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The importance of a network design plan

A well-designed network is a successful network. This tip helps systems integrators and value-added resellers (VARs) understand how a design plan contributes to a predictable and consistent network.

A well-designed network is a successful network. This tip, reposted courtesy of, helps systems...

integrators and value-added resellers (VARs) understand how a design plan contributes to a predictable and consistent network.

IP network design: A learning guide
Learn more about the process of designing a predictable and consistent IP network with our Learning Guide.

A competent network design is the foundation upon which all successful network implementations are built. This article focuses on the design of IP-based networks, due to the prevalence of IP as the de-facto standard desktop protocol. The applications that a state-of-the-art IP network supports have become increasingly diverse in nature. Along with traditional data applications, IP has become a transport mechanism for real-time applications such as voice, video and multimedia. As a result of the heterogeneous nature of modern applications, the design of IP internetworks has never been more challenging.

The need for a design plan

Most IP internetworks can be thought of as falling within one of two categories in relation to their design. There are the networks that have clearly been well designed and there are those that have merely been pieced together over time. The perceptible difference between these two types of networks illustrates the importance of good design. A network that has been well designed is characterized by predictability and consistency in relation to each of the following areas:


A consistently high level of performance is observed in relation to the major network performance parameters. These parameters might include application response time and the variation in response time.


The network should provide a resilient platform for the applications that it supports. A highly specified network might have to meet an availability target of 99% for all applications with a 'zero-downtime' requirement for mission critical applications. Ideally, the failure of any one link or networking device along the client-to-server path should not result in the loss of a client-server session. Automatic failover to an alternate path should occur within a time-interval that is short enough to minimize the affect on existing sessions. This time-interval is called the convergence time, which can be defined as the duration from a network topology change (such as the loss of a link) occurring until each device on the network is aware of the change. Well-designed networks are characterized by consistently low convergence times.


A scalable network is capable of adequately supporting growth without having to be radically re-designed. Growth in terms of the number of users, the number of network nodes or sites must be catered to along with the possible addition of new applications and the increased bandwidth consumption that they entail. To obtain a feeling for how scalable your customer's network is, ask yourself the following questions: What if there was twice the number of users, twice the number of nodes, new applications that demanded twice the bandwidth? A scalable network can accommodate this growth and change without requiring a significant overhaul of its infrastructure. The fundamental network topology and the technology employed should not have to be redesigned in order to accommodate growth. New nodes and users can be added to a scalable network in a simple building block approach. The new nodes, for example, should simply entail the addition of a new section or block to an existing structure, which is the core or backbone of the network. Increased bandwidth demands should be accommodated by appropriately augmenting the LAN and WAN bandwidth as necessary. Certain operational upgrades may also be required during the network lifetime such as increased memory and processing power on the network routers and switches. However, what should not be required is a radical overhaul of the network infrastructure in order to support projected growth during the network's lifetime. This is, after all, one of the fundamental reasons why a network plan is put in place to begin with.

Running costs

There is no getting away from the fact that cost is the most fundamental driver behind the network design process. Networks must not only meet a certain technical specification, they must also be cost-effective in their design and implementation. The main cost component to owning a network is usually the WAN costs from the service provider. That is the cost of the frame relay, ATM, leased lines or ISDN technology. Network designs are typically characterized by a trade-off of cost versus performance and availability. For example, more bandwidth may be required to ensure optimum application performance; however, there is usually a cut-off point where purchasing more bandwidth is no longer cost-effective. Similarly, backup circuits or ISDN may be required to ensure resilience along the client-to-server data path in the event of a failure on the primary data path. This backup technology must be of similar speed to the primary link in order to avoid degraded service during a fault condition. It is an economic decision for the customer whether or not degraded service can be tolerated during a fault scenario.

A well-designed network should not only be cost-effective to operate; it should also be characterized by relatively consistent running costs. One of the best illustrations of the importance of consistent and predictable running costs relates to the question of support costs. The second largest component to the cost of owning a network (after WAN costs) is the cost of support. It is also the most overlooked cost element mainly because it is notoriously difficult to quantify. For example, it may be decided to follow a private-managed implementation of ATM in order to reduce the WAN costs that would be incurred from an ATM service provider. While this would undoubtedly reduce WAN costs, it would also result in increased support costs. A significant level of expertise is required to support a private ATM network. Hiring and retaining such expertise is expensive. However, without having such expertise in-house the cost of network support is likely to be even greater with the need for external consultants and other third parties being used to 'fill the gaps' and ensure a smooth daily operation.

About the author
Cormac Long is the author of
IP Network Design and Cisco Internetworking and Troubleshooting.

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