Specifications

Quality of Service Overview
May 09, 2011 Page 8 of 38
TxQ Scheduling
TxQscanbeconfiguredforTxQscheduling,alsoreferredtoasweightedfairqueuing.See
WeightedFairQueuingonpage 9foradetaileddiscussionofweightedfairqueuing.See
PreferentialQueueTreatmentforPacketForwardingonpage 8foradetaileddiscussionofall
queuetreatmenttypessupported.
TxQschedulingisconfigured
inCoSportconfigurationusingthearb‐sliceorarb‐percentage
options.Thearb‐sliceoptionsegmentstheTxQschedulingtimeslicepoolbynumericvalues.The
arb‐percentageoptionsegmentstheTxQschedulingtimeslicepoolbyapercentageofthepool.
WhenconfiguringTxQschedulingavalueis
specifiedforallqueuesinTxQorderfromlowestto
highest.A0isenteredforanyqueue(configurableorLLQ)notimeslicesareallocatedto.All
entriesinaconfigurationmustadduptoeitherthetotalnumberofslicessupportedor100percent
dependinguponthechosenoption.
Usetheshowport‐configtxqcommandtodisplaythetotal
numberofslicessupportedforyourdevice.Bydefault,thetotalnumberoftimeslicesisspecified
forthehighestuserconfigurable(non‐LLQ)queue.
IfyouareusingadefaultTxQconfigurationforthisportgroup(youareneither
remappingCoS
prioritiesnorTxQs),TxQschedulingcanbeconfigureddirectlyinCoSportconfigurationwithout
CoSsettings,reference,orport‐resourceconfiguration.
Preferential Queue Treatment for Packet Forwarding
Therearethreetypesofpreferentialqueuetreatmentsforpacketforwarding:strictpriority,
weightedfair,andhybrid.
Strict Priority Queuing
WithStrictPriorityQueuing,ahigherpriorityqueuemust beemptybeforealowerpriorityqueue
cantransmitanypackets.StrictpriorityqueuingisdepictedinFigure 2.Inboundpacketsenteron
theupperleftandproceedtotheappropriatequeue,basedupontheTxQconfigurationinthe
CoS.Outboundpackets
exitthequeuesonthelowerright.Atthistimeonlyqueue3packetsare
forwarded.Thiswillbetrueuntilqueue3iscompletelyempty.Queue2packetswillthenbe
forwarded.Queue1packetswillonlyforwardifbothqueue2 andqueue 3areempty.Queue0
packets
willonlyforwardifallotherqueuesareempty.Strictpriorityqueuingassuresthatthe
highestpriorityqueuewithanypacketsinitwillget100percentofthebandwidthavailable.This
isparticularlyusefulforoneormoreprioritylevelswithlowbandwidthandlowtolerancefor
delay.The
problemwithstrictpriorityqueuingisthatshouldthehigherlevelqueuesneverfully
empty,lowerlevelqueu escanbestarvedofbandwidth.