Specifications

About QoS
12-12 Configuring Quality of Service
RoamAboutSwitchAmarksthepacketbasedonthepacket’sinternalCoSvalue.Inthis
example,theegressinterfaceisinaVLANandhasan802.1QVLANtag.Therefore,the
RoamAboutSwitchmarksboththe802.1pvalue(with7)andthetunnelheader’sDSCPvalue
(with56).Roam AboutSwitchA
sendsthepackettoRoamAboutSwitchBontheIPtunnel
thatconnectsthetwoswitches.
4. RoamAboutSwitch B receivesthepacketfromtheLayer3cloud.Thepackethasan802.1Q
VLANtag,sotheRoamAboutSwitchclassifiesthepacketbymappingits802.1pvalue(inthis
example,7)to
thematchinginternalCoSvalue(also7).However,becausethepacketalsohas
anon‐zerovalueintheDSCPfieldofthetunnelheader,theRoamAboutSwitchreclassifies
thepacketbymappingtheDSCPvalue(56)toaninternalCoSvalue(7)instead.
5. RoamAboutSwitch B encapsulatesthe packetin
anIPtunnelpacketandmarkstheDSCP
valueinthetunnelheaderbasedonthepacket’sinternalCoSvalue.Inthisexample,the
RoamAboutSwitchmarksthetunnelheaderwithDSCP56.RoamAboutSwitchBsendsthe
packettoAPBontheIPtunnelthatconnectsthem.
6. APB
receivesthepacketand doesthefollowing:
•MapstheDSCPvalueinthetunnelheader(56)toaninternalCoSvalue(7).
•Marksthepacket’sservicetypebasedontheinternalCoSvalue(7).
•Placesthepacketinaforwardingqueue(Voice)basedontheinternalCoSvalue(7).
Inthisexample,
theAPplacesthepacketintheVoiceforwardingqueue.TheVoicequeuehas
statisticallymoreaccesstotheairthantheotherqueues,sotheuser’svoicetrafficreceives
prioritytreatment.
SVP QoS Mode
TheSVPQoSmodeoptimizesforwardingofSVPtrafficbysettingtherandomwaittimeanAP
radiowaitsbeforetransmittingthetrafficto0microseconds.
Normally,anAPradiowaitsanadditionalnumberofmicrosecondsfollowingthefixedwaittime,
beforeforwardingaqueuedpacketorframe.Eachforwardingqueue
hasadifferentrangeof
possiblerandomwaittimes.TheVoicequeuehasthenarrowestrange,whereastheBackground
andBestEffortqueueshavethewidestrange.TherandomwaittimesensurethattheVoicequeue
getsstatisticallymoreaccesstotheairthanthe otherqueues.
Bysettingthe
randomwaittimeto0forSVP,theSVPQoSmodeprovidesSVPtrafficthegreatest
possibleaccesstotheair,onastatisticalbasis.TheQoSmodeaffectsforwardingofSVPtraffic
only.Therandomwaittimesforothertypesoftrafficarethesameasthoseusedwhenthe
QoS
modeisWMM.
Note: An ACL can override a packet’s marking. If a packet matches a permit ACL mapped
to the outbound traffic direction, Distributed AP, or user VLAN, and the ACL sets the CoS
value, the tunnel header’s DSCP value is marked based on the CoS value in the ACL
instead.