- Enterasys Security Router User's Guide

Mechanisms Providing QoS
XSR User’s Guide 12-5
Configuring CBWFQ
CBWFQ is configured using the bandwidth command. It provides a minimum bandwidth
guarantee during congestion. For example, policy-map keyser guarantees 30 percent of the
bandwidth to class sosay and 60 percent of the bandwidth to class intrigue. If one class uses less of
the requested share of bandwidth, the excess bandwidth may be used by the other class.
XSR(config)#policy-map keyser
XSR(config-pmap<keyser>)#class sosay
XSR(config-pmap-c<sosay>)#bandwidth percent 30
XSR(config-pmap<keyser>)#class intrigue
XSR(config-pmap-c<intrigue>)#bandwidth percent 60
Measuring Bandwidth Utilization
A quick and easy way to monitor bandwidth utilization is as follows: define an empty policy map,
apply the policy to the serial sub-interface, and display the output of
show policy-map. Enter the
following commands:
XSR(config)#policy-map test
XSR(config-pmap<test>)#interface serial 1/1:0
XSR(config-subif<S1/1.0>)#service-policy output test
XSR(config-subif<S1/1.0>)#exit
XSR(config)#show policy-map interface serial 1/0:0
Describing Priority Queues
Priority Queues (PQ) extend absolute (strict) priority to certain traffic. Higher priority packets are
sent before lower priority packets, and lower priority packets are sent before any non-priority
packets. Priority queuing ensures that applications which cannot tolerate much delay (e.g., voice
and video traffic) are serviced before non-time critical applications (e.g., FTP).
Traffic assigned to priority queues is rate-limited so the queue’s presence would not “starve” low
priority packets and fair queues. The XSR supports up to four priority queues per interface,
labeled high, medium, low, and normal. They are characterized by the following rules:
• High priority queues are emptied before low priority queues.
• PQ bandwidth is controlled using a traffic policer to rate-limit it
Configuring Priority Queues
The priority command configures priority queuing for certain packets based on the traffic class.
When you specify priority (using the following commands) for a class, it takes a bandwidth
argument affording maximum bandwidth. The following commands configure priority queuing:
• policy-map policy-name
• class class-name
• priority priority-level kbps [burst-size]
Be aware that bandwidth guarantees come into play when an interface is congested, at which time
traffic class guarantees bandwidth equal to the specified rate. The
priority command
implements a maximum bandwidth guarantee. If the priority class does not use its bandwidth, the
Note: If priority queues are configured to take up almost the entire bandwidth of the interface or
PVC, CBWFQ and control packets will get no actual bandwidth and may be blocked.