Specifications

Quality of Service Overview
May 09, 2011 Page 9 of 38
Figure 2 Strict Priority Queuing Packet Behavior
Low Latency Queuing
ALowLatencyQueue(LLQ)isanon‐configurablestrictpriorityqueue.LLQsaredesignedto
guardagainst:
• Packetloss
•Delay
• Jitter
LLQhardwareresourcescannotbeconfigured,butapolicycanbeconfiguredforaCoSthatis
mappedtoanLLQ.Inthisway,trafficassociatedwithhighvaluereal‐time
voiceorvideopackets
canbemappedtoanLLQ.TheLLQprioritywilldeterminewhenmappedtrafficwillbeserviced
relativetoothertraffic.Forexample,S‐Seriesqueues0,9,and10areLLQs.Ifavoicepolicyis
mappedtoaCoSwithaTxQreferencethat
isinturnmappedtoqueue9,thisvoicetrafficwillbe
servicedassoonasqueue10isemptyandwillcontinuetobeservicedaheadofanylowerpriority
queueuntilthereisnotrafficleftinqueue9.
LLQsarehardwaredependent.Notallhardwaredevicessupportlow
latencyqueuing.Usethe
showcosport‐configtxqcommandtodisplayLLQsforagivenmodule.
Weighted Fair Queuing
Withweightedfairqueuing,queueaccesstobandwidthisdividedupbypercentagesofthetime
slicesavailable.Forexample,if100percentisdividedinto64timeslices,andeachqueueis
configuredfor25percent,eachqueuewillget16timeslices,afterwhichthenextlowestpriority
queue
willgetthenext16,andsoon.Shouldaqueueemptybeforeusingitscurrentshareoftime
slices,theremainingtimeslicesaresharedwithallremainingqueues.Figure 3depictshow
weightedfairqueuingworks.Inboundpacketsenterontheupperleftoftheboxandproceedto
theappropriatepriorityqueue.Outboundpacketsexitthequeuesonthelowerright.Queue3has