N ETWORK D ESIGN M ETHODOLOGY T OP -D OWN N ETWORK D ESIGN 1.

NETWORK DESIGN METHODOLOGY

TOP-DOWN NETWORK DESIGN

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Top-Down Network Design, 3rd Edition

Priscilla Oppenheimer

Designing and Supporting Computer Networks (CCNA)

Four phases in the top-down network design methodology

I: Identifying Your Customer's Needs and Goals

II: Logical Network Design

III: Physical Network Design

IV: Testing, Optimizing, and Documenting Your Network Design

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PART 1IDENTIFYING YOUR CUSTOMER’S NEEDS AND GOALS

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Analyzing Business Goals and Constraints Analyzing Technical Goals and Tradeoffs Characterizing the Existing Internetwork Characterizing Network Traffic

CHAPTER ONEANALYZING BUSINESS GOALS AND CONSTRAINTS

Systematic, Top-down network design methodology

Analysing your customer’s business objectives

Analysing the business constrains; budgets, timeframes, workplace politics.

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NETWORK DESIGN

Good network design must recognizes customer’s requirements.

Network design choices and tradeoffs must be made when designing the logic network before any physical devices are selected.

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STRUCTURED NETWORK DESIGN

Four fundamental network design goals: Scalability Availability Security Manageability

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STRUCTURED NETWORK DESIGN

Core Layer: connects Distribution Layer devices Distribution Layer: interconnects smaller LANs Access Layer: provides connections for hosts

and end devices

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START FROM THE TOP

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Application

Presentation

Session

Transport

Network

Data Link

PhysicalLayer 1

Layer 7

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

SYSTEMS DEVELOPMENT LIFE CYCLES(SDLC)

Typical systems are developed and continue to exist over a period of time, often called a systems development life cycle (SDLC).

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Analyze requirements

Develop logical design

Develop physical design

Test, optimize, and document

design

Monitor and optimize network

performance

Implement and test network

TOP-DOWN NETWORK DESIGN STEPS

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systems development life cycle (SDLC).

THE PDIOO NETWORK LIFE CYCLE

Plan

Design

Implement

Operate

OptimizeRetire

Plan Design Implement Operate Optimize

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NETWORK DESIGN STEPS

Phase 1 – Analyze RequirementsAnalyze business goals and constraintsAnalyze technical goals and tradeoffsCharacterize the existing networkCharacterize network traffic

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Phase 2 – Logical Network DesignDesign a network topologyDesign models for addressing and namingSelect switching and routing protocolsDevelop network security strategiesDevelop network management strategies

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Phase 3 – Physical Network DesignSelect technologies and devices for

campus networksSelect technologies and devices for

enterprise networks

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Phase 4 – Testing, Optimizing, and Documenting the Network DesignTest the network designOptimize the network designDocument the network design

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BUSINESS GOALS

Increase revenue Reduce operating costs Improve communications Shorten product development cycle Expand into worldwide markets Build partnerships with other companies Offer better customer support or new

customer services

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RECENT BUSINESS PRIORITIES Mobility Security Resiliency (fault tolerance) Business continuity after a disaster Network projects must be prioritized

based on fiscal goals Networks must offer the low delay

required for real-time applications such as VoIP

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BUSINESS CONSTRAINTS

Budget Staffing Schedule Politics and policies

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COLLECT INFORMATION BEFORE THE FIRST MEETING

Before meeting with the client, whether internal or external, collect some basic business-related information

Such asProducts produced/Services suppliedFinancial viabilityCustomers, suppliers, competitorsCompetitive advantage

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MEET WITH THE CUSTOMER

Try to getA concise statement of the goals of

the project What problem are they trying to solve? How will new technology help them be

more successful in their business? What must happen for the project to

succeed?

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MEET WITH THE CUSTOMERGet a copy of the organization chart

This will show the general structure of the organization It will suggest users to account for It will suggest geographical locations to account for

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MEET WITH THE CUSTOMERGet a copy of the security policy

How does the policy affect the new design? How does the new design affect the policy? Is the policy so strict that you (the network

designer) won’t be able to do your job?Start cataloging network assets that security

should protect Hardware, software, applications, and data Less obvious, but still important, intellectual

property, trade secrets, and a company's reputation

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THE SCOPE OF THE DESIGN PROJECT

Small in scope?Allow sales people to access network via a VPN

Large in scope?An entire redesign of an enterprise network

Use the OSI model to clarify the scopeNew financial reporting application versus new

routing protocol versus new data link (wireless, for example)

Does the scope fit the budget, capabilities of staff and consultants, schedule?

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GATHER MORE DETAILED INFORMATION

ApplicationsNow and after the project is completed Include both productivity applications and

system management applications User communities Data stores Protocols Current logical and physical architecture Current performance

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SUMMARY

Systematic approach Focus first on business requirements and

constraints, and applications Gain an understanding of the customer’s

corporate structure Gain an understanding of the customer’s

business style

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REVIEW QUESTIONS

What are the main phases of network design per the top-down network design approach?

What are the main phases of network design per the PDIOO approach?

Why is it important to understand your customer’s business style?

What are some typical business goals for organizations today?

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CHAPTER TWO

ANALYZING TECHNICAL GOALS AND TRADEOFFS

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Copyright 2010 Cisco Press & Priscilla Oppenheimer

TECHNICAL GOALS

Scalability Availability Performance Security Manageability Usability Adaptability Affordability

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Your lab report should reflect some of these goals of your own designed network.

SCALABILITY Scalability refers to the ability to grow Some technologies are more scalable

Flat network designs, for example, don’t scale well

Try to learnNumber of sites to be addedWhat will be needed at each of these sitesHow many users will be addedHow many more servers will be added

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AVAILABILITY

Availability can be expressed as a percent uptime per year, month, week, day, or hour, compared to the total time in that periodFor example:

24/7 operation Network is up for 165 hours in the 168-hour week Availability is 98.21%

Different applications may require different levels

Some enterprises may want 99.999% or “Five Nines” availability

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AVAILABILITY

Availability can also be expressed as a mean time between failure (MTBF) and mean time to repair (MTTR)

Availability = MTBF/(MTBF + MTTR)For example:

The network should not fail more than once every 4,000 hours (166 days) and it should be fixed within one hour

4,000/4,001 = 99.98% availability

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AVAILABILITY

DOWNTIME IN MINUTES

4.32

1.44

.72

.01

30

10

5

.10

1577(26

H)

99.70%

52699.90%

26399.95%

599.999%

Per Hour Per Day Per Week Per Year

.18

.06

.03

.0006

.29 2 10599.98% .012

99.999% AVAILABILITY MAY REQUIRE TRIPLE REDUNDANCY

Can the customer afford this?37

Enterprise

ISP 1 ISP 2 ISP 3

SERVER FARMS

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Many enterprise networks provide users with Internet-accessible services, such as email and e-commerce. The availability and security of these services are crucial to the success of a business.Managing and securing numerous distributed servers at various locations within a business network is difficult. Recommended practice centralised servers in server farms. Server farms are typically located in computer rooms and data centres.

BENEFITS OF CREATING A SERVER FARM

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1. Network traffic enters and leaves the server farm at a defined point. This arrangement makes it easier to secure, filter and prioritise traffic.

2. Redundant, high-capacity links can be installed to the servers and between the server farm network and the main LAN. This configuration is more cost-effective than attempting to provide a similar level of connectivity to servers distributed throughout the network.

3. Load balancing and failover can be provided between servers and between networking devices.

4. The number of high-capacity switches and security devices is reduced, helping to lower the cost of providing services.

NETWORK PERFORMANCE Common performance factors include

BandwidthThroughputBandwidth utilizationOffered loadAccuracyEfficiencyDelay (latency) and delay variationResponse time

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BANDWIDTH VS. THROUGHPUT

Bandwidth and throughput are not the same

Bandwidth is the data carrying capacity of a circuit, fixed.

Usually specified in bits per second-bps

Throughput is the quantity of error free data transmitted per unit of time

Measured in bps, Bps, or packets per second (pps) Depend on offered load, access method and error

rate

Throughput < Bandwidth 43

BANDWIDTH, THROUGHPUT, LOAD

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Offered Load

Throughput

Actual

Idea

l

100 % of Capacity

100 % of Capacity

OTHER FACTORS THAT AFFECT THROUGHPUT

The size of packets Inter-frame gaps between packets Packets-per-second ratings of devices that forward

packets Client speed (CPU, memory, and HD access speeds) Server speed (CPU, memory, and HD access speeds) Network design MAC Protocols (ALOHA 18.4%) Distance Errors Time of day, etc., etc., etc.

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THROUGHPUT VS. GOODPUT Are you referring to bytes per second,

regardless of whether the bytes are user data bytes or packet header bytesOr are you concerned with application-layer

throughput of user bytes, sometimes called “goodput” In that case, you have to consider that bandwidth is

being “wasted” by the headers in every packet

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PERFORMANCE (CONTINUED)

EfficiencyHow much overhead is required to deliver an

amount of data?How large can packets be?

Larger better for efficiency (and goodput) But too large means too much data is lost if a

packet is damaged How many packets can be sent in one bunch

without an acknowledgment?

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EFFICIENCY

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Small Frames (Less Efficient)

Large Frames (More Efficient)

DELAY FROM THE USER’S POINT OF VIEW

Response TimeA function of the

application and the equipment the application is running on, not just the network

Most users expect to see something on the screen in 100 to 200 milliseconds

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DELAY FROM THE ENGINEER’S POINT OF VIEW

Propagation delayA signal travels in a cable at about 2/3 the

speed of light in a vacuum (3×108 m/s) Transmission delay (also known as

serialization delay)Time to put digital data onto a transmission

line For example, it takes about 5 ms to output a 1,024

byte packet on a 1.544 Mbps T1 line Packet-switching delay Queuing delay

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QUEUING DELAY AND BANDWIDTH UTILIZATION

Number of packets in a queue increases exponentially as utilization increases

Queue depth = utilization/(1- utilization)

0

3

6

9

12

15

0.5 0.6 0.7 0.8 0.9 1

Average Utilization

Ave

rag

e Q

ue

ue

De

pth

EXAMPLE A packet switch has 5 users, each offering

packets at a rate of 10 packets per second The average length of the packets is 1,024

bits The packet switch needs to transmit this

data over a 56-Kbps WAN circuitLoad = 5 x 10 x 1,024 = 51,200 bpsUtilization = 51,200/56,000 = 91.4%Average number of packets in queue =

(0.914)/(1-0.914) = 10.63 packets

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SECURITY

Focus on requirements first Detailed security planning later

(Chapter 8) Identify network assets

Including their value and the expected cost associated with losing them due to a security problem

Analyze security risks

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MANAGEABILITY

Fault management Configuration management Accounting management Performance management Security management

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USABILITY

Usability: the ease of use with which network users can access the network and services

Networks should make users’ jobs easier

Some design decisions will have a negative affect on usability:Strict security, for example

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ADAPTABILITY

Avoid incorporating any design elements that would make it hard to implement new technologies in the future

Change can come in the form of new protocols, new business practices, new fiscal goals, new legislation

A flexible design can adapt to changing traffic patterns and Quality of Service (QoS) requirements

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AFFORDABILITY

A network should carry the maximum amount of traffic possible for a given financial cost

Affordability is especially important in campus network designs

WANs are expected to cost more, but costs can be reduced with the proper use of technologyQuiet routing protocols, for example

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MAKING TRADEOFFS (EXAMPLE)

Scalability 20 Availability 30 Network performance 15 Security 5 Manageability 5 Usability 5 Adaptability 5 Affordability 15Total (must add up to 100) 100

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SUMMARY

Continue to use a systematic, top-down approach

Don’t select products until you understand goals for scalability, availability, performance, security, manageability, usability, adaptability, and affordability

Tradeoffs are almost always necessary

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REVIEW QUESTIONS

What are some typical technical goals for organizations today?

How do bandwidth and throughput differ? How can one improve network efficiency? What tradeoffs may be necessary in order

to improve network efficiency?

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CHAPTER THREE

CHARACTERIZING THE EXISTING INTERNETWORK

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WHAT’S THE STARTING POINT? According to Abraham Lincoln:

“If we could first know where we are and whither we are tending, we could better judge what to do and how to do it.”

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WHERE ARE WE?

Characterize the exiting internetwork in terms of: Its infrastructure

Logical structure (modularity, hierarchy, topology) Physical structure

Addressing and namingWiring and mediaArchitectural and environmental constraintsHealth

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DIAGRAM A PHYSICAL NETWORK AND DOCUMENT THE EXISTING NETWORK

Network documentation: Logical and physical diagrams Floor plans Complete lists for equipments and

applications Current network configuration files

GET A NETWORK MAP (PHYSICAL)

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Gigabit Ethernet

Eugene Ethernet20 users

Web/FTP server

Grants PassHQ

Gigabit Ethernet

FEP (Front End Processor)

IBMMainframe

T1

MedfordFast Ethernet

50 users

RoseburgFast Ethernet

30 usersFrame Relay

CIR = 56 KbpsDLCI = 5

Frame RelayCIR = 56 Kbps

DLCI = 4

Grants PassHQ

Fast Ethernet75 users

InternetT1

DIAGRAM A PHYSICAL NETWORK AND DOCUMENT THE EXISTING NETWORK

Identify and document the strengths and weaknesses of the existing network

Focus on finding ways to overcome weaknesses

CHARACTERIZE ADDRESSING AND NAMING

IP addressing for major devices, client networks, server networks, and so on

Any addressing oddities, such as discontiguous subnets?

Any strategies for addressing and naming?For example, sites may be named using airport

codes San Francisco = SFO, Oakland = OAK In LSBU, T-tower block; K-keyworth building; B-

Borough road building; L- london road building

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DISCONTINUOUS SUBNETS – make problems for some routing protocols

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Area 1Subnets 10.108.16.0 -

10.108.31.0

Area 0Network

192.168.49.0

Area 2Subnets 10.108.32.0 -

10.108.47.0

Router A Router B

CHARACTERIZE THE WIRING AND MEDIA

Single-mode fiber Multi-mode fiber Shielded twisted pair (STP) copper Unshielded-twisted-pair (UTP) copper Coaxial cable Microwave Laser Radio Infra-red

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ARCHITECTURAL CONSTRAINTS

Make sure the following are sufficientAir conditioningHeatingVentilationPowerProtection from electromagnetic interferenceDoors that can lock

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ARCHITECTURAL CONSTRAINTS

Make sure there’s space for:Cabling conduits Patch panelsEquipment racksWork areas for technicians installing and

troubleshooting equipment

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CHECK THE HEALTH OF THE EXISTING INTERNETWORK

Performance Availability Bandwidth utilization Accuracy Efficiency Response time Status of major routers, switches, and

firewalls74

CHARACTERIZE AVAILABILITY

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Enterprise

Segment 1

Segment 2

Segment n

MTBF MTTRDate and Duration of Last Major Downtime

Cause of Last Major Downtime

Fix for Last Major Downtime

Mean time between failures (MTBF)Mean time to recovery (MTTR)

Network Utilization

0 1 2 3 4 5 6 7

17:10:00

17:07:00

17:04:00

17:01:00

16:58:00

16:55:00

16:52:00

16:49:00

16:46:00

16:43:00

16:40:00

Tim

e

Utilization

Series1

NETWORK UTILIZATION IN MINUTE INTERVALS

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Network Utilization

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

17:00:00

16:00:00

15:00:00

14:00:00

13:00:00

Tim

e

Utilization

Series1

NETWORK UTILIZATION IN HOUR INTERVALS

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BANDWIDTH UTILIZATION BY PROTOCOL

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Protocol 1

Protocol 2

Protocol 3

Protocol n

Relative Network Utilization

Absolute Network Utilization

Broadcast Rate

Multicast Rate

CHARACTERIZE PACKET SIZES

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CHARACTERIZE RESPONSE TIME

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Node A

Node B

Node C

Node D

Node A Node B Node C Node D

X

X

X

X

CHECK THE STATUS OF MAJOR ROUTERS, SWITCHES, AND FIREWALLS

show buffers show environment show interfaces show memory show processes show running-config show version

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Use Cisco IOS show command

TOOLS

Protocol analyzers Multi Router Traffic Grapher (MRTG) Remote monitoring (RMON) probes Cisco Discovery Protocol (CDP) Cisco IOS NetFlow technology CiscoWorks

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SUMMARY

Characterize the exiting internetwork before designing enhancements

Helps you verify that a customer’s design goals are realistic

Helps you locate where new equipment will go

Helps you cover yourself if the new network has problems due to unresolved problems in the old network

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REVIEW QUESTIONS

What factors will help you decide if the existing internetwork is in good enough shape to support new enhancements?

When considering protocol behavior, what is the difference between relative network utilization and absolute network utilization?

Why should you characterize the logical structure of an internetwork and not just the physical structure?

What architectural and environmental factors should you consider for a new wireless installation?

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CHAPTER FOUR

CHARACTERIZING NETWORK TRAFFIC

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NETWORK TRAFFIC FACTORS

Traffic flow Location of traffic sources and data

stores Traffic load Traffic behavior Quality of Service (QoS) requirements

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USER COMMUNITIES, a set of worker who use a particular application or set of applications.

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User Community Name

Size of Community (Number of Users)

Location(s) of Community

Application(s) Used by Community

DATA STORES (SINKS), an area in a network where application layer data resides. Server, or any device where large quantities of data are stored.

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Data Store Location Application(s) Used by User Community(or Communities)

TRAFFIC FLOW, involves identifying and characterizing individual traffic flows between traffic source and stores.

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Destination 1 Destination 2 Destination 3Destination MB/sec MB/secMB/sec MB/sec

Source 1

Source 2

Source 3

Source n

TRAFFIC FLOW EXAMPLE

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Administration

Business and Social Sciences

Math and Sciences

50 PCs 25 Macs50 PCs

50 PCs30 PCs

30 Library Patrons (PCs) 30 Macs and 60 PCs in Computing Center

Library and Computing Center

App 1 108 KbpsApp 2 60 KbpsApp 3 192 KbpsApp 4 48 KbpsApp 7 400 KbpsTotal 808 Kbps

App 1 48 KbpsApp 2 32 KbpsApp 3 96 KbpsApp 4 24 KbpsApp 5 300 KbpsApp 6 200 KbpsApp 8 1200 KbpsTotal 1900 Kbps

App 1 30 KbpsApp 2 20 KbpsApp 3 60 KbpsApp 4 16 KbpsTotal 126 Kbps

App 2 20 KbpsApp 3 96 KbpsApp 4 24 KbpsApp 9 80 KbpsTotal 220 Kbps

Arts and Humanities

Server Farm

10-Mbps Metro Ethernet to Internet

TYPES OF TRAFFIC FLOW

Terminal/host Client/server Thin client Peer-to-peer Server/server Distributed computing

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TRAFFIC FLOW FOR VOICE OVER IP

The flow associated with transmitting the audio voice is separate from the flows associated with call setup and teardown. The flow for transmitting the digital

voice is essentially peer-to-peer.Call setup and teardown is a

client/server flow A phone needs to talk to a server or

phone switch that understands phone numbers, IP addresses, capabilities negotiation, and so on.

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IDENTIFYING APPLICATION IMPACTS ON NETWORK DESIGN

File transfer and email applications: Unpredictable bandwidth usage Large packet size Centralization of file and mail servers in a

secure location Redundancy to ensure reliable service

IDENTIFYING APPLICATION IMPACTS ON NETWORK DESIGN

HTTP and web traffic: Network media Redundancy Security

NETWORK APPLICATIONSTRAFFIC CHARACTERISTICS

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Name of Application

Type of Traffic Flow

Protocol(s) Used by Application

User Communities That Use the Application

Data Stores (Servers, Hosts, and so on)

Approximate Bandwidth Requirements

QoS Requirements

TRAFFIC LOAD To calculate whether capacity is

sufficient, you should know:The number of stationsThe average time that a station is idle

between sending framesThe time required to transmit a message

once medium access is gained That level of detailed information can be

hard to gather, however

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SIZE OF OBJECTS ON NETWORKS

Terminal screen: 4 Kbytes Simple e-mail: 10 Kbytes Simple web page: 50 Kbytes High-quality image: 50Mbytes Database backup: 1Gbytes or more

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TRAFFIC BEHAVIOR

Broadcasts All ones data-link layer destination address

FF: FF: FF: FF: FF: FF Doesn’t necessarily use huge amounts of

bandwidth But does disturb every CPU in the broadcast

domain Multicasts

First bit sent is a one 01:00:0C:CC:CC:CC (Cisco Discovery Protocol)

Should just disturb NICs that have registered to receive it

Requires multicast routing protocol on internetworks 98

NETWORK EFFICIENCY

Frame size Protocol interaction Windowing and flow control Error-recovery mechanisms

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QOS REQUIREMENTS

ATM service specificationsConstant bit rate (CBR)Realtime variable bit rate (rt-VBR)Non-realtime variable bit rate (nrt-VBR)Unspecified bit rate (UBR)Available bit rate (ABR)Guaranteed frame rate (GFR)

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QOS REQUIREMENTS PER IETF (Internet Engineering Task Force, develops and promotes Internet standards, It is an open standards organization, with no formal membership or membership requirements.)

IETF integrated services working group specificationsControlled load service

Provides client data flow with a QoS closely approximating the QoS that same flow would receive on an unloaded network

Guaranteed service Provides firm (mathematically provable) bounds

on end-to-end packet-queuing delays101

QOS REQUIREMENTS PER IETF

IETF differentiated services working group specificationsRFC 2475 IP packets can be marked with a

differentiated services codepoint (DSCP) to influence queuing and packet-dropping decisions for IP datagrams on an output interface of a router

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HOW QUALITY OF SERVICE IS IMPLEMENTED ON THE LAN/WAN

Where QoS can be implemented to affect traffic flow:

Layer 2 devices Layer 3 devices

DOCUMENT THE NETWORK REQUIREMENTS OF SPECIFIC CATEGORIES OF APPLICATIONS

Estimate the volume of application traffic during the initial design phase.

Document projected applications and associated hardware in a network diagram.

SUMMARY

Continue to use a systematic, top-down approach

Don’t select products until you understand network traffic in terms of:FlowLoadBehaviorQoS requirements

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REVIEW QUESTIONS

List and describe six different types of traffic flows.

What makes traffic flow in voice over IP networks challenging to characterize and plan for?

Why should you be concerned about broadcast traffic?

How do ATM and IETF specifications for QoS differ?

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OF PART 1 107

N ETWORK D ESIGN M ETHODOLOGY T OP -D OWN N ETWORK D ESIGN 1.

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