can
configure class-default as any other CBWFQ, except that you cannot assign
bandwidth to it.
Queuing and Services
Once traffic has been classified, it is dropped into different queues so that
each class of traffic can be treated differently (priority, bandwidth etc.). The
following describes two queue types used in the XSR: Class Based Weight
Fair Queuing and Priority Queuing. They are mutually exclusive - only one
type of queue may be applied to one class. But, they may be mixed in a
policy-map when applied to different classes.
Describing Class-Based Weight Fair Queuing
The configured bandwidth of a class is the bandwidth delivered to the class
during congestion. The higher the bandwidth, the more likely the packet is
being transmitted under congested conditions. If there is no data on a
particular queue, then its share of the bandwidth will be divided and shared
among the active queues in proportion to their specified bandwidth.
CBWFQ specifies the exact amount of bandwidth to allocate for a specific
class, or queue, of traffic. Taking into account available bandwidth on the
interface, you can configure up to 64 classes and control distribution among
them. If excess bandwidth is available, it is divided among other CBWFQs in
proportion to their configured bandwidths.
When bandwidth is specified as an absolute number, it is used to calculate the
weight of the class. In such a case, the sum of bandwidth for all classes,
including priority classes, should not exceed the link bandwidth otherwise
the bandwidth for the default class will be zero causing a traffic blockage and
packet pileup in the queue.
NOTE
For each policy-map, only one type of bandwidth, percentage or absolute
bandwidth, can be used for all the CBWFQ classes inside the policy-map.