User`s guide

XSR User’s Guide 215
Chapter 10 Mechanisms to Provide QoS
Configuring Quality of Service
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
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
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.