2006 Cisco Systems, Inc. All rights reserved. 3.2: Implementing QoS.

© 2006 Cisco Systems, Inc. All rights reserved.

3.2: Implementing QoS


Objectives Describe the need for QoS as it relates to various types

of network traffic.

Identify QoS mechanisms.

Describe the steps used to implement QoS.


What Is Quality of Service? Two Perspectives

The user perspectiveUsers perceive that their applications are performing properly

Voice, video, and data

The network manager perspectiveNeed to manage bandwidth allocations to deliver the desired application performance

Control delay, jitter, andpacket loss


Different Types of Traffic Have Different Needs

Application Examples

Sensitivity to QoS Metrics

Delay Jitter Packet Loss

Interactive Voice and Video Y Y Y

Streaming Video N Y Y

Transactional/ Interactive Y N N

Bulk DataEmail

File TransferN N N

Need to managebandwidth allocations

Real-time applications especially sensitive to QoS

Interactive voiceVideoconferencing

Causes of degraded performanceCongestion lossesVariable queuing delays

The QoS challengeManage bandwidth allocations to deliver the desired application performanceControl delay, jitter, and packet loss


Cisco IOS QoS Tools Congestion management:

PQ CQWFQ CBWFQ

Queue managementWRED

Link efficiencyLink fragmentation and interleave RTP and CRTP

Traffic shaping and traffic policing

QoS Toolbox


Priority Queuing

PQ puts data into four levels of queues: high, medium, normal, and low.


Priority Queuing

Priority output queuing allows a network administrator to define four priorities of traffic---high, normal, medium, and low---on a given interface.

As traffic comes into the router, it is assigned to one of the four output queues.

Packets on the highest-priority queue are transmitted first.


Priority Queuing ctd..

When that queue empties, traffic on the next highest-priority queue is transmitted, and so on.

This mechanism assures that during congestion, the highest-priority data does not get delayed by lower-priority traffic.

However, if the traffic sent to a given interface exceeds the bandwidth of that interface, lower-priority traffic can experience significant delays.


Custom Queuing

CQ handles traffic by assigning a specified amount of queue space to each class of packet and then servicing up to 17 queues in a round-robin fashion.


Custom Queuing Custom queuing allows a customer to reserve a

percentage of bandwidth for specified protocols.

Customers can define up to 10 output queues for normal data and an additional queue for system messages such as LAN keepalive messages (routing packets are not assigned to the system queue).

The routers service each queue sequentially, transmitting a configurable percentage of traffic on each queue before moving on to the next one.

Custom Queuing guarantees that mission-critical data is always assigned a certain percentage of the bandwidth, but also assures predictable throughput for other traffic.


Custom Queuing … To provide this feature, routers determine how many

bytes should be transmitted from each queue, based on the interface speed and the configured percentage.

When the calculated byte count from a given queue has been transmitted, the router completes transmission of the current packet and moves on to the next queue, servicing each queue in a round-robin fashion.


Weighted Fair Queuing

•WFQ makes the transfer rates and interarrival periods of active high-volume conversations much more predictable.


Weighted Fair Queuing (Flow based)

Flow-Based WFQ: Creating Fairness Among Flows

For situations in which it is desirable to provide consistent response time to heavy and light network users alike without adding excessive bandwidth, the solution is flow-based WFQ (commonly referred to as just WFQ).

It is a flow-based queuing algorithm that creates bit-wise fairness by allowing each queue to be serviced fairly in terms of byte count.


WFQ …. For example, if queue 1 has 100-byte packets and

queue 2 has 50-byte packets, the WFQ algorithm will take two packets from queue 2 for every one packet from queue 1.

This makes service fair for each queue: 100 bytes each time the queue is serviced.

WFQ ensures that queues do not starve for bandwidth and that traffic gets predictable service.

Low-volume traffic streams that comprise the majority of traffic, receive increased service, transmitting the same number of bytes as high-volume streams.

This behavior results in what appears to be preferential treatment for low-volume traffic, when in actuality it is creating fairness.


Weighted Random Early Detection

•WRED provides a method that stochastically discards packets if congestion begins to increase.


Implementing QoS

Step 1: Identify types of traffic and their requirements.

Step 2: Divide traffic into classes.

Step 3: Define QoS policies for each class.


Step 1: Identify Types of Traffic and Their Requirements Network audit: Identify traffic on the network.

Business audit: Determine how important each type of traffic is for business.

Service levels required: Determine required response time.


Step 2: Define Traffic Classes

Scavenger Class

Less than Best Effort


Step 3: Define QoS Policy A QoS policy is a

network-wide definition of the specific levels of QoS that are assigned to different classes of network traffic.


Quality of Service OperationsHow Do QoS Tools Work?

Classification and Marking

Queuing and (Selective) Dropping

Post-Queuing Operations


Self Check1. What types of applications are particularly sensitive to

QoS issues?

2. What is WFQ? How is it different than FIFO?

3. What are the 3 basic steps involved in implementing QoS?

4. What is Scavenger Class?


Summary QoS is important to both the end user and the network

administrator. End users experience lack of QoS as poor voice quality, dropped calls or outages.

Network traffic differs in its ability to handle delay, jitter and packet loss. Traffic sensitive to these issues requires priority treatment. QoS measures can provide priority to sensitive traffic, while still providing services to more resilient traffic.

Implementing QoS involves 3 basic steps: identify the types of traffic on your network, divide the traffic into classes, and define a QoS policy for each traffic class.

2006 Cisco Systems, Inc. All rights reserved. 3.2: Implementing QoS.

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