The network design methodology presented in this section is derived from the Cisco Prepare, Plan, Design, Implement, Operate, and Optimize (PPDIOO) methodology, which reflects a network’s lifecycle. The following sections describe the PPDIOO phases and their relation to the network design methodology, and the benefits of the lifecycle approach to network design. Subsequent sections explain the design methodology in detail.

Design as an Integral Part of the PPDIOO Methodology

Figure: PPDIOO Network Lifecycle Influences Design

Prepare phase: The Prepare phase involves establishing the organizational (business) requirements, developing a network strategy, and proposing a high-level conceptual architecture, identifying technologies that can best support the architecture. Financial justification for the network strategy is established by assessing the business case for the proposed architecture.

■ Plan phase: This phase involves identifying the network requirements, which are based on the goals for the network, where the network will be installed, who will require which network services, and so forth. The Plan phase also involves assessing the sites where the network will be installed and any existing networks, and performing a gap analysis to determine if the existing system infrastructure, sites, and operational environment can support the proposed system. A project plan helps manage the tasks, responsibilities, critical milestones, and resources required to implement the changes to the network. The project plan should align with the scope, cost, and resource parameters established in the original business requirements. The output of this phase is a set of network requirements.

■ Design phase: The initial requirements determined in the Plan phase drive the network design specialists’ activities. These specialists design the network according to those initial requirements, incorporating any additional data gathered during network analysis and network audit (when upgrading an existing network) and through discussion with managers and network users. The network design specification that is produced is a comprehensive detailed design that meets current business and technical requirements and incorporates specifications to support availability, reliability, security, scalability, and performance. This design specification provides the basis for the implementation activities.

■ Implement phase: Implementation and verification begins after the design has been approved. The network and any additional components are built according to the design specifications, with the goal of integrating devices without disrupting the existing network or creating points of vulnerability.

■ Operate phase: Operation is the final test of the design’s appropriateness. The Operate phase involves maintaining network health through day-to-day operations, which might include maintaining high availability and reducing expenses. The fault detection and correction and performance monitoring that occur in daily operations provide initial data for the network lifecycle’s Optimize phase.

■ Optimize phase: The Optimize phase is based on proactive network management, the goal of which is to identify and resolve issues before real problems arise and the organization is affected. Reactive fault detection and correction (troubleshooting) are necessary when proactive management cannot predict and mitigate the failures. In the PPDIOO process, Optimize phase might lead to network redesign if too many network problems or errors arise, if performance does not meet expectations, or if new applications are identified to support organizational and technical requirements.

Although Design is one of the six PPDIOO phases, all the other phases influence design decisions, and the Design phase interacts closely with them, as follows:

■ The requirements derived from the Prepare and Plan phases are the basis for network design.

■ The Implement phase includes the initial verification of the design on the actual network.

■ During the Operate and Optimize phases, the final decision is made about the appropriateness of the design, based on network analysis and any problems that arise. The network might have to be redesigned to correct any discovered errors.

Benefits of the Lifecycle Approach to Network Design

The network lifecycle approach provides many benefits, including the following:

■ Lowering the total cost of network ownership:

• Identifying and validating technology requirements
• Planning for infrastructure changes and resource requirements
• Developing a sound network design aligned with technical requirements and business goals
• Accelerating successful implementation
• Improving the efficiency of the network and of the staff supporting it
• Reducing operating expenses by improving the efficiency of operation processes and tools

■ Increasing network availability:

• Assessing the state of the network’s security and its ability to support the proposed design
• Specifying the correct set of hardware and software releases and keeping them operational and current
• Producing a sound operational design and validating network operation
• Staging and testing the proposed system before deployment
• Improving staff skills
• Proactively monitoring

improving business agility:

• Establishing business requirements and technology strategies
• Readying sites to support the system to be implemented
• Integrating technical requirements and business goals into a detailed design and demonstrating that the network is functioning as specified
• Expertly installing, configuring, and integrating system components
• Continually enhancing performance

■ Accelerating access to applications and services:

• Assessing and improving operational preparedness to support current and planned network technologies and services
• Improving service-delivery efficiency and effectiveness by increasing availability, resource capacity, and performance
• Improving the availability, reliability, and stability of the network and the applications running on it
• Managing and resolving problems affecting the system and keeping software applications current.

Design methodology

The design methodology presented here includes three basic steps; some of the design methodology steps are intrinsic to the PPDIOO Design phase, whereas other steps are related to other PPDIOO phases:

Step 1 Identify customer requirements: In this step, which is typically completed during the PPDIOO Prepare phase, key decision makers identify the initial requirements. Based on these requirements, a high-level conceptual architecture is proposed.

Step 2 Characterize the existing network and sites: The Plan phase involves characterizing sites and assessing any existing networks, and performing a gap analysis to determine whether the existing system infrastructure, sites, and operational environment can support the proposed system.

Characterization of the existing network and sites includes site and network audit and network analysis. During the network audit, the existing network is thoroughly checked for integrity and quality. During the network analysis, network behavior (traffic, congestion, and so forth) is analyzed.

Step 3 Design the network topology and solutions: In this step, the detailed design of the network is created. Decisions are made about networked infrastructure, infrastructure services, and applications. The data for making these decisions is gathered during the first two steps.

When the design is complete, the design implementation process is executed; this process includes the following steps:

Step 1 Plan the implementation: During this step, the implementation procedures are prepared in advance to expedite and clarify the actual implementation. Cost assessment is also undertaken at this time. This step is performed during the PPDIOO Design phase.

Step 2 Implement and verify the design: The actual implementation and verification of the design take place during this step by building a network. This step maps directly to the Implement phase of the PPDIOO methodology.

Step 3 Monitor and optionally redesign: The network is put into operation after it is built. During operation, the network is constantly monitored and checked for errors. If troubleshooting problems become too frequent or even impossible to manage, a network redesign might be required; this can be avoided if all previous steps have been completed properly. This step is, in fact, a part of the Operate and Optimize phases of the PPDIOO methodology.