The availability of multigigabit campus switches from Cisco presents customers the opportunity to build extremely high-performance networks

If the right network design approach is followed, performance and reliability are easy to achieve. Unfortunately, some alternative network

design approaches can result in a network with lower performance, reliability, and manageability. With so many features available, and with

so many permutations and combinations possible, it is easy to go astray. This paper is the result of Cisco’s experience with many different

customers and it represents a common sense approach to network design that will result in simple, reliable, manageable networks.

The conceptual approach followed in this paper has been used successfully in routed and switched networks around the world for many

years. This hierarchical approach is called the “multilayer design.” The multilayer design is modular and capacity scales as building blocks

are added. A multilayer campus intranet is highly deterministic, which makes it easy to troubleshoot as it scales. Intelligent Layer 3 services

reduce the scope of many typical problems caused by misconfigured or malfunctioning equipment. Intelligent Layer 3 routing protocols such

as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP) handle load balancing and fast convergence.

The multilayer model makes migration easier because it preserves the existing addressing plan of campus networks based on routers and

hubs. Redundancy and fast convergence to the wiring closet are provided by Hot Standby Router Protocol (HSRP). Bandwidth scales from

Fast Ethernet to Fast EtherChannel and from Gigabit Ethernet to Gigabit EtherChannel. The model supports all common campus protocols.

The multilayer model will be described, along with two main scalability options appropriate for building-sized networks up to large

campus networks. Five different backbone designs with different performance and scalability are also presented. In this paper the term

backbone is used to represent the switches and links in the core of the network through which all traffic passes on its way from client to server.

Network design requires extensive practical experience combined with a

theoretical understanding of the technologies and how they relate to one

another. Hands-on experience is particularly critical and this is often overlooked.

An engineer who does not have extensive network support experience

is, in my view, not yet equipped to work in design.

The tools that enable you to achieve the design goals are encompassed in

the technology itself. You need to have a good knowledge and understanding

of topics like scalable routing protocols, cost-effective WAN transport

technology, and network management.

Network design models should not be trusted. No network design tool or

model on the market is realistically applicable to anything beyond a simple

network regardless of what the sales engineers for the various vendors tell

you. For this reason, it is recommended that you do laboratory work and

perform some proof of concept tests. Designs must be performed in a lab

rather than as a theoretical paper exercise. The multitude and interaction

of so much technology is simply too complex to verify in anything other than

a real-life test bed.

Figure 1-1 displays a network design flowchart that provides an approximate

guideline that could be used to approach the basic steps to be followed

during the design process:

The final chapter of this book, “Network Security” examines the security of

IP networks. In the chapter, the requirement to incorporate the company’s

security policy into the network design is made apparent. Security risks and

vulnerabilities must be addressed at the network design stage. The tools and

procedures used to secure a network are also integral to its design.

A disaster recovery plan should be developed in conjunction with any significant

network design or redesign. A disaster is a precisely defined term that

has a different meaning for different companies and their networks.The complete

and outright failure of all core resources is an example of a disaster scenario.

This is not to be confused with the design of a network that is resilient

against common failures on a communications link or a central router.

The need for network resilience is driven by the application availability

requirements. After ascertaining the availability requirements of each

application, a plan must be put in place to ensure that this availability can

be provided. A resilient design must provide full resilience along the clientto-

server data path. This entails achieving the following:

Most network designs are characterized by a tradeoff between cost and

availability. Providing a truly resilient design for all aspects of the network

in many cases will cause the network budget to be exceeded. It is then a

question of prioritizing and defining the exact level of resilience that will be

provided for each application and on each part of the network.

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