WAN Design

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WAN Design

• Chapter 3

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WAN Design Requirements

• WAN communication is often called a service
  because the network provider often charges users
  for the WAN services it provides.
• All technologies and features used in WANs are
  developed to meet the following design
  requirements
• Optimize WAN bandwidth.
• Minimize cost.
• Maximize the effective service to the end users.

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WAN Design Requirements

• Traditional shared-media networks are now being
  overtaxed because of the following
• Network usage has increased.
• Application software evolution is continually
  more demanding.
• Applications increasingly require distinct
  network qualities of service.
• An unprecedented number of connections are being
  established.
• The explosive growth of corporate intranets and
  extranets has created a greater demand for
  bandwidth.
• The increased use of enterprise servers continues
  to grow to serve the business needs of
  organizations.

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WAN Design Requirements

• New WAN infrastructures must be
• more complex.
• based on new technologies.
• able to handle an ever-increasing (and rapidly
  changing) application mix with required and
  guaranteed service levels.
• Within the next five years, theres an expected
  300 traffic increase.

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WAN Design Goals

• Two primary goals drive WAN design and
  implementation
• Application availability
• Networks carry application information between
  computers.
• If the applications are not available, the
  network is not doing its job.
• Total cost of ownership
• Information Systems (IS) department budgets often
  run in the millions of dollars.
• More businesses rely on electronic data,
  therefore the costs of computing resources will
  continue to rise.
• A well-designed WAN can help to balance these
  objectives.
• When properly implemented, the WAN infrastructure
  can optimize application availability and allow
  the cost-effective use of existing network
  resources.

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WAN Design Goals

• In general, WAN design needs to take into account
  three general factors
• Environmental variables
• Includes 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
• Consist of network reliability, traffic
  throughput, and host/client computer speeds (for
  example, network interface cards and hard drive
  access speeds).
• Networking variables
• Includes the network topology, line capacities,
  and packet traffic.

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WAN Design Models

• Network designs tend to follow one of two general
  design strategies
• Mesh
• The network topology is flat.
• All routers perform essentially the same
  functions, and there is usually no clear
  definition of where specific functions are
  performed.
• Expansion of the network tends to proceed in a
  haphazard, arbitrary manner.
• Hierarchical
• The structure the network is organized in layers,
  each of which has one or more specific functions.
  
• Data traffic flows based on source / destination
  addressing.

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Hierarchical WAN Design Model

• Benefits to using a hierarchical model include
  the following
• Easier to implement.
• Easier to manage.
• Easier to troubleshoot.
• Improved scalability.
• Predictability.
• Protocol support.

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Three Layer Design
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3 Hierarchical WAN Design Layers

• A hierarchical network design includes the
  following three layers
• The CORE layer
• Provides optimal transport between sites.
• The DISTRIBUTION layer
• Provides policy-based connectivity.
• The ACCESS layer
• Provides workgroup and user access to the
  network.
• Could also be called the 3 levels of a router
  hierarchy.

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Core Layer

• Provides fast WAN area connections between remote
  sites, tying a campus networks together in a
  corporate or enterprise WAN.
• Is usually implemented as a WAN.
• Needs redundant paths.
• Can withstand individual circuit outages and
  continue to function.
• Links are point-to-point.
• There are rarely any hosts in the core layer.
• Should not perform any filtering slows down
  performance.
• Core services (for example, T1/T3, Frame Relay,
  SMDS) typically are leased from a telecom service
  provider.

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Distribution Layer

• Provides network services to multiple LANs within
  a WAN environment.
• This is where the WAN backbone network is found,
  and it is typically based on Fast Ethernet.
• This is implemented on large sites and is used to
  interconnect buildings.
• Provides boundary definition, and it is the layer
  at which packet manipulation occurs.
• Can be summarized as the layer that provides
  policy-based connectivity.

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Distribution Layer

• Can include several functions, such as the
  following
• Address or area aggregation.
• Departmental or workgroup access to the core
  layer.
• Broadcast/multicast domain definition.
• Virtual LAN (VLAN) routing.
• Any media transitions that need to occur.
• Security.
• Provides policy-based connectivity.
• Not putting end stations on the backbone frees up
  the backbone to act strictly as a transit path
  for traffic between workgroups or campus-wide
  servers.

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Access Layer

• The access layer is usually a LAN or a group of
  LANs, typically Ethernet or Token Ring, that
  provide users with frontline access to network
  services.
• Almost all hosts are attached to the network,
  including servers of all kinds and user
  workstations.
• Allows logical segmentation of the network and
  grouping of users based on their function.
• Can also use access control lists or filters to
  further optimize the needs of a particular set of
  users.
• Workgroup servers should be located here.
• The main goal of the Access layer is to isolate
  the broadcast traffic between the individual
  workgroups, segments, or LANs.

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Access Layer

• In the campus environment, access-layer functions
  can include the following
• Shared bandwidth.
• Switched bandwidth.
• MAC-layer filtering.
• Microsegmentation.
• The access layer connects users into LANs, and
  LANs into WAN backbones or WAN links.

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Other Designs
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Other Designs

• A three-layer model can meet the needs of most
  enterprise networks.
• However, a two-layer design may be adequate or
  even a single layer flat network.
• A hierarchical structure should still be planned
  or maintained to allow these network designs to
  expand to three layers as the need arises.

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One Layer Design

• In a 1 layer design, the key design decision
  becomes the placement of servers
• They can be distributed across multiple LANs
• Or concentrated in a central server farm
  location.
• A one-layer design is 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

• In a two-layer design, a WAN link is used to
  interconnect separate sites.
• Inside the site, multiple LANs may be
  implemented, with each LAN segment being its own
  broadcast domain.

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