User`s guide

XSR User’s Guide 213
Chapter 10 Mechanisms to Provide QoS
Configuring Quality of Service
Describing the Policy Map
The policy statement in a QoS policy-map specifies how traffic defined by the
traffic class-map will be treated. Each class in policy-map has to be assigned
to one of the two types of queues: CBWFQ or Priority Queue. This includes
specifying the following:
 The
bandwidth command assigns traffic from this class to a Class- Based
Weight Fair Queue (CBWFQ) with the specified bandwidth. A CBWFQ
shares the output link with other CBWFQs on the same link in
proportion to its specified bandwidth or weight. During congestion,
queues are serviced (assigned bandwidth) in proportion to their weight.
When uncongested, a queue can borrow bandwidth from other queues.
 The priority command assigns traffic from this class a Priority
Queue (PQ) and sets the parameter for the queue. Priority queues
provide guaranteed bandwidth - they always receive the bandwidth
requested. Priority class is not allowed to send more than its
guaranteed bandwidth and excess traffic is discarded. Unused
priority bandwidth is picked up by the class-default class.
For classes that are assigned to CBWFQ you can control the maximum rate of
traffic sent or received on a port as follows:
 The
police command controls traffic received by a queue by
defining the action taken for packets that conform or exceed the
specified rate. You may drop the packet, change its IP precedence or
DSCP setting, or forward it without modification.
Both CBWFQ and Priority Queues can control queue size and the type of
congestion avoidance mechanism, as well as mark packets as follows:
 The
set ip precedence, set ip dscp commands mark a packet by
setting the IP precedence or DSCP field. The Differentiated Services
Field is defined in RFCs-2474 and 2475.
 The
queue-limit command specifies or modifies the maximum
number of packets the queue can hold before tail drop for TCP/IP
traffic for a class policy configured in a policy map.
 The
random-detect command sets Random Early Detect (RED), a
congestion avoidance mechanism that slows traffic by randomly
dropping packets when congestion exists.
Traffic not assigned to a class in the policy-map is assigned to class-default
which is always created and assigned as a CBWFQ. Bandwidth for the class-