CCNA 4 Chapter 3

1
CCNA 4 Chapter 3

• WAN Design

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First Things First

• You must understand the customer when designing a
  network
• If time permits, a performance analysis of the
  network should be done on an existing network

3
WAN Design First Steps

• When designing a WAN - know who has the authority
  to make decisions
• A good internetwork design will provide the
  greatest resources at the least cost
• The final internetwork design should reflect the
  corporate structure and include business critical
  traffic
• WAN traffic is expected to
• increase 300 in the next
• five years

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WAN Design First Steps

• Sensitivity Testing
• Sensitivity testing is the breaking of stable
  links and observing the results

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Goals of WAN Design

• Two primary goals drive WAN design
• Application availability - Networks carry
  application information between computers. If the
  applications are not available to network users,
  the network is not doing its job.
• Total cost of ownership - As large businesses
  increasingly rely on electronic data for managing
  business activities, the associated costs of
  computing resources will continue to rise.
• A well-designed WAN can help to balance these
  objectives.

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New Requirements

• New network requirements that have put a burden
  on traditional shared-media include
• Network usage has increased
• Network costs continue to increase
• Growth of corporate Intranets Extranets

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3 Factors in WAN Design

• Environmental variables - including the location
  of hosts, servers, terminals, and other end
  nodes the projected traffic for the environment
  and the projected costs for delivering different
  service levels.
• Performance constraints such as network
  reliability, traffic throughput, and host/client
  computer speeds (for example, network interface
  cards and hard drive access speeds).
• Networking variables including network
  topology, line capacities, and packet traffic.

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WAN Technologies
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Analyzing Networks

• Important to understand worst case traffic load
• The most expensive and time consuming method of
  obtaining user utilization information is
  administering a human factors test
• Important to understand applications types of
  traffic problems they cause, business
  requirements, technical requirements and
  performance requirements
• The users level of training has no effect on
  availability - the availability measures the
  usefulness of the network, and can be increased
  by adding more resources

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Networking Models

• Mesh - network topology is flat all routers
  perform essentially the same functions, and there
  is usually no clear definition of where specific
  functions are performed
• Hierarchical - network is organized in layers
  that each have one or more specific functions

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Advantages of Using theOSI Model in WAN Design

• Allows the network to be designed in layers
• Uses layers to simplify the task required for
  internetworking
• Design elements can
• be replicated as the
• network grows

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Hierarchical Benefits

• Scalability - can grow larger without sacrificing
  control or manageability
• Ease of troubleshooting - because the functions
  of the individual layers are well defined, the
  isolation of problems in the network is less
  complicated
• Predictability - using functional layers is
  fairly predictable
• Easiest to implement

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Hierarchical Design

• A router is used for data path decision points
• The Network layer of the OSI model can be used to
  control data traffic patterns

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One-Layer Design Model

• Implemented on smaller networks
• Typically implemented if there are only a few
  remote locations in the company, and access to
  applications is mainly done via the local LAN to
  the site file server
• Each site is its own broadcast domain

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Two-Layer Design

• A WAN link is used to interconnect separate sites

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Three-Layer Model Components

• Core layer - provides fast wide-area connections
  between geographically remote sites, tying a
  number of campus networks together in a corporate
  or enterprise WAN
• Distribution layer - gives network services to
  multiple LANs within a WAN environment
• Access layer - is usually a LAN or group of LANs,
  typically Ethernet or Token Ring, that provide
  users with front-line access to network services

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Core Layer

• Responsible for providing a fast path between
  remote sites
• Responsible for providing redundant paths

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Distribution Layer

• Policy-based connectivity implemented
• Access to services controlled at this layer (do
  NOT confuse this with the access layer)
• Where the WAN backbone network is found, and is
  typically based on Fast Ethernet
• Enterprise servers should be placed at this layer

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Access Layer

• Usually a LAN or group of LANs, typically
  Ethernet or Token Ring
• Where users with common interests are grouped,
  almost all hosts are attached to the network
• Broadcast traffic is isolated
• Workgroup servers should be placed at this layer

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Alternatives to Dedicated WAN Links

• It is common for remote sites to access the
  WAN core layer by using WAN technologies other
  than dedicated links.

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WAN Threaded Case Study

• Question
• What type of design should be used with the
  Washington School District?

Answer Two-Layer Hierarchical Design
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WAN Threaded Case Study

• Question
• What type of WAN technology will be implemented
  at the district WAN core?

• Answer
• Core routers are connected via 4 T1 leased lines
  (dedicated) implementing PPP.

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WAN Threaded Case Study

• Question
• Where will Frame Relay be implemented in the
  district WAN?

• Answer
• Frame Relay will be implemented on the District
  Office core router for Internet access.

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WAN Threaded Case Study

• Question
• Where will ISDN be implemented in the district
  WAN?

• Answer
• ISDN will be used by the Community School to
  connect to a core router. This connection does
  not require full-time connectivity.

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CCNA 4 Chapter 3

• WAN Design
• THE END