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WAN Availability and QoS

Chapter Description

In this sample chapter from CCNP Enterprise Design ENSLD 300-420 Official Cert Guide: Designing Cisco Enterprise Networks, you will review WAN methodologies and WAN availability with deployment models using MPLS, hybrid, and Internet designs. It also covers backup connectivity, failover designs, QoS strategies, and designing end-to-end QoS policies.

WAN Design Methodologies


WAN design methodologies should be used when designing enterprise edge networks. Some keys to WAN design are the following processes:

  • Identifying the network requirements: This includes reviewing the types of applications, the traffic volume, and the traffic patterns in the network.

  • Assessing the existing network: This involves reviewing the technologies used and the locations of hosts, servers, network equipment, and other end nodes.

  • Designing the topology: This is based on the availability of technology as well as the projected traffic patterns, technology performance, constraints, and reliability.

When designing the WAN topology, remember that the design should describe the functions that the enterprise modules should perform. The expected service levels provided by each WAN technology should be explained. WAN connections can be characterized by the cost of renting the transmission media from the service provider to connect two or more sites.

New network designs should be flexible and adaptable to future technologies and should not limit the customer’s options going forward. For example, collaboration applications such as VoIP and video are common now, and most enterprise network designs should be able to support them. The customer should not have to undergo major hardware upgrades to implement these types of technologies. The ongoing support and management of the network is another important factor, and the design’s cost-effectiveness is important as well.

Table 9-2 lists key design principles that can help serve as the basis for developing network designs.


Table 9-2 Key Design Principles

Design Principle


High availability

Redundancy through hardware, software, and connectivity


Modularity with additional devices, services, and technologies


Measures to protect business data


Enough capacity and bandwidth for applications


Ease of managing and maintaining the infrastructure

Standards and regulations

Compliance with applicable laws, regulations, and standards


Appropriate security and technologies given the budget

High availability is what most businesses and organizations strive for in sound network designs. The key components of application availability are response time, throughput, and reliability. Real-time applications such as voice and video are not very tolerant of jitter and delay.

Table 9-3 identifies various application requirements for data, voice, and video traffic.


Table 9-3 Application Requirements for Data, Voice, and Video Traffic


Data File Transfer

Interactive Data Application

Real-Time Voice

Real-Time Video

Response time


Within a second

One-way delay less than 150 ms with low delay and jitter

Minimum delay and jitter

Throughput and packet loss tolerance





Downtime (high reliability = low downtime)





Response Time

Response time is a measure of the time between a client user request and a response from the server host. An end user will be satisfied with a certain level of delay in response time. However, there is a limit to how long the user will wait. This amount of time can be measured and serves as a basis for future application response times. Users perceive the network communication in terms of how quickly the server returns the requested information and how fast the screen updates. Some applications, such as a request for an HTML web page, require short response times. On the other hand, a large FTP transfer might take a while, but this is generally acceptable.


In network communications, throughput is a measure of data transferred from one host to another in a given amount of time. Bandwidth-intensive applications have a greater impact on a network’s throughput than does interactive traffic such as a Telnet session. Most high-throughput applications involve some type of file-transfer activity. Because throughput-intensive applications have longer response times, you can usually schedule them when time-sensitive traffic volumes are lower, such as after hours.


Reliability is a measure of a given application’s availability to its users. Some organizations require rock-solid application reliability, such as five nines (99.999%); this level of reliability has a higher price than most other applications. For example, financial and security exchange commissions require nearly 100% uptime for their applications. These types of networks are built with a large amount of physical and logical redundancy. It is important to ascertain the level of reliability needed for a network that you are designing. Reliability goes further than availability by measuring not only whether the service is there but whether it is performing as it should.

Bandwidth Considerations

Table 9-4 compares several WAN technologies in terms of speeds and media types.


Table 9-4 Physical Bandwidth Comparison

WAN Connectivity

Bandwidth: Up to 100 Mbps

Bandwidth: 1 Gbps to 10 Gbps


Fast Ethernet

Gigabit Ethernet, 10 Gigabit Ethernet


Fast Ethernet

Gigabit Ethernet, 10 Gigabit Ethernet, SONET/SDH, dark fiber




LTE/LTE Advanced

802.11n/ac Wave1/Wave2

LTE Advance Pro/5G

A WAN designer must engineer the network with enough bandwidth to support the needs of the users and applications that will use the network. How much bandwidth a network needs depends on the services and applications that will require network bandwidth. For example, VoIP requires more bandwidth than interactive Secure Shell (SSH) traffic. A large number of graphics or CAD drawings require an extensive amount of bandwidth compared to file or print sharing information being transferred on the network. A big driver in increasing demand for bandwidth is the expanded use of collaboration applications that utilize video interactively.

When designing bandwidth for a WAN, remember that implementation and recurring costs are important factors. It is best to begin planning for WAN capacity early. When the link utilization reaches around 50% to 60%, you should consider increases and closely monitor the capacity. When the link utilization reaches around 75%, immediate attention is required to avoid congestion problems and packet loss that will occur when the utilization nears full capacity.

QoS techniques become increasingly important when delay-sensitive traffic such as VoIP is using the limited bandwidth available on the WAN. LAN bandwidth, on the other hand, is generally inexpensive and plentiful; in the age of robust real-time applications, however, QoS can be necessary. To provide connectivity on the LAN, you typically need to be concerned only with hardware and implementation costs.

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