Top-Down Network Design
Chapter One
Analyzing Business Goals and Constraints
Copyright 2010 Cisco Press & Priscilla Oppenheimer
Top-Down Network Design
• Network design should be a complete process that matches business needs to
available technology to deliver a system that will maximize an organization’s success.
– In the LAN area it is more than just buying a few devices.
– In the WAN area it is more than just calling the phone company.
Start at the Top
• Don’t just start connecting the dots.
• Analyze business and technical goals first.
• Explore divisional and group structures to find out who the network serves and where they reside.
• Determine what applications will run on the network and how those applications behave on a network.
• Focus on Layer 7 and above first.
Layers of the OSI Model
Application Presentation
Session Transport
Network Data Link
Physical
Layer 1 Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Structured Design
• A focus is placed on understanding data flow, data types, and processes that access or change the data.
• A focus is placed on understanding the location and needs of user communities that access or change data and processes.
• Several techniques and models can be used to characterize the existing system, new user requirements, and a structure for the future system.
• A logical model is developed before the physical model.
– The logical model represents the basic building blocks, divided by function, and the structure of the system.
– The physical model represents devices and specific technologies and implementations.
Systems Development Life Cycles
• SDLC: Does it mean Synchronous Data
Link Control or Systems Development Life Cycle?
• The latter for the purposes of this class!
• Typical systems are developed and continue to exist over a period of time, often called a systems development life cycle (SDLC).
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
Network Design Steps
• Phase 1 – Analyze Requirements
– Analyze business goals and constraints – Analyze technical goals and tradeoffs – Characterize the existing network
– Characterize network traffic
Network Design Steps
• Phase 2 – Logical Network Design
– Design a network topology
– Design models for addressing and naming – Select switching and routing protocols
– Develop network security strategies
– Develop network management strategies
Network Design Steps
• Phase 3 – Physical Network Design
– Select technologies and devices for campus networks
– Select technologies and devices for enterprise networks
Network Design Steps
• Phase 4 – Testing, Optimizing, and Documenting the Network Design
– Test the network design
– Optimize the network design – Document the network design
The PDIOO Network Life Cycle
Plan
Design
Implement Operate
Optimize
Retire
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
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
Business Constraints
• Budget
• Staffing
• Schedule
• Politics and policies
Collect Information Before the First Meeting
• Before meeting with the client, whether internal or external, collect some basic business-related information
• Such as
– Products produced/Services supplied – Financial viability
– Customers, suppliers, competitors – Competitive advantage
Meet With the Customer
• Try to get
– A 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?
Meet With the Customer
• What will happen if the project is a failure?
– Is this a critical business function?
– Is this project visible to upper management?
– Who’s on your side?
Meet With the Customer
• Discover any biases
– For example
• Will they only use certain company’s products?
• Do they avoid certain technologies?
• Do the data people look down on the voice people or vice versa?
– Talk to the technical and management staff
Meet With the Customer
– Get 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
Meet With the Customer
– Get 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
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 scope
– New 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?
Gather More Detailed Information
• Applications
– Now 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
Network Applications
Name of Application
Type of Application
New
Application?
Criticality Comments
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
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?
Top-Down Network Design
Chapter Two
Analyzing Technical Goals and Tradeoffs
Copyright 2010 Cisco Press & Priscilla Oppenheimer
Technical Goals
• Scalability
• Availability
• Performance
• Security
• Manageability
• Usability
• Adaptability
• Affordability
Scalability
• Scalability refers to the ability to grow
• Some technologies are more scalable
– Flat network designs, for example, don’t scale well
• Try to learn
– Number of sites to be added
– What will be needed at each of these sites – How many users will be added
– How many more servers will be added
Availability
• Availability can be expressed as a percent uptime per year, month, week, day, or hour, compared to the total time in that period
– For 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
Availability
Downtime in Minutes
4.32 1.44 .72 .01
30 10 5 .10
1577 99.70%
526 99.90%
263 99.95%
5 99.999%
Per Hour Per Day Per Week Per Year
.18 .06 .03 .0006
.29 2 105
99.98% .012
99.999% Availability May Require Triple Redundancy
Enterprise
ISP 1 ISP 2 ISP 3
• Can the customer afford this?
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
Network Performance
• Common performance factors include
– Bandwidth – Throughput
– Bandwidth utilization – Offered load
– Accuracy – Efficiency
– Delay (latency) and delay variation – Response time
Bandwidth Vs. Throughput
• Bandwidth and throughput are not the same thing
• Bandwidth is the data carrying capacity of a circuit
• Usually specified in bits per second
• Throughput is the quantity of error free data transmitted per unit of time
• Measured in bps, Bps, or packets per second (pps)
Bandwidth, Throughput, Load
Offered Load T
h r o u g h p u t
Actual
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
• Protocols
• Distance
• Errors
• Time of day, etc., etc., etc.
Throughput Vs. Goodput
• You need to decide what you mean by throughput
• Are you referring to bytes per second,
regardless of whether the bytes are user data bytes or packet header bytes
– Or 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
Performance (continued)
• Efficiency
– How 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?
Efficiency
Small Frames (Less Efficient)
Large Frames (More Efficient)
Delay from the User’s Point of View
• Response Time
– A 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
Delay from the Engineer’s Point of View
• Propagation delay
– A signal travels in a cable at about 2/3 the speed of light in a vacuum
• 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
Queuing Delay and Bandwidth Utilization
• Number of packets in a queue increases exponentially as utilization increases
0 3 6 9 12 15
0.5 0.6 0.7 0.8 0.9 1
Average Utilization
Average Queue Depth
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 circuit
– Load = 5 x 10 x 1,024 = 51,200 bps – Utilization = 51,200/56,000 = 91.4%
– Average number of packets in queue =
(0.914)/(1-0.914) = 10.63 packets
Delay Variation
• The amount of time average delay varies
– Also known as jitter
• Voice, video, and audio are intolerant of delay variation
• So forget everything we said about maximizing packet sizes
– There are always tradeoffs
– Efficiency for high-volume applications versus low and non-varying delay for multimedia
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
Network Assets
• Hardware
• Software
• Applications
• Data
• Intellectual property
• Trade secrets
• Company’s reputation
Security Risks
• Hacked network devices
– Data can be intercepted, analyzed, altered, or deleted
– User passwords can be compromised – Device configurations can be changed
• Reconnaissance attacks
• Denial-of-service attacks
Manageability
• Fault management
• Configuration management
• Accounting management
• Performance management
• Security management
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
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
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
technology
– Quiet routing protocols, for example
Network Applications
Technical Requirements
Name of Application
Cost of Downtime
Acceptable MTBF
Acceptable MTTR
Throughput Goal
Delay Must be Less Than:
Delay Variation Must be Less Than:
Making Tradeoffs
• Scalability 20
• Availability 30
• Network performance 15
• Security 5
• Manageability 5
• Usability 5
• Adaptability 5
• Affordability 15
Total (must add up to 100) 100
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
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?
Top-Down Network Design
Chapter Three
Characterizing the Existing Internetwork
Copyright 2010 Cisco Press & Priscilla Oppenheimer
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.”
Where Are We?
• Characterize the existing internetwork in terms of:
– Its infrastructure
• Logical structure (modularity, hierarchy, topology)
• Physical structure
– Addressing and naming – Wiring and media
– Architectural and environmental constraints – Health
Get a Network Map
Gigabit Ethernet
Eugene Ethernet 20 users
Web/FTP server
Grants Pass HQ Gigabit Ethernet
FEP (Front End Processor)
IBM Mainframe T1
Medford Fast Ethernet
50 users
Roseburg Fast Ethernet
30 users Frame Relay
CIR = 56 Kbps DLCI = 5
Frame Relay CIR = 56 Kbps
DLCI = 4
Grants Pass HQ Fast Ethernet
75 users
Internet T1
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
Discontiguous Subnets
Area 1
Subnets 10.108.16.0 - 10.108.31.0
Area 0 Network 192.168.49.0
Area 2
Subnets 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
Telecommunications Wiring Closet
Horizontal Wiring
Work-Area Wiring Wallplate
Main Cross-Connect Room (or Main Distribution Frame)
Intermediate Cross-Connect Room (or Intermediate Distribution Frame)
Building A - Headquarters Building B
Vertical Wiring (Building Backbone)
Campus Backbone
Campus Network Wiring
Architectural Constraints
• Make sure the following are sufficient
– Air conditioning – Heating
– Ventilation – Power
– Protection from electromagnetic interference – Doors that can lock
Architectural Constraints
• Make sure there’s space for:
– Cabling conduits – Patch panels
– Equipment racks
– Work areas for technicians installing and troubleshooting equipment
Issues for Wireless Installations
• Reflection
• Absorption
• Refraction
• Diffraction
Check the Health of the Existing Internetwork
• Performance
• Availability
• Bandwidth utilization
• Accuracy
• Efficiency
• Response time
• Status of major routers, switches, and firewalls
Characterize Availability
Enterprise
Segment 1
Segment 2
Segment n
MTBF MTTR
Date and Duration of Last Major Downtime
Cause of Last Major
Downtime
Fix for Last Major
Downtime
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
Time
Utilization
Series1
Network Utilization in Minute
Intervals
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
Time
Utilization
Series1
Network Utilization in Hour
Intervals
Bandwidth Utilization by Protocol
Protocol 1
Protocol 2
Protocol 3
Protocol n
Relative Network Utilization
Absolute Network Utilization
Broadcast Rate
Multicast Rate
Characterize Packet Sizes
Characterize Response Time
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
Tools
• Protocol analyzers
• Multi Router Traffic Grapher (MRTG)
• Remote monitoring (RMON) probes
• Cisco Discovery Protocol (CDP)
• Cisco IOS NetFlow technology
• CiscoWorks
Summary
• Characterize the existing 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
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?
Top-Down Network Design
Chapter Four
Characterizing Network Traffic
Copyright 2010 Cisco Press & Priscilla Oppenheimer
Network Traffic Factors
• Traffic flow
• Location of traffic sources and data stores
• Traffic load
• Traffic behavior
• Quality of Service (QoS) requirements
User Communities
User
Community Name
Size of
Community (Number of Users)
Location(s) of Community
Application(s) Used by
Community
Data Stores
Data Store Location Application(s) Used by User Community(or Communities)
Traffic Flow
Destination 1 Destination 2 Destination 3 Destination
MB/sec MB/sec MB/sec MB/sec
Source 1
Source 2
Source 3
Source n
Traffic Flow Example
Administration
Business and Social Sciences
Math and Sciences
50 PCs 25 Macs
50 PCs
50 PCs 30 PCs
30 Library Patrons (PCs) 30 Macs and 60 PCs in Computing Center
Library and Computing Center
App 1 108 Kbps App 2 60 Kbps App 3 192 Kbps App 4 48 Kbps App 7 400 Kbps Total 808 Kbps
App 1 48 Kbps App 2 32 Kbps App 3 96 Kbps App 4 24 Kbps App 5 300 Kbps App 6 200 Kbps App 8 1200 Kbps Total 1900 Kbps App 1 30 Kbps
App 2 20 Kbps App 3 60 Kbps App 4 16 Kbps Total 126 Kbps App 2 20 Kbps
App 3 96 Kbps App 4 24 Kbps App 9 80 Kbps Total 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
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.
Network Applications
Traffic Characteristics
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 stations
– The average time that a station is idle between sending frames
– The time required to transmit a message once medium access is gained
• That level of detailed information can be hard to gather, however
Size of Objects on Networks
• Terminal screen: 4 Kbytes
• Simple e-mail: 10 Kbytes
• Simple web page: 50 Kbytes
• High-quality image: 50,000 Kbytes
• Database backup: 1,000,000 Kbytes or more
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
Network Efficiency
• Frame size
• Protocol interaction
• Windowing and flow control
• Error-recovery mechanisms
QoS Requirements
• ATM service specifications
– Constant 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)
QoS Requirements per IETF
• IETF integrated services working group specifications
– Controlled 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 delays
QoS Requirements per IETF
• IETF differentiated services working group specifications
– RFC 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
Summary
• Continue to use a systematic, top-down approach
• Don’t select products until you understand network traffic in terms of:
– Flow – Load
– Behavior
– QoS requirements
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?
Top-Down Network Design
Chapter Six
Designing Models for Addressing and Naming
Copyright 2010 Cisco Press & Priscilla Oppenheimer
Guidelines for Addressing and Naming
• Use a structured model for addressing and naming
• Assign addresses and names hierarchically
• Decide in advance if you will use
– Central or distributed authority for addressing and naming
– Public or private addressing
– Static or dynamic addressing and naming
Advantages of Structured Models for Addressing & Naming
• It makes it easier to
– Read network maps
– Operate network management software
– Recognize devices in protocol analyzer traces – Meet goals for usability
– Design filters on firewalls and routers – Implement route summarization
Public IP Addresses
• Managed by the Internet Assigned Numbers Authority (IANA)
• Users are assigned IP addresses by Internet service providers (ISPs).
• ISPs obtain allocations of IP addresses from their appropriate Regional Internet Registry (RIR)
Regional Internet Registries (RIR)
• American Registry for Internet Numbers (ARIN) serves North America and parts of the Caribbean.
• RIPE Network Coordination Centre (RIPE NCC) serves Europe, the Middle East, and Central Asia.
• Asia-Pacific Network Information Centre (APNIC) serves Asia and the Pacific region.
• Latin American and Caribbean Internet Addresses Registry (LACNIC) serves Latin America and parts of the Caribbean.
• African Network Information Centre (AfriNIC) serves Africa.
Private Addressing
• 10.0.0.0 – 10.255.255.255
• 172.16.0.0 – 172.31.255.255
• 192.168.0.0 – 192.168.255.255
Criteria for Using Static Vs.
Dynamic Addressing
• The number of end systems
• The likelihood of needing to renumber
• The need for high availability
• Security requirements
• The importance of tracking addresses
• Whether end systems need additional information
– (DHCP can provide more than just an address)
The Two Parts of an IP Address
Prefix Host
32 Bits
Prefix Length
Prefix Length
• An IP address is accompanied by an indication of the prefix length
– Subnet mask – /Length
• Examples
– 192.168.10.1 255.255.255.0 – 192.168.10.1/24
Subnet Mask
• 32 bits long
• Specifies which part of an IP address is the
network/subnet field and which part is the host field
– The network/subnet portion of the mask is all 1s in binary.
– The host portion of the mask is all 0s in binary.
– Convert the binary expression back to dotted-decimal notation for entering into configurations.
• Alternative
– Use slash notation (for example /24) – Specifies the number of 1s
Subnet Mask Example
• 11111111 11111111 11111111 00000000
• What is this in slash notation?
• What is this in dotted-decimal notation?
Another Subnet Mask Example
• 11111111 11111111 11110000 00000000
• What is this in slash notation?
• What is this in dotted-decimal notation?
One More Subnet Mask Example
• 11111111 11111111 11111000 00000000
• What is this in slash notation?
• What is this in dotted-decimal notation?
Designing Networks with Subnets
• Determining subnet size
• Computing subnet mask
• Computing IP addresses
Addresses to Avoid When Subnetting
• A node address of all ones (broadcast)
• A node address of all zeros (network)
• A subnet address of all ones (all subnets)
• A subnet address of all zeros (confusing)
– Cisco IOS configuration permits a subnet address of all zeros with the ip subnet-zero command
Practice
• Network is 172.16.0.0
• You want to divide the network into subnets.
• You will allow 600 nodes per subnet.
• What subnet mask should you use?
• What is the address of the first node on the first subnet?
• What address would this node use to send to all devices on its subnet?
More Practice
• Network is 172.16.0.0
• You have eight LANs, each of which will be its own subnet.
• What subnet mask should you use?
• What is the address of the first node on the first subnet?
• What address would this node use to send to all devices on its subnet?
One More
• Network is 192.168.55.0
• You want to divide the network into subnets.
• You will have approximately 25 nodes per subnet.
• What subnet mask should you use?
• What is the address of the last node on the last subnet?
• What address would this node use to send to all devices on its subnet?
IP Address Classes
• Classes are now considered obsolete
• But you have to learn them because
– Everyone in the industry still talks about them!
– You may run into a device whose configuration is affected by the classful system
Classful IP Addressing
Class First First Byte Prefix Intent
Few Bits Length
A 0 1-126* 8 Very large networks
B 10 128-191 16 Large networks
C 110 192-223 24 Small networks
D 1110 224-239 NA IP multicast
E 1111 240-255 NA Experimental
*Addresses starting with 127 are reserved for IP traffic local to a host.
Class Prefix Number of Addresses Length per Network
A 8 224-2 = 16,777,214
B 16 216-2 = 65,534
C 24 28-2 = 254
Division of the Classful Address
Space
Classful IP is Wasteful
• Class A uses 50% of address space
• Class B uses 25% of address space
• Class C uses 12.5% of address space
• Class D and E use 12.5% of address space
Classless Addressing
• Prefix/host boundary can be anywhere
• Less wasteful
• Supports route summarization
– Also known as
• Aggregation
• Supernetting
• Classless routing
• Classless inter-domain routing (CIDR)
• Prefix routing
Supernetting
• Move prefix boundary to the left
• Branch office advertises 172.16.0.0/14
172.16.0.0
172.17.0.0
172.18.0.0
172.19.0.0
Branch-Office Networks Enterprise Core
Network Branch-Office Router
172.16.0.0/14 Summarization
Second Octet in Decimal Second Octet in Binary
16 00010000
17 00010001
18 00010010
19 00010011
Discontiguous Subnets
Area 1
Subnets 10.108.16.0 - 10.108.31.0
Area 0 Network 192.168.49.0
Area 2
Subnets 10.108.32.0 - 10.108.47.0
Router A Router B
A Mobile Host
Subnets 10.108.16.0 - 10.108.31.0
Router A Router B
Host 10.108.16.1
IPv6 Aggregatable Global Unicast Address Format
• FP Format Prefix (001)
• TLA ID Top-Level Aggregation Identifier
• RES Reserved for future use
• NLA ID Next-Level Aggregation Identifier
• SLA ID Site-Level Aggregation Identifier
• Interface ID Interface Identifier
3 13 8 24 16 64 bits
FP TLA
ID
RES NLA
ID
SLA ID
Interface ID
Public topology Site
Topology
Upgrading to IPv6
• Dual stack
• Tunneling
• Translation
Guidelines for Assigning Names
• Names should be
– Short
– Meaningful – Unambiguous – Distinct
– Case insensitive
• Avoid names with unusual characters
– Hyphens, underscores, asterisks, and so on
• Maps names to IP addresses
• Supports hierarchical naming
– example: frodo.rivendell.middle-earth.com
• A DNS server has a database of resource
records (RRs) that maps names to addresses in the server’s “zone of authority”
• Client queries server
– Uses UDP port 53 for name queries and replies – Uses TCP port 53 for zone transfers
Domain Name System (DNS)
DNS Details
• Client/server model
• Client is configured with the IP address of a DNS server
– Manually or DHCP can provide the address
• DNS resolver software on the client
machine sends a query to the DNS
server. Client may ask for recursive
lookup.
DNS Recursion
• A DNS server may offer recursion, which allows the server to ask other servers
– Each server is configured with the IP address of one or more root DNS servers.
• When a DNS server receives a response from another server, it replies to the resolver client software. The server also caches the information for future
requests.
– The network administrator of the authoritative DNS server for a name defines the length of time that a non-
authoritative server may cache information.
Summary
• Use a systematic, structured, top-down approach to addressing and naming
• Assign addresses in a hierarchical fashion
• Distribute authority for addressing and naming where appropriate
• IPv6 looms in our future
Review Questions
• Why is it important to use a structured model for addressing and naming?
• When is it appropriate to use IP private addressing versus public addressing?
• When is it appropriate to use static versus dynamic addressing?
• What are some approaches to upgrading to IPv6?
Top-Down Network Design
Chapter Seven
Selecting Switching and Routing Protocols
Copyright 2010 Cisco Press & Priscilla Oppenheimer
Switching and Routing Choices
• Switching
– Layer 2 transparent bridging (switching) – Multilayer switching
– Spanning Tree Protocol enhancements – VLAN technologies
• Routing
– Static or dynamic
– Distance-vector and link-state protocols – Interior and exterior
– Etc.
Selection Criteria for Switching and Routing Protocols
• Network traffic characteristics
• Bandwidth, memory, and CPU usage
• The number of peers supported
• The capability to adapt to changes quickly
• Support for authentication
Making Decisions
• Goals must be established
• Many options should be explored
• The consequences of the decision should be investigated
• Contingency plans should be made
• A decision table can be used
Example Decision Table
Transparent Bridging (Switching) Tasks
• Forward frames transparently
• Learn which port to use for each MAC address
• Flood frames when the destination
unicast address hasn’t been learned yet
• Filter frames from going out ports that don’t include the destination address
• Flood broadcasts and multicasts
Switching Table on a Bridge or Switch
MAC Address Port
1 2 3 08-00-07-06-41-B9
00-00-0C-60-7C-01 00-80-24-07-8C-02
Cisco Spanning Tree Protocol Enhancements
• PortFast
• UplinkFast and Backbone Fast
• Unidirectional link detection
• Loop Guard
Redundant Uplinks
Access Layer
Distribution Layer
Core Layer
Switch A
Switch B Switch C
Primary Uplink
Secondary Uplink
X X
X = blocked by STP
• If a link fails, how long will STP take to recover?
• Use UplinkFast to speed convergence
Protocols for Transporting VLAN Information
• Inter-Switch Link (ISL)
– Tagging protocol – Cisco proprietary
• IEEE 802.1Q
– Tagging protocol – IEEE standard
• VLAN Trunk Protocol (VTP)
– VLAN management protocol
Selecting Routing Protocols
• They all have the same general goal:
– To share network reachability information among routers
• They differ in many ways:
– Interior versus exterior – Metrics supported
– Dynamic versus static and default – Distance-vector versus link-sate – Classful versus classless
– Scalability
Interior Versus Exterior Routing Protocols
• Interior routing protocols are used within an autonomous system
• Exterior routing protocols are used between autonomous systems
Autonomous system (two definitions that are often used):
“A set of routers that presents a common routing policy to the internetwork”
“A network or set of networks that are under the administrative control of a single entity”
Routing Protocol Metrics
• Metric: the determining factor used by a routing algorithm to decide which route to a network is better than another
• Examples of metrics:
– Bandwidth - capacity – Delay - time
– Load - amount of network traffic – Reliability - error rate
– Hop count - number of routers that a packet must
travel through before reaching the destination network – Cost - arbitrary value defined by the protocol or
administrator
Routing Algorithms
• Static routing
– Calculated beforehand, offline
• Default routing
– “If I don’t recognize the destination, just send the packet to Router X”
• Cisco’s On-Demand Routing
– Routing for stub networks
– Uses Cisco Discovery Protocol (CDP)
• Dynamic routing protocol
– Distance-vector algorithms – Link-state algorithms
Static Routing Example
RouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.20.2 Send packets for subnet 50 to 172.16.20.2 (Router B)
e0 e0 e0
s0 s1
s0 s0
Router A Router B Router C
Host A Host B Host C
172.16.10.2 172.16.30.2 172.16.50.2
172.16.20.1 172.16.40.1
172.16.10.1 172.16.30.1 172.16.50.1
172.16.20.2 172.16.40.2
Default Routing Example
RouterA(config)#ip route 0.0.0.0 0.0.0.0 172.16.20.2 If it’s not local, send it to 172.16.20.2 (Router B)
e0 e0 e0
s0 s1
s0 s0
Router A Router B Router C
Host A Host B Host C
172.16.10.2 172.16.30.2 172.16.50.2
172.16.20.1 172.16.40.1
172.16.10.1 172.16.30.1 172.16.50.1
172.16.20.2 172.16.40.2
Distance-Vector Routing
• Router maintains a routing table that lists known networks, direction (vector) to each network, and the distance to each network
• Router periodically (every 30 seconds, for example) transmits the routing table via a
broadcast packet that reaches all other routers on the local segments
• Router updates the routing table, if necessary, based on received broadcasts
Distance-Vector Routing Tables
Router A Router B
172.16.0.0 192.168.2.0
Network Distance Send To 172.16.0.0 0 Port 1 192.168.2.0 1 Router B
Network Distance Send To 192.168.2.0 0 Port 1 172.16.0.0 1 Router A Router A’s Routing Table Router B’s Routing Table
Link-State Routing
• Routers send updates only when there’s a change
• Router that detects change creates a link-state advertisement (LSA) and sends it to neighbors
• Neighbors propagate the change to their neighbors
• Routers update their topological database if necessary
Distance-Vector Vs. Link-State
• Distance-vector algorithms keep a list of
networks, with next hop and distance (metric) information
• Link-state algorithms keep a database of routers and links between them
– Link-state algorithms think of the internetwork as a graph instead of a list
– When changes occur, link-state algorithms apply Dijkstra’s shortest-path algorithm to find the
shortest path between any two nodes
Choosing Between Distance- Vector and Link-State
Choose Distance-Vector
• Simple, flat topology
• Hub-and-spoke topology
• Junior network administrators
• Convergence time not a big concern
Choose Link-State
• Hierarchical topology
• More senior network administrators
• Fast convergence is critical
