User`s guide

XSR User’s Guide 217
Chapter 10 Mechanisms to Provide QoS
Configuring Quality of Service
Assign the class frost to the priority queue:
XSR(config)#policy-map frame1
XSR(config-pmap<frame1>)#class frost
XSR(config-pmap-c<frost>)#priority high 20
XSR(config-pmap-c<frost>)#queue-limit 30
Describing Traffic Policing
While it is possible to precisely control the output rate of all traffic using
CBWFQ and priority queues with maximum link bandwidth, practically
speaking, this is rarely done. Typically, you identify certain critical
applications, assign QoS values and bandwidth to them, and let the
remainder of traffic take whatever bandwidth is left.
The XSR’s implementation of traffic policing provides this benefit:
 Packet marking through IP precedence or DSCP value setting -
Packet marking partitions your network into multiple priority levels.
Configuring Traffic Policing
To successfully configure Traffic Policing, you must create a traffic class and
attach the traffic policy to an interface or DLCI. The
police command
specifies the following options:
 Bandwidth, burst and excess burst values
 Action to take for traffic that conforms or exceeds the specified rate
This is how the policer works. It maintains two token buckets, one holding
tokens for normal burst and the other for excess burst. The policing algorithm
handles token refilling and burst checking.
Token buckets are refilled every time a new packet arrives. The specified
bandwidth and the interval between the arrival time of the new packet and
that of the previous packet are used to calculate the number of tokens to refill
the buckets. The formula is as follows:
Refill Token Bytes = (Bandwidth * Interval) / 8