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Quality of Service (QoS)
CIS 187 Multilayer Switched Networks
CCNP
Rick Graziani
Spring 2009
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Overview
Previously an organization would use separate networksfor: Voice
Video
data traffic
Now common practice to combinethese into a single multi-service network
in which the varied traffic types coexist.
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Overview
QoS Issues over non-QoS networks: Stop-startand choppyInternet streaming video performance
Harsh audiowhen using Internet based IP phone
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Quality of Service
defined
QoSrefers to the abil i ty o f a network to provide imp roved serv iceto selected network traff ic over var ious und er lying techn olog iesinclud ing Frame Relay, ATM, Ethernet and IP-routed netwo rks .
QoS features provide improved and more predictable network serviceby offering the following services:
Dedicated bandwidth Improved loss characteristics
Congestion management and Avoidance
Traffic Shaping
Prioritization of traffic
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Quality of Service defined
The goal is to move information from one point to anotherand the characteristics that define the quality of this
movement are: Delay
Delay Variation (also known as Jitter)
Loss
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Loss
Lossrefers to the percentage of packets that fail toreach their dest inat ion .
Loss can result from: Errors in the network
Corrupted frames
Congested networks
s
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Loss
Packet loss in a healthy networkare actually deliberately dropped
by networking devices to avoid congestion. (later) TCP: TCPs retransmission mechanism
UDP: Some loss may be acceptable
As a guide, a highly available network should suffer less than 1% lossand for voice traffic the loss should approach 0%.
TCP Header
UDP Header
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Delay or latency
Delayor latencyrefers to the t ime it takes for a packet to travelfrom the sourc e to the dest inat ion.
Fixed delays Serialization and encoding/decoding.
For example, a bit takes a fixed 100ns to exit a 10Mb Ethernet
interface. Variable delays
Congestion and time packets spend in network bufferswaitingfor access to the media.
As a design rule the total time it takes a voice packet to cross the
network should be less than 150ms (ms, millisecond = 1,000thof asecond).
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Delay variation or jitter
Delay variationorjitteris the dif ference in the delay t im es ofcon secut iv e packets.
Ajitter bufferused to smooth out arrival times. Increases total network delay.
In general, traffic requiring low latency also requires a minimumvariation in latency.
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Delay variation or jitter
As a design rule, voice networks cannot cope with more than 30ms ofjitter.
Jitter in excess of 30ms will result in degraded audio performance.
Excessive jitterin a streaming video environment will result in: Jerky motion
Loss of video quality
Loss of video
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Network availability
Highly availablenetwork uses: Redundancy
Dynamic routing protocols
Hot Standby Routing Protocol (HSRP)
Spanning Tree Protocol (STP)
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Provisioning
Bandwidthis not listed as an element of QoS. Inadequate bandwidthinflates latency It is not possible to meet QoS requirements if network LAN and WAN links
have insufficient bandwidth simply adding bandwidth, (also known as over-provisioning) will not solve the problem.
Over-provisioned network: Good News:Less likely to be congested
Bad News:If it does become congested, the network may not performas wellas a lower bandwidth networkthat makes use of QoS features.
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Quality of Service
requirements for data
Some traffic can usually tolerate lower QoS levels.
Relativepriority modeldivides traffic into four classes: Gold(Mission-Critical)Transactional, software Silver(Guaranteed-Bandwidth)Streaming video, messaging,
intranet
Bronze(Best-Effort and Default class)Internet browsing, E-Mail
Less-than-Best-Effort(Optional; higher-drop preferences)FTP,backups, and applications (MySpace, YouTube, KaZaa)
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Quality of Service
requirements for voice
Voice qualityis directly affected by allthree QoS quality factors:
Loss
Delay
delay variation
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Quality of Service requirements for video
Streaming videoapplications have more lenientQoSrequirements due to application buffering.
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Quality of Service requirements for video
QoS needs of videoconferencingtraffic are similar to those forvoice.
Lossshould be no more than 1%
One-way latencyshould be no more than 150-200ms
Average jittershould be no more than 30ms
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Quality of Service mechanisms
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Quality of Service mechanisms
Once the QoS requirements of the network have been defined, anappropriate service model must be selected. A service modelis a general approach or a design philosophy for
handling the competing streams of traffic within a network.
There are three service models from which to chose;
Best-effort Integrated
Differentiated
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Best-Effort service
Best effortis a single service model in which an appl icat ion send s data: Whenever i t must
In any quant i ty Without request ing p ermis sion o r f i rs t inform ing the netwo rk
For best-effort service, the network delivers data if it can, without anyassurance of:
Reliability
delay
throughput
(relative time of arrival)
(single interface outbound queue)
(one packet at a time)
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Best-Effort service
Cisco IOS QoSimplements best-effort service isFIFOqueuing.
FIFOis the defaul t method of q ueuing for LAN and high sp eedWAN interfaces on sw itches and routers. Best-effort service is suitable:
General file transfers
E-mail
Web browsing
(relative time of arrival)
(single interface outbound queue)
(one packet at a time)
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Integrated services model
Integrated serviceor IntServ
The application requests a
specific kind of service from
the network before it sends
data.
The Cisco IOS IntServ modelmakes use of the IETF ResourceReservation Protocol (RSVP)
Used by applications to signal
their QoS requirements to the
router. Drawbacks Not scalable
Require continuous signalling
from network devices
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Integrated services model
Routers, in conjunction with RSVP are able to use intelligent queuingmechanisms to provide two types of services.
Guaranteed Rate Service, which allows applications to reserve bandwidth tomeet their requirements.
For example, a Voice over IP (VoIP) application can reserve 32 Mbps end-to-end using this kind of service.
Cisco IOS QoS uses weighted fair queuing (WFQ) with RSVPto provide
this kind of service Controlled Load Service, which allows applications to have low delay andhigh throughput even during times of congestion.
For example, adaptive real-time applications such as playback of arecorded conference can use this kind of service.
Cisco IOS QoS uses RSVP with Weighted Random Early Detection(WRED)to provide this kind of service.
FYI
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Differentiated services model
Differentiated Serviceor DiffServarchitecture Emerging standard from the IETF.
Each packet is classi f ied upon entry into the netwo rk.
These are represented using the Type of Service (ToS)field. IPpacket header:
IP precedenceor
Differential Services Code Point (DSCP).
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Differentiated services model
Once packets are classified at the edge by Access layer switches
Border routers
Unlike the IntServ model,DiffServ does not requ irenetwork app l icat ions be QoS aware.
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Traffic marking
Data Link Layer:
Ethernet frame has no fieldsto signify its QoSrequirements.
ISL or 802.1Q/P provides a 3 bit Class of Service
(CoS) field.
Gives Layer 2 switches the ability to prioritize traffic.
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Traffic marking
At the Network layeran IP packet contains: ToS:
IP-Precedencefield Differentiated Services Code Point (DSCP) fields.
Either of these can be used to signify the QoSrequirements of an IP packet.
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Traffic marking
The decision of whether to mark traffic at layers 2 or 3 or bothis not
trivial and should be made after consideration of the following points: Layer 2marking of frames can be performed for non IP traffic.
Layer 2marking of frames is the only QoS option available forswitches that are not IP aware
Layer 3marking will carry the QoS information end-to-end
Older IP equipmentmay not understand DSCP
Layer 2
Layer 3
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CoS
The 3 bit CoSfield present allows eight levelsof priority. 0 lowest priority to 7 highest priority Switchesset a layer 2 CoS valuefor traffic based on
their ingressport
Router translatethe CoS value intoan equivalent IPPrecedence or DSCP value
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ToS
ToS IPDSCPvalue is the first 6 bits
IP Precedencevalue is the first 3 bits
The IP Precedence value is actually part of the IP DSCP value. Therefore, both values cannot be set simultaneously.
DSCP supersedes IP Precedence. A maximum of:
8different IP precedencemarkings
64different IP DSCPmarkings
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Modular QoS command lineinterface (CLI)
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Modular QoS command line interface (CLI)
The Modular QoS Command Line Interfaceor MQCis central toCiscos model for implementing IOS based QoS solutions.
The MQC breaks down the tasks associated with QoS into modulesthat:
Identify traffic flows
Classify traffic flows as belonging to a common class of QoS.
Apply QoS policies to that class
Define the interfaces on which the policy should be enforced
The modular nature of MQC allows the reuse of common trafficclasses and policies. This simplifies the configuration, makes it moreefficient to implement changes and reduces the chances of errors.
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Example Modular QoS CLI
Interface
service-policy
outputpolicy1
Interface
service-policy
outputpolicy1
Interface
service-policy
outputpolicy2
policy-mappolicy1
class class1bandwidth
queue-limit
random-detect
class class2
bandwidth
queue-limit
random-detect
policy-mappolicy2
class class1bandwidth
queue-limit
random-detect
class class3
bandwidth
queue-limit
random-detect
class-map class1
match input-interface
class-map class2
match access-group
access-list
class-map class3
match input-interface
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Classification of trafficThe class-map
The class-map command is used to define a traffic class. The purpose of a traffic class is to classi fy o r ident i fy traff ic that
should be given a part icular QoS.
Traff ic that matches a certain crit er ia.
A traffic class contains three major elements: Name
Series of match commands
Ifmore than onematch command exists in the traffic class an
instruction on how to evaluate these match commands.
Switch(config)# class-map cisco
Switch(config-cmap)#
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Classification of trafficThe class-map
Match commands are used to specify various criteria for classifyingpackets.
If a packet matchesthe specified criteria: Packet is considered a member of the class
Packet is forwarded according to the QoSspecifications set inthe traffic policy
Packets that fail to meet any of the matching criteria: Classified as members of the default traffic class
Subject to a separate traffic policy
Switch(config)# class-map cisco
Switch(config-cmap)# match access-group name test
In the example below, any traffic that is permitted by the named ACL test will
be considered part of the traffic class known as cisco.
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Classification of trafficThe class-map
If match-any is specified as the evaluation instruction, the traffic beingevaluated by the traffic class must match on e of the speci f ied
criter ia.
If match-allis specified as the evaluation instruction, the traffic beingevaluated by the traffic class must m atch all of the specif ied cr i ter ia.
Switch(config)# class-mapmatch-anycisco
Switch(config-cmap)# match access-group name test
Switch(config-cmap)# match interface fastethernet 0/1
If traffic matchesa permit statement in the ACL test orthe traffic
originates from FastEthernet 0/1then it will be considered to be part of
the class of traffic known as cisco.
Defining the QoS policy The
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Defining the QoS policyThe
policy-map
The policy-map command is used to create a traffic policy. The purpose of a traffic policy is to conf igu re the QoS features
that should be associated with the traffic that has been classified ina user-specified traffic class.
A traffic policy contains three elements: Policy Name
Traffic class(specified with the class command)
QoS policiesto be applied to each class
Switch(config)# policy map policy1
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The policy-mapshown creates a traffic policy namedpolicy1.
The policy applies to alltraffic classified or identifiedby the previously defined traffic-class cisco
Specifies that traffic in this example should be
allocated bandwidth of 3000 kbps. Any traffic which does notbelong to the class cisco
forms part of the catch-all class-defaultclass
Will be given a default bandwidth of 2000 kbps.
Switch(config)# policy-mappolicy1
Switch(config-pmap)# class cisco
Switch(config-pmap-c)# bandwidth 3000
Switch(config-pmap)# class class-defaultSwitch(config-pmap-c)# bandwidth 2000
Applying the policy to an interface The
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Applying the policy to an interface The
service-policy
The service policy command is used to attach the trafficpolicy, as specified with the policy-mapcommand, to aninterface.
Can be applied to packets enteringor leavingthe
interface.
Switch(config)# interface fastethernet 0/1
Switch(config-if)# service-policy outputpolicy1
Applying the policy to an interface The
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Applying the policy to an interface The
service-policy
All packets leaving the specified interfaceare evaluated accordingto the criteria specified in the traffic policy named policy1.
Switch(config)#interface fastethernet 0/1
Switch(config-if)#service-policy outputpolicy1
Switch(config-if)#exit
Applying the policy to an interface The
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Applying the policy to an interface The
service-policy
Any traffic which does not belong to the class cisco forms part of the catch-allclass-defaultclass will be given a default bandwidth of 2000 kbps.
Switch(config)#interface fastethernet 0/1
Switch(config-if)#service-policy outputpolicy1
Switch(config)#policy-mappolicy1
Switch(config-pmap)#class cisco
Switch(config-pmap-c)#bandwidth 3000
Switch(config-pmap)#class class-default
Switch(config-pmap-c)#bandwidth 2000
Switch(config)# class-map match-any cisco
Switch(config-cmap)# match access-group name test
Switch(config-cmap)# match interface fastethernet 0/1
Attach the traffic
policyto an interface
Identify the QoS
featuresof a Policy
using classes
Identify the traffic or traffic flows
Classify traffic
flows as
belonging to a
common class
of QoS.
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IP Precedence and DSCP
IP P d
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IP Precedence
3 bits = 8 possibilities. Network control and Internetwork controlclasses are
usually reserved for router-generated packets such as
routing updates, ICMP messages, etc.
To protect packets that are necessary for the health of
the network.
Only 6 usable classes for production.
DSCP
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DSCP
The Differentiated Service Code Pointis aselecto r forrou ter 's per-hop behaviors .
DSCP (like IP Precedence) can be used to providedifferential treatment to packets.
Up to 64 different aggregates/classes can be supported Default DSCP = 000 000
P H B h i
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Per Hop Behavior
Behavior Aggregate (BA) -A collection of packets that have the sameDSCP value (also called a codepoint) and crossing in a particulardirection.
Per Hop Behavior (PHB)- The packet scheduling, queuing, policing,or shaping behavior of a node on any given packet belonging to a BA,and as configured by a Service Level Agreement (SLA) or policy.
To date, four standard PHBs are available to construct a DiffServ-enabled network and achieve coarse-grained, end-to-end CoS andQoS.
IP Packet
IP Packet
IP Packet
IP Packet
Same
DSCP
Value
Cl S l t PHB (D fi d i RFC 2474)
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Class-Selector PHBs (Defined in RFC-2474)
To preserve backward compatibility with the IP-precedence scheme: DSCP values of the form `xxx000,'
These codepoints are called class-selector codepoints.
These PHBs retain almost the same forwarding behavior as nodesthat implement IP-precedence based classification and forwarding.
These PHBs ensure that DS-compliant nodes can co-exist with IP-precedence aware node.
DSCP IP Precedence
111000 (56) Range = 56 thru 63 111(7)Network Control110 000 (48) Range = 48 thru 55 110(6)Internetwork Control
101000 (40) Range = 40 thru 47 101(5)Critical
100000 (32) Range = 32 thru 39 100(4)Flash Override
011 000 (24) Range = 24 thru 31 011(3)Flash
010000 (16) Range = 16 thru 23 010(2) - Immediate
001000 (8) Range = 8 thru 15 001 (1) - Priority
000 000 (0) Range = 0 thru 7 000 (0) - Routine
E dit d F di d A d F di
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Expedited Forwarding and Assured Forwarding
Expedited Forwarding (EF)PHB defines a premium service for video and VoIP. Recommended DSCP is 101110
Assured Forwarding (AF)PHB defines a method by which BAs can be givendifferent forwarding assurances. The AFxy PHB defines four AFxclasses: AF1, AF2, AF3, and AF4.
Each class is assigned a certain amount of buffer space and interfacebandwidth, dependent on the SLA with the Service Provider/policy.
Within each AFx class (AFxy)it is possible to specify 3 drop precedencevalues.
Packets in AF13
will get dropped
before packets in
AF12, before
packets in AF11.
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Classification at the Access Layer
Classification at
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the Access Layer
QoS should be implemented end-to-endwithin a network. Best to classify traffic as soon as possible. Frames and packets can be marked as important by using:
Layer 2 Class of Service (CoS)
Layer 3 the IP Precedence/Differentiated Services Code Point
(DSCP)
Layer 2
Layer 3
Trusting the CoS
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Trusting the CoS
If Edge device(IP phone or application) is capable of setting theCoS bitsthen other devices must decide whether to trust the deviceor not.
The default action of switches: Notto trust edge devices
Any frames that enter the switch have their CoS re-writtento the
lowest priority of 0. Ifthe edge device can be trusted:
Default behaviourmust be overridden
Access switch must be configured to simply switch the frameleaving the CoS bits untouched.
Configuring CoS trust using the IOS
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Configuring CoS trust using the IOS
Depending on the switch model it may be necessary to first activate
QoSusing the command:
switch(config)#mls qos
Requiredon both the Catalyst 3550and 6500.
The Catalyst 2950has QoS enabled by default.
Configuring CoS trust using the IOS
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Configuring CoS trust using the IOS
The trust is configuredon the switch port using the command:
switch(config-if)#mls qos trust cos
Any ISL or 802.1Q/P frames that enter the switch portwill now have its CoS passed, untouched, through
the switch.
If an untagged frame arrives at the switch port,the switch will assign adefault CoSto the frame
before forwarding it.
Default CoS = 0
Can be changed using the interface configuration
command:
switch(config-if)#mls qos cos default-cos
default-cos is a number between 0 and 7
Assigning CoS on
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g g
a per-port basis
switch(config-if)#mls qos trust cosswitch(config-if)#mls qos cos default-cos
If the incoming frame has a CoS, maintain the same CoS. If the incoming frame has no CoS (0), apply the default CoS.
Re-writing the
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g
CoS
May be desirablenot to trust any CoS value that maybe present in frames sourced from an edge device.
Override parameter- ignores any existing CoS value Apply the default value.
Switch(config-if)#mls qos cos override
switch(config-if)#mls qos cos default-cos
Using a MAC ACL to assign a DSCP value
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Using a MAC ACL to assign a DSCP value
It is not always possible to classify the CoS of a frame, based on
an ingress (incoming) port.
Ingress port is connected to multiple hosts through a hub
Simple workgroup switch that does not support QoS classification
Using a MAC ACL to assign a DSCP value
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Using a MAC ACL to assign a DSCP value
Switch(config)#mac access-list extended name
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
Example
Set the DSCP field of packets coming from a single IP-Phone (called receptionphone) within a switched network.
IP-Phone MAC address is 000.0a00.0111
Switch(config)#mac access-list extended receptionphone
Switch(config-ext-macl)#permit host 000.0a00.0111 any
Create the condition criteria.
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
A class-map is used to link the identified traffic to a particular class ofservice.
In this case a class of servicecalled ipphone is created.
Switch(config)#class-map match-all ipphone
Switch(config-cmap)#match access-group name receptionphone
Identify the traffic or traffic flows
Switch(config)#mac access-list extended receptionphone
Switch(config-ext-macl)#permit host 000.0a00.0111 any
Create the condition criteria.
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
The creation of the class-map can be verified with the show class-mapcommand
Switch#show class-map
Class Map match-any class-default (id 0)
Match any
Class Map match-all ipphone(id 2)
Match access-group name receptionphone
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
Now a policy map is used to define the action that shouldbe taken on any traffic that forms part of that class.
In this case the policy will be called inbound-accesslayerand the action is to set DSCP for the packets to 40.
Switch(config)#policy-map inbound-accesslayer
Switch(config-pmap)#class ipphone
Switch(config-pmap-c)#set ip dscp 40
Identify the QoS features of a Policy
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
CoS 0 1 2 3 4 5 6 7
DSCP 0 8 16 24 32 40 48 56
IP
Prec
0 1 2 3 4 5 6 7
Cisco Switches support mapping
DSCP or IP Precedence
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
The show policy-mapcommand can be used to verify any policy-mapconfiguration.
Switch#show policy-map
Policy Map inbound-accesslayer
class ipphone
set ip dscp 40
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
In this case the policy will be applied to all theinterfaces so that QoS will be maintained regardless of
the interface the IP-Phone is connected to.
Switch(config)#interface range fastethernet 0/1 - 24
Switch(config-if-range)#service-policy inputinbound-
accesslayer
Attach the traffic policy to an interface.
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
The showmls qos interface command can be used to determine thepolicies that are bound to a particular interface on the switch.
Switch#show mls qos interface fastethernet 0/1
FastEthernet0/1Attached policy-map for Ingress: inbound-accesslayer
trust state: not trusted
trust mode: not trusted
COS override: dis
default COS: 0
pass-through: none
trust device: none
Configuring DSCP using a MAC ACL
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Configuring DSCP using a MAC ACL
Switch(config)#interface range fastethernet 0/1 - 24
Switch(config-if-range)#service-policy input inbound-
accesslayer
Switch(config)#policy-map inbound-accesslayer
Switch(config-pmap)#class ipphone
Switch(config-pmap-c)#set ip dscp 40
Switch(config)#class-map match-all ipphone
Switch(config-cmap)#match access-group name receptionphone
Switch(config)#mac access-list extended receptionphone
Switch(config-ext-macl)#permit host 000.0a00.0111 any
Attach the traffic policy to an interface.
Identify the QoS features of a Policy
Identify the traffic or traffic flows
Create the condition criteria.
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Another Example (FYI)
Using an IP ACL to define the DSCP ord
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precedence
Using the Modular QoS Command Line Interface (MQC) it is possibleto classify traffic based on its IP or TCP properties.
Scenario: In order to prevent large FTP downloads from disruptingmore critical services, the network administrator wishes to tag all FTP
packets entering an access-layer switch with either:
An IP Precedence of 0 (low) or
A DSCP of 0 (low) so that the traffic can be subjected to QoSpolicies within the network.
In this case an IP ACL will be used to identify the packets.
Switch(config)#ip access-list extended 100
Switch(config-ext-nacl)#permit tcp any any eq ftp
Create the condition criteria.
Using an IP ACL to define the DSCP ord
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precedence
Traffic is classified as reducedservice if it is permitted by the
access list.
Switch(config)#class-map reducedservice
Switch(config-cmap)#match access-group 100
Identify the traffic or traffic flows
Using an IP ACL to define the DSCP ord
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precedence
Policy-map is used to set the DSCP to 0for this class of traffic.
Switch(config)#policy-map inbound-accesslayer
Switch(config-pmap)#class reducedservice
Switch(config-pmap-c)#set ip dscp 0
Identify the QoS features of a Policy
Using an IP ACL to define the DSCP ord
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precedence
Alternatively the IP precedencecan be set using the following policy-map. Note:
Both the Catalyst 2950 and the Catalyst 3550 support the setting ofthe DSCP.
The 3550 does support the setting of IP precedence. The 2950 does notsupport the setting of IP precedence.
This is not a serious problem as the IP Precedence field forms thefirst 3 bits of the DSCP. Thus by choosing and setting theappropriate DSCP value, the IP Precedence can still be set.
Switch(config)#policy-map inbound-accesslayer
Switch(config-pmap)#class reducedservice
Switch(config-pmap-c)#set ip precedence 0
Identify the QoS features of a Policy
Using an IP ACL to define the DSCP ord
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precedence
Having now defined the action to be taken on FTP packets, the onlyremaining step is to tell the switch which interfaces to apply the policy
to.
In this case the policy will be applied to all the interfaces so that QoSwill be maintained regardless of the interface an FTP source may be
connected to.
Switch(config)#interface range fastethernet 0/1 - 24
Switch(config-if-range)#service-policy input inbound-
accesslayer
Attach the traffic policy to an interface.
Using an IP ACL to define the DSCP ord
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precedence
Switch(config)#ip access-list extended 100
Switch(config-ext-nacl)#permit tcp any any eq ftp
Switch(config)#class-map reducedservice
Switch(config-cmap)#match access-group 100
Switch(config)#policy-map inbound-accesslayer
Switch(config-pmap)#class reducedserviceSwitch(config-pmap-c)#set ip dscp 0
Switch(config)#interface range fastethernet 0/1 - 24
Switch(config-if-range)#service-policy input inbound-
accesslayer
Attach the traffic policy to an interface.
Identify the QoS features of a Policy
Identify the traffic or traffic flows
Create the condition criteria.
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Scheduling
Suggested Readings
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gg g
Queuing overview
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Q g
A protocol-dependent switching process handles trafficarriving at a router interface.
This process includes delivery of traffic to an outgoinginterface buffer.
First-in, first-out (FIFO)queuing is the classic algorithmfor packet transmission.
Queuingi
*
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overview
Cisco IOS software offers three alternative queuing options: Weighted fair queuing (WFQ)
Class-based weighted fair queuing (CBWFQ)- IOS 12.2 and later
Low latency queuing (LLQ)- IOS 12.2 and later
Queuing methods discussed in previously in CCNP, and have beenreplaced somewhat by CBWFQ and LLQ
Custom Queuing replaced by CBWFQ
Priority Queuing replaced by LLQ
Effective use of traffic prioritization*
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p
Generalizations on Queuing:
If there is no congestion on the WAN link, traffic prioritization isnot necessary.
If a WAN link is constantly congested, traffic prioritization may not
resolve the problem.
Adding bandwidth might be the appropriate solution.
Establishing a queuing policy*
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g q g p y
Goalis to deploy and maintain a single enterprise network thatsupports a variety of: Applications
Organizations
Technologies
User expectations Result: Provide all users with an appropriate level of service, while
continuing to support mission-critical applications.
Choosing a Cisco IOS queuing options*
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g q g p
Typically, voice and video have the lowesttolerance for delay.
WFQ
Priority
LLQ (PQ/CBFQ)
Custom
CBWFQ
*
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Configuring Weighted FairQueuing
FIFOFirst InFirst Out*
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81
FIFOqueuing is in effect, traffic is transmitted in the order receivedwithout regard for bandwidth consumption or the associated delays.
Packet trainsare groups of packets that tend to move togetherthrough the network. These packet trains can consume all available bandwidth, and
other traffic flows back up behind them.
(relative time of arrival)
(single interface outbound queue)
(one packet at a time)
FQFair Queuing*
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g
Fair Queuingis notan optionon Cisco routers. Allows packets that are ready to be transmitted to leave, even if
they started to arrive after another packet. Complete packets that are ready to be transmitted leave first. Remember, packets may enter the output buffer from a variety of input
interfaces.
(single interface outbound queue)
(one packet at a time)
Weighted fair queuing overview*
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Weighted fair queuing (WFQ) is an automated methodthat provides fairbandwidth allocation to all network traffic.
Provides traffic priority management that dynamically sorts traffic intoconversations, or flows.
Then breaks up a stream of packetswithin each conversation to ensure thatbandwidth is shared fairly between individual conversations.
There are four typesof weighted fair queuing: Flow-basedDefault (WFQ)
Distributed - Runs on Versatile Interface Processor (not discussed)
Class-basedNext section
Distributed class-based(Not discussed)
Packet 3 is queued before packets 1 or2 because packet 3 is a small packet in
a low-volume conversation
Small packet in low-volume conversation arrives 3rd
Weighted fair queuing overview*
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Flow Based WFQschedules delay-sensitive traffic to the front of a queueto reduce response time, and also shares the remaining bandwidth fairlyamong high-bandwidth flows.
By breaking up packet trains, WFQ assures that: Low-volume trafficis transferred in a timely fashion.
Gives low-volume traffic, such as Telnet sessions, priority over high-volume traffic, such as File Transfer Protocol (FTP) sessions.
Gives concurrent file transfers balanced useof link capacity.
Automaticallyadaptsto changing network traffic conditions.
(single interface outbound queue)
(one packet at a time)
Weighted fair queuing overview*
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Weighted fair queuing is enabled by default for physicalinterfaces whose bandwidth is less than or equal to T1/E1,or 1.544 Mbps/2.048 Mbps.
WFQ default on T1/E1
and slower.
FIFO default on fasterthan T1/E1.
T1 T3
Weighted fair queuing operation*
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86
The WFQ sorting of traffic into flows isbased on packet headeraddressing.
Common conversation discriminatorsare as follows (based on ahash):
Source/destination network address
Source/destination Media Access Control (MAC) address
Source/destination port or socket numbers
Frame Relay data-link connection identifier (DLCI) value
Quality of service/type of service (QoS/ToS) value
The router determines what the actual flows are, not theadministrator.
Packet 3 is queued before packets 1 or
2 because packet 3 is a small packet in
a low-volume conversation
Small packet in low-volume conversation arrives 3rd
Weighted fair queuing operation*
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WFQ assigns a weight to each flow. Lower weights are served first.
Small, low-volume packetsare given priorityover large, high-volume conversation packets.
Flow Based WFQ algorithm allocates a separate queue for eachconversation.
WFQ is IP Precedence-aware.
This is only pertinent if the IP precedence bit is used
Coming next
Weighted fair queuing*
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WFQstarts by sorting trafficthat arrives on an egress interface intoconversation flows.
The router determineswhat the actual flowsare
The administrator cannot influence this decision. Conversations are based on a hash (combination) of: Source/destination network address
Source/destination Media Access Control (MAC) address
Source/destination port or socket numbers
Frame Relay data-link connection identifier (DLCI) value
Quality of service/type of service (QoS/ToS) value
(relative time of arrival)
(single interface outbound queue)
10141517
Flow #3
Flow #2
Flow #1
Weighted fairqueuing
*
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queuing IP ToS bits are used to determine
which packet gets priority.
Simplification: Dispatch = Finish time x Weight Weight = 32768/(IP Prec + 1)
IP Precedence Weight12.0(5)T and later Our Value
0 32768 81 16384 7
2 10920 6
3 8192 5
4 6552 4
5 5456 3
6 4680 2
7 4096 1
Weighted fair queuing*
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FIFOLargest first, then medium, then smallest FQSmallest first, then medium, thenlargest
WFQMultiplier is used, weight = 32768/(IP Prec + 1) To keep it simple we will use our values and leave out somedetails.
Lowest value wins!
Higher IP Precedence gets a lower value (weight)
(relative time of arrival)
(single interface outbound queue, IP PrecOur Value)
10141517
Flow #3
Flow #2
Flow #1 0 - 8
3 - 5
0 - 8
Weighted fair queuing*
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Dispatch = Finish time x Our Value (weight)
First packet: 17 x 8 = 136 Last
Second packet: 15 x 5 = 75 Lowest
Third packet: 14 x 8 = 112 Next lowest
(relative time of arrival)
(single interface outbound queue, IP PrecOur Value)
10141517
Flow #3
Flow #2
Flow #1 0 - 8
3 - 5
0 - 8
3 - 50 - 80 - 8
Lowest wins!
Weighted fair queuing*
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What if a flow has contains packets with different IP Precedencebits?
Problem is that high-priority packet, 3-5, cannot be dispatched untilafter the large packet in front of it (same flow) leaves.
Packets within a flow are handled FIFO.
(relative time of arrival)
(single interface outbound queue, IP PrecOur Value)
10141517
Flow #3
Flow #2
Flow #1 0 - 8
3 - 5
0 - 8
3 - 50 - 80 - 8
3 - 5
20
3 - 5
Must wait for previous
packet in flow to leave.
Handled using FIFO.
*
FYI
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Configuring
weightedfair queuing
The congestive-discard-thresholdis the number of messages toqueue for high-volume traffic.
In other words, the maximum number of packets in a conversation heldin a queue before they are discarded.
1 to 512
Default is 64 packets.
Router(config-if)#fair-queue {congestive-discard-threshold}
I have more than 128
packets! No more come
into this queue.
*
FYI
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Configuring
weightedfair queuing
The congestive-discard-thresholdapplies only to high volumeconversations that have more than one message in the queue.
The discard policy tries to control conversations that would monopolize
the link. If an individual conversation queue contains more messages than the
congestive discard threshold, that conversation will not have any new
messages queued until that queues content drops below one-fourth of
the congestive discard value.
I have more than 128
packets! No more come
into this queue until .
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*
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Class-Based Weighted Fair
Queuing
Class Based WFQ*
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WFQ separates packets into flows and applies a weight to high-priority
packets so they can leave first. CBWFQ adds a level of administrator control to WFQ. The same WFQ process is followed, the difference is that the
administrator can control how packets are divided into the
conversation or flows.
(relative time of arrival)
(single interface outbound queue, IP PrecOur Value)
10141517
Flow #3
Flow #2
Flow #1 0 - 8
3 - 5
0 - 8
3 - 50 - 80 - 8
3 - 5
20
3 - 5
WFQ
Class Based WFQ
*
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Scenario: the administrator has decided that all high-priority trafficshould reside in the same flow, regardless of any other conditions thatmight place them into separate flows, such as Source/destination
network address, Source/destination Media Access Control (MAC)address, etc.
The WFQ algorithm is still at work, but the queue definition is nowunder control.
CBWFQcan be used to guarantee that f lows receive adequate
bandw idth def ined by the admin istrator .
(relative time of arrival)
(single interface outbound queue, IP PrecOur Value)
10141517
Flow #3
Flow #2
Flow #1 0 - 8
3 - 5
0 - 83 - 5
20
3 - 5 3 - 53 - 50 - 80 - 8
3 - 50 - 80 - 83 - 5
WFQ
CBWFQ
Class-based weighted fair queuing overview*
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Class-based weighted fair queuing (CBWFQ) extends the standardWFQ functionality to provide support for user-defined trafficclasses.
By using CBWFQ, network managers candefine traffic classesbased onseveral match criteria, including:
Protocols
Access Control Lists (ACLs)
Input interfaces
CBWFQ
FIFO Queues*
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A FIFO queue is reserved for each class, and traffic belonging to aclass is directed to the queue for that class.
More than one IP flow, or conversation", can belong to a class. Once a class has been defined according to its match criteria, the
characteristics can be assigned to the class.
To characterize a class: assign the bandwidth maximum packet limit
The bandwidthassigned to a class is the guaranteed bandwidth givento the class during congestion.
CBWFQ
Class233 1
*
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CBWFQ(not you) assigns a weightto each configured class instead of eachflow.
Weight is proportional to the bandwidth(you) configuredfor each class.
Weightis equal to the interface bandwidth divided by the class bandwidth o rcan be configured as a percentage. Weight= Interface bandwidth / class bandwidth
32= 2,048 kbps / 64 kbps (2,048 kbps = 2 Mbps)
16= 2,048 kbps / 128 kbps
64= 2,048 kbps / 32 kbps
A class with a higher bandwidth value will have a lower weight
Highest BW
Lowest weight
Highest priority
Router(config)# policy-map policy1
Router(config-pmap)# class class1
Router(config-pmap-c)#bandwidth 64
Router(config-pmap-c)# queue-limit 30
Router(config-pmap-c)# exit
Router(config-pmap)# class class2
Router(config-pmap-c)#bandwidth 128
Router(config-pmap-c)# exit
Bandwidth is configured in the policy-
map class (later)
CBWFQ Class 233 1*
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By default, the total amount of bandwidth allocated for all classes mustnot exceed 75 percentof the available bandwidth on the interface.
The other 25 percent is used for control and routing traffic.
This is why when you configure a T1 link (and slower), you only get
75% of the bandwidth, unless you turn off queuing.
Highest BW
Lowest weight
Highest priority
CBWFQ Class 233 1*
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The queue limitmust also be specified for the class.
Themaximum number of packets allowed to accumulate in the queuefor the class.
After limit is met packets are droppedsee Tail Drop and WRED.
Packets belonging to a class are subject to the bandwidth and queue limits thatare configured for the class.
Highest BW
Lowest weight
Highest priority
Router(config)# policy-map policy1
Router(config-pmap)# class class1
Router(config-pmap-c)# bandwidth 64
Router(config-pmap-c)# queue-limit 30
CBWFQ versus flow-based WFQ
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Bandwidth allocationCBWFQ allows the administratorto specify the exact amount of bandwidth to be allocated
for a specific class of traffic. Up to 64 classes, and can control distribution among
them.
Class
233 1
Highest BW
Lowest weight
Highest priority
CBWFQ and taildrops
* Hey, these packets are coming infaster than I can send them out!For now I will store some of them in
my output buffer.
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Packet bursts or flows demanding high bandwidth can causecongestion when packets arrive at an output port faster than they can
be transmitted.
The router tries to handle short-term congestions bypacket
buffering. Packet buffering has a cost of delay and jitter, but the packets are
not dropped.
JitterAny distortion of a signal or image caused by poorsynchronization.
p y p
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CBWFQ and taildrops
* Now there are more packets than I can storein my output buffer and I cant send them outfast enough. Guess, I have to start dropping
later packets until I have room in my buffer.
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p
Tail drop. A router simply discards any packet that arrives at the tail end
of a queue that has completely used up its packet-holdingresources.
Default queuing responseto congestion.
Tail drop treats all traffic equally and does not differentiate betweenclasses of service.
Full
CBWFQ andtail drops
* I didnt receive an ACK for my last several TCPsegments. TCP says I have to go into slow start andchange my window size to 512 bytes. I can then
begin to increase it exponentially until I reach the
receivers advertised window size.
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108
When using tail drop, the router drops all traffic that exceeds the queuelimit.
Many TCP sessionsthen simultaneously go into a slow start.
This reduces the TCP window size. Consequently,traffic temporarily slowsas much as possible. As congestion is reduced, window sizes begin to increasein
response to the available bandwidth.
p receiver s advertised window size.
Full
All TCP hosts with non-
ACKed segments gointo TCP Slow Start.
Now, there is very
little traffic that
needs to be sent
out that interface.
CBWFQ and tail drops*
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This activity creates a condition called global synchronization.
Global synchronizationmanifests when: Multiple TCP hosts reduce their transmission rates in response to
packet dropping, and then increase their transmission rates after thecongestion is reduced.
The most important point is that the waves of transmissionknown as globalsynchronization will result in significant link under-utilization.
Time
Full
Tail Drops
1. Traffic flowsenter the
queue at
different times
2. When aggregateload exceeds queue
Tail drops cause
synched TCP window
reduction.
3. Under use causessynched TCP window
expansion.
4. This causes moreTail drop and window
size oscillations.
Bandwidth overused
then underused.
Queue
overused
Queue
underused
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Weighted Random Early Detect (WRED)*
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Tail dropsare a passive queue management mechanism.
Random Early Detection (RED)and Weighted REDare alternatives to taildrops for CBWFQ. Active queue management mechanisms (RED and WRED) drop packets
before congestion occurs.
This is to prevent tail drops and the ups and downs from global TCPsynchronization.
My buffer is not full, but I am going to use Random Early
Detection (RED) and start dropping some packets. This will
help keep global synchronization of TCP slow start fromhappening.
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Weighted Random Early Detect (WRED)*
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113
The WRED algorithm is constantly updated with thecalculated average queue size, which is based on the
recent history of queue sizes.
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WRED*
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115
Based on the profileand the average queue size, WRED calculatesthe probabilityfor droppingthe current packet and either drops it orpasses it to the output queue.
If the queue is already full, the packet is tail-dropped.
Otherwise, it is eventually sent out on the interface.
WRED monitors the average queue depth in the router and determineswhen to begin packet drops based on the queue depth.
When the average queue depth crosses the user-specifiedminimum threshold, WRED begins to drop both TCP and UDPpackets with a certain probability.
WRED*
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The packet drop probabilityis based on the minimum threshold, maximumthreshold, and mark probability denominator.
When the average queue depth is above the minimum threshold, RED startsdropping packets.
The rate of packet drop increases linearly as the average queue size
increases until the average queue size reaches the maximum threshold. The mark probability denominatoris the fraction of packets dropped when
the average queue depth is at the maximum threshold.
For example, if the denominator is 512, one out of every 512 packets isdropped when the average queue is at the maximum threshold.
When the average queue size is above the maximum threshold, all packetsare dropped.
WRED*
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117
If the average queue depth ever crosses the user-specified maximumthreshold, then WRED reverts to tail drop, and all incoming packetsmight be dropped.
The idea behind using WRED is to maintain the queue depth at alevel somewhere between the minimum and maximum thresholds,
and to implement different drop policies for different classes oftraffic.
WRED is only useful when the bulk of the traffic is TCP traffic.
With TCP, dropped packets indicate congestion, so the packetsource reduces its transmission rate.
CBWFQ Using WRED Packet DropExample
*
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In the following example, the class map class1 is created and definedto use the input interface FastEthernet0/1 as a match criterion to
determine if packets belong to the class.
Next, the policy map policy1 is defined to contain policy specificationfor class1, which is configured for WRED packet drop.
Router(config)# class-map class1
Router(config-cmap)# match input-interface FastEthernet0/1
Router(config)# policy-mappolicy1
Router(config-pmap)# class class1
Router(config-pmap-c)# bandwidth 1000Router(config-pmap-c)# random-detect
Router(config)# interface serial0/0
Router(config-if)# service-policy outputpolicy1
Amount of bandwidth in
proportion of the link.
Weight = int bw/ class bw
Enables WRED
Low Latency Queuing (LLQ)*
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The Low Latency Queuing (LLQ)feature provides strict priorityqueuing for class-based weighted fair queuing (CBWFQ), reducing
jitter in voice conversations.
Configured by theprioritycommand, strict priority queuing gives
delay-sensitive data, such as voice, preferential treatment over othertraffic.
With this feature, delay-sensitive data is sent first, before packets inother queues are treated.
LLQis also referred to as priority queuing/class-based weighted fairqueuing (PQ/CBWFQ) because it is a combination of the two
techniques.
LLQ*
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120
CBWFQ (without PQ, non-LLQ)),the weight for a packet belonging toa specific class is derived from the bandwidth assigned to the classduring configuration.
The bandwidth assigned to the packets of a class determines theorder in which packets are sent.
All packets are serviced equally, based on weight.
No class of packets may be granted strict priority.
This scheme poses problems for voice and video traffic that is largelyintolerant of delay, especially variation in delay.
LLQ
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In the event of congestion or when bandwidth has expired, priorityisused to drop packets.
Voice trafficqueued to the priority queue is UDP-basedand,therefore, notadaptive to the early packet drop characteristic of
WRED. Because WRED is ineffective, you cannot use the WRED random-
detectcommand with theprioritycommand.
No
RED/WRED
LLQ*
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122
Although it is possible to enqueue various types of real-time traffic tothe strict priority queue, Cisco recommends that only voice trafficbe directed to it.
Configuring LLQ
*
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123
When theprioritycommand is specified for a class, it uses abandwidth argument that gives maximum bandwidth in kilobits persecond (kbps).
This parameter is used to specify the maximum amount of bandwidthallocated for packets belonging to the class configured with the
prioritycommand (during times of congestion). The bandwidth parameter guarantees bandwidth to the priority class
and restrains the flow of packets from the priority class.
Note: There is also amax-reserved-bandwidthcommand that conbe used, so the priority queue does not starve the remaining queues.
and
LLQ Example*
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124
router(config)# access-list 102permit udp host 10.10.10.10 host10.10.10.20 range 16384 20000
router(config)# access-list 102permit udp host 10.10.10.10 host10.10.10.20 range 53000 56000
router(config)# class-map voice
router(config-cmap)# match access-group 102
router(config)# policy-mappolicy1
router(config-pmap)# class voice
router(config-pmap-c)# priority 50
router(config-pmap)# class bar
router(config-pmap-c)# bandwidth 20
router(config-pmap)# class class-default
router(config-pmap-c)# fair-queue
router(config)# interface atm1/0
router(config-subif)# pvc 0/102
router(config-subif-vc)# service-policy outputpolicy1
A strict priority queue(with a guaranteedallowed bandwidth of50 kbps) is reserved
for traffic that is sentfrom the sourceaddress (10.10.10.10)to the destinationaddress (10.10.10.20),in the range of ports
16384 through 20000and 53000 through56000.
Suggested Readings
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Quality of Service (QoS)
CIS 187 Multilayer Switched Networks
CCNP
Rick Graziani
Spring 2009