Chabot College
Cisco Networking Academy
WAN Design
Semester 4, Chapter 3
Table of Contents
Go There!
WAN Design Requirements
Go There!
Gathering & Analyzing
Requirements
Go There!
The Three-Layer WAN model
Go There!
WAN Layer Functions
WAN Design
Requirements
Table of Contents
Network Demand
WANs need to be developed to meet the
following requirements:
 Optimize WAN bandwidth
 Minimize cost
 Maximize the effective service to end users
Network Demand
LANs & shared media networks are being
overtaxed because...
 Network cost continues to escalate
 Network usage has increased
Application requirements increasingly demand more network
services (i.e., “push” “technologies)
Increased use of enterprise servers
The number on intra- and extranets continues to rise
LANs connected through WANs is expected to
increase WAN traffic 300% in the next 5 years.
LAN/WAN Integration
LANs and WANs, previously logically
separated, must now be fully integrated for
seamless performance.
The LAN/WAN network (or corporate internet)
now must be able to handle...
 Voice traffic (VoIP)
 Bandwidth intensive multimedia applications
Video conferencing
On-line training

Increased business critical data access
Overriding Goal in WAN Design
Minimize Cost While
Increasing Network
Availability
Gathering & Analyzing
Requirements
Table of Contents
Factors Affecting Design
Environmental Variables
 Where are all the nodes?
Performance Constraints
 What level of reliability? Host/client speeds? Traffic
throughput?
Networking Variables
 What’s the topology? What is the traffic’s characteristics?
Traffic Characterization is critical to successful
WAN design and implementation, but it is
seldom done.
Traffic Characterization
Types of Traffic
 Voice/fax
 Client/Server data
 Messaging
 File transfers
 Batch data
 Network overhead
 Multimedia
Traffic Characteristics
 Peak & Avg. Volumes
 Connectivity & volume flows
 Connection orientation
 Latency tolerance
 Network availability tolerance
 Error rate tolerance
 Priority
 Protocol type
 Avg. packet length & MTUs
Gathering User Requirements
In general, users primarily want application
availability in their networks. This includes...
 Response Time -- time between entry of a command and
execution of the command
 Throughput-intensive apps. -- such as file-transfers and
batch operations scheduled during low traffic periods
 Reliability -- some apps require nearly 100% uptime such
as NASDAQ and emergency services.
Assessing User Requirements
Three methods to assess user needs:
 User community profiles--determine the needs of various
user groups within the organization; crucial 1st step
 Interviews, focus groups, and surveys--used to establish a
baseline for building the network
 Human factors tests--most expensive & time consuming of
the three; sampling of users interacting with the network
from a controlled lab environment to determine user
tolerance to various levels of service
Factors That Affect Availability
Throughput
Response Time
Access to Services
You can increase availability by adding more
resources (i.e. bandwidth, servers, etc.), but
this drives up cost
Network design seeks to provide the greatest
availability for the least cost.
Analyzing Requirements
Sensitivity Testing
 Evaluate how a network will behave under certain
conditions.
 Involves breaking stable links and observing the results
how is traffic rerouted
speed of convergence
is connectivity lost?
is some traffic sensitive to the break?

Increase traffic loads to media saturation point and observe
results.
The Three-Layer
WAN Model
Table of Contents
The Importance of Layers
Designing networks using the OSI model
 Allows the network to be designed in layers
 Uses layers to simplify the tasks required for
internetworking
 Design elements can be replicated as the network grows
Therefore, networks should be designed using
a hierarchical model.
 Unfortunately, most networks are thrown together into a
mesh (“a mess!”) with little or no vision of future needs.
Benefits of Hierarchical Design
Scalability
 allows for future growth without sacrificing control or functionality
Ease of Implementation
 logically constructed layers specify the functions of each layer
Ease of troubleshooting
 well-defined functions at each layer aid in the isolation of problems
Predictability
 behavior of functional layers can be estimated and planned for
Protocol support
 allows easier implementation of future technologies because the
network has been logically constructed
Manageability
 All the above aids net. admin. in overall management of the network
The Hierarchical Design Model
The three layers are...
 Core layer--provides transport between remote sites
 Distribution layer--provides policy-based connectivity
 Access layer--provides workgroup/user access to network
Core Layer
Fast WAN connections between remote sites
 Core links are normally point-to-point with no host
devices
 Core services include:

T1/T3
Frame Relay
ATM
SMDS
Distribution Layer
Provides WAN services to multiple LANs
 Usually the campus backbone
 Uses Fast Ethernet (or Gigabit Ethernet)
 Used on large sites to interconnect buildings

Access Layer
Usually a LAN or group of LANs
 Gives access to specific users and workgroups
 This layer is where all hosts (including servers) are attached to
the network
 We study the design of this layer in Semester 3, LAN Design

WAN Layer Functions
Table of Contents
Core Layer Functions
Optimize Transport Between
Remote Sites
 Redundant paths to guard against circuit
outages
 Provide load sharing and rapid
convergence when link states change
 Efficient use of bandwidth by...
Implementing scalable routing protocols and
Blocking local traffic access to the core
Distribution Layer Functions
Policy-Based Connectivity
 Boundary definition & packet manipulation
 Control access to services of the core layer and other
distribution layer routers
 VLAN routing
 Address aggregation (i.e., subnets) & route optimization
 ACLs and other security measures
Access Layer Functions
Workgroup & User Access to the Network
 Isolation of Broadcast Traffic
 Shared and Switched Bandwidth
 MAC-layer filtering
 Microsegmentation
Other Hierarchical Options
One-Layer Design
 Only a few remote sites
need to be connected
 Servers are placed in farms
or in each workgroup to
reduce traffic on the
backbone
Two-Layer Design
 WAN link is used to
interconnect separate sites
 Link does not have to be
dedicated. An alternative
would be ISDN.
Hierarchical Design Advantages
Controlling data traffic patterns through
source/destination network layer addressing
 A packet only needs to travel up the hierarchy as far as it needs to
find the destination.
 With good design, most traffic would be contained in the access
layer with users accessing their workgroup servers
Server Placement
 Enterprise Servers needed by all workgroups should be placed in the
Distribution Layer (e.g. email, DNS, etc.)
 Workgroup Servers needed by a unique set of users should be
placed in the Access Layer, preferably in the same broadcast domain
as the users.
Table of Contents
End Slide Show