Cisco Networking Academy Internetworking Technologies
Instructor: Prabul, CCNA
 
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9. WAN Design

Top 10 Graphics:

  1. Benifits of a Hierarchical Design Model
  2. Three-Layer Model Components
  3. Core-Layer Functions
  4. Distribution-Layer Functions
  5. Access-Layer Functions
  6. One-Layer Design -- Distributed
  7. Two-Layer Design
  8. Adding Frame Relay and ISDN WAN Links
  9. Traffic Pattern in a Two-Layer Hierarchy
  10. Server Placement

The requirement analysis for WAN design are quite similar to that of LAN design. The most scalable design for Wide Area Network implementation is a 'Hierarchical model' with each layer performing a particular function. Enterprise WANs can be made up of several different WAN technologies. Placement of servers is very critical in order to control traffic patterns across the WAN.

Step 1... - Analyze Requirements (of the network and its users)

  • Business issues
  • Technology issues
  • Administrative issues
  • Gather Data -
    • Corporate Structure
    • Business information flow
    • Applications in use
    • Current topology
    • Performance characteristics of current network
    • Determine if documented policies are in place
    • Mission-critical data
    • Mission-critical operations
    • Approved protocols and platforms
    • Control versus distributed authority
  • Business requirements
  • Technical requirements
  • New applications or business operations
  • Availability requirements -
    • Throughput
    • Response time
    • Access to resources

Design Rule: First and foremost you must understand the customer.

Design Rule: Find out what availability means to your customer.

Step 2... - Analyse Network Load Requirements

  • Client/Server applications
  • Host/terminal applications
  • Routing protocols
  • Reqularly scheduled services, such as file backup
  • Estimate worst-case traffic load during the busiest times for users and during regularly scheduled network services

Design Rule: Before developing an internetwork structure and provisioning hardware, determine the network traffic load.

Design Rule: Evaluate applications that cause traffic problems

Benefits of a Hierarchical Design Model:

  1. Scalability
  2. Ease of implementation
  3. Ease of troubleshooting
  4. Predictability
  5. Protocol Support
  6. Manageability

Scalability - Networks that follow the hierarchical model can grow much larger without sacrificing control or manageability because functionality is localized and potential problems can be recognized more easily. An example of a very large-scale hierarchical network design is the Public Switched Telephone Network (PSTN).

Ease of implementation - A hierarchical design assigns clear functionality to each layer, thereby making network implementation easier.

Ease of troubleshooting - Because the functions of the individual layers are well defined, the isolation of problems in the network is less complicated. Temporarily segmenting the network to reduce the scope of a problem also is easier.

Predictability - The behavior of a network using functional layers is much more predictable, which makes capacity planning for growth considerably easier; this design approach also facilitates modeling of network performance for analytical purposes.

Protocol support - The mixing of current and future applications and protocols will be much easier on networks that follow the principles of hierarchical design because the underlying infrastructure is already logically organized.

Manageability - All of the listed benefits contribute to greater manageability of the network.

Three-Layer Model Components:

  1. Core layer
  2. Distribution layer
  3. Access layer

Core layer - Provides fast wide-area connections between geographically remote sites, tying a number of 'campus' networks together in a corporate or enterprise WAN. Core links are usually point-to-point, and there are rarely any hosts in the core layer. Core services are typically leased from a telecom service provider (for example, T1/T3, Frame Relay, SMDS, etc.).

Distribution layer - Refers to the distribution of network services to multiple LANs within a campus network environment. This layer is where the 'campus backbone' network is found, typically based on Fast Ethernet. This layer is implemented on sites that are large and is used to interconnect buildings.

Access layer - Usually a LAN or a group of LANs, typically Ethernet or Token Ring, that provide users with frontline access to network services. The access layer is where almost all hosts are attached to the network, including servers of all kinds and user workstations.

NOTE: A layer is identified as a point in the network where an OSI reference model Layer 3 (network layer) boundary occurs. The three layers are bounded by Layer 3 devices or other devices that provide separation into broadcast domains.

Three-Layer Model Functions:

  1. Core-Layer (optimized transport between remote sites)
    • Redundant paths
    • Load sharing
    • Rapid convergence
    • Efficient use of bandwidth
    • Usually implemented as a WAN
  2. Distribution-Layer (Policy-based connectivity)
    • Control access to services
    • Define path metrics
    • Control network advertisements
    • Include the campus backbone with all its connecting routers
    • No end stations (such as servers) on the backbone
    • A transit path for traffic between workgroups
  3. Access-Layer (Connects workgroups to backbones)
    • Provides logical segmentation of the network
    • Group users with common interests
    • Isolate broadcast traffic from the workgroup
    • Connects users to LANs, and LANs to campus backbones or WANs

One-Layer Design: (Distributed)

  • A single-layer design will do in many smaller networks. A key design decision becomes the placement of servers: they may be distributed across multiple LANs, or concentrated in a central 'server farm' location.
  • The one-layer design is typically implemented where there are only a few remote locations in the company and access to applications are mainly done via the local LAN to the site file server.
  • Each site is its own broadcast domain.

Two-Layer Design:

  • A WAN link is used to interconnect separate sites.
  • VLANs may be implemented to create separate logical networks without requiring additional routers.
  • Multiple LANs may be implemented with each LAN segment being its own broadcast domain.
  • The router becomes a concentration point from WAN links.

Adding Frame Relay and ISDN to WAN links:

  • Different WAN technologies can be used to access the WAN core.
  • Entry points should be established on a router that is directly connected to the WAN core.

Advantages of Hierarchical WAN design:

  • Provides a method for controlling data 'traffic patterns'
    • By putting Layer 3 routing points throughout the network
    • Routers are data path decision points.
    • Data traffic will flow up the hierarchy only as far as it needs to to find the destination host.
  • Server Placement:
    • Enterprise servers vs. Workgroup servers
    • Placement of servers based on users
    • Moving servers to correct locations frees up WAN bandwidth

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