Design Principles

Cisco has developed the Plan-Design-Implement-Operate-Optimize (PDIOO) network life cycle to describe the multiple phases through which a network passes. This life cycle is illustrated in Figure 1-2, and the phases are briefly described as follows:

• Plan phase The detailed network requirements are identified, and the existing network is reviewed.

• Design phase The network is designed according to the initial requirements and additional data gathered during analysis of the existing network. The design is refined with the client.

• Implement phase The network is built according to the approved design.

• Operate phase The network is operational and is being monitored. This phase is the ultimate test of the design.

• Optimize phase During this phase, issues are detected and corrected, either before problems arise or, if no problems are found, after a failure has occurred. Redesign might be required if too many problems exist.

• Retirement phase Although not part of the PDIOO acronym, this phase is necessary when part of the network is outdated or is no longer required.

Figure 1-2. PDIOO Network Life Cycle Includes Many Design Aspects^[1]

The PDIOO cycle describes all the phases of a network's life. The task of designing a network is obviously an integral part of this life cycle and influences all phases. For example, the implemen-tation phase might involve a prototype, which can help validate the design.

Network design should include the following tasks, as illustrated in Figure 1-3:

• Determine requirements

• Analyze the existing network, if one exists

• Prepare the preliminary design

• Complete the final design development

• Deploy the network

• Monitor, and redesign if necessary

• Maintain documentation (as a part of all the other tasks)

Figure 1-3. Network Design Is an Ongoing Process

These tasks, and their relationship to the PDIOO phases, are examined in the following sections.

Determining Requirements

Determining the network requirements is a part of the PDIOO Plan phase. Many types of requirements must be considered, including those that are related to technical and business issues. Any factors that might restrict the design must also be identified. In the case where an existing network is in place, these constraints can be related to what is already there and how the new network must be phased in to allow continuous operation.

Technical requirements and restraints can include the following items:

• Applications that are to run on the network

• Internet connections required

• Addressing restrictions, for example, the use of private Internet Protocol (IP) version 4 (IPv4) addresses

• Support for IP version 6 (IPv6) addresses

• Other protocols that are to run on the network (for example, routing protocols)

• Cabling requirements

• Redundancy requirements

• Use of proprietary equipment and protocols

• Existing equipment that must be supported

• Network services required, including quality of service (QoS) and wireless

• How security is to be integrated into the network

• Network solutions required (for example, voice traffic, content networking, and storage networking)

• Network management

• Support for existing applications while new ones are being phased in

• Bandwidth availability

Key Point

An intelligent network service supports network applications. For example, security and QoS are not ultimate goals of or applications on a network; rather, they are necessary services that enable other applications. Note that some of these services, such as security, are now integral parts of any well-designed network.

Intelligent network solutions are network-based applications. These network solutions require the support of the network services. Voice communication is an example of an intelligent network solution; it requires QoS for optimal operation.

Requirements and restrictions related to business issues can include the following:

• Budget Capital (for new equipment) and operating (for ongoing expenses).

• Schedule This could include the phasing out of older applications, hiring of new personnel, and so forth.

• People Considerations include who will install and operate the network, what skills they have, whether they require training, whether any of these tasks will be outsourced, and so forth.

• Legal Issues include any restrictions on the use and storage of data collected, whether the organization has contractual obligations or opportunities related to the network (for example, long-term maintenance or lease contracts), and so forth.

• History Factors include examining the existing network's structure and determining whether any person or group will block changes or additions.

• Policies Consider whether current organizational policies might restrict the network design.

As discussed earlier in the house-addition analogy, extracting requirements is a difficult task.

Key Point

Requirements must be clear and deterministic (verifiable); in other words, at the end of the project, you should easily be able to determine whether a requirement has been met.

For example, a customer might say that the new network must help reduce overall costs. This goal must be translated into requirements that can be implemented and measured. For example, reducing costs could mean that a web-based ordering system replaces call-center ordering, or it could mean that unreliable equipment is replaced. Each of these options has its own initial and operating costs, so you must understand what the network owner means when he states his goals.

Each of the requirements should also be assessed for its importance, and a weighting factor should be assigned so that if conflicts arise (for example, an inadequate budget), the most important requirements can be met.

Analyzing the Existing Network

If this is a redesign of an existing network, the current network must be analyzed and understood. As noted earlier, an existing network is likely to restrict the network design in some manner; for example, the existing cabling might not be optimal but might have to be kept for cost reasons. Analyzing the existing network is typically done during the Optimize phase of the existing network; it could also be considered as part of the Plan phase for the new network.

You should analyze the network to determine both what is good and what should be changed. For example, the network might include virtual private network (VPN) connections so that employees can access corporate files through the Internet (VPN is discussed in Chapter 4, "Network Security Design"). If the organization is satisfied with this feature, this portion of the network might not have to be changed.

Key Point

While examining documentation about the existing network and discussing it with users, administration staff, and other stakeholders is important, you should also do an audit of the network. The audit can identify items such as the protocols that are running (both those that are known to be running and those that have not been reported), the devices installed and their configurations, the utilization of these devices, and the bandwidth on key WANs).

Many operating systems include commands that display information about device hardware and software. For example, the show version command in the Cisco Internet Operating System (IOS) displays information related to the version of the software and the amount of memory available. Additional tools, such as protocol analyzers, might be necessary to gather other information.

Preparing the Preliminary Design

Preliminary design involves considering all the network requirements and constraints (including the budget), and determining viable alternative solutions. The network owner is then consulted, and together an optimal solution is chosen; this solution is later developed into the final design. Both the preliminary design and final design are done during the PDIOO Design phase.

Two models that can be used for network design are examined in the "Modular Network Design" section, later in this chapter. Whichever model is used, a top-down approach (rather than a bottom-up approach) is recommended.

Key Point

A top-down approach to network design means that requirements are considered first, with the applications and network solutions that will run on the network driving the design.

A bottom-up approach would first select devices, features, cabling, and so on, and then try to fit the applications onto this network. A bottom-up approach can lead to redesign if the applications are not accommodated properly. This approach can also result in increased costs by including features or devices that are not required and would have been excluded had the network requirements analysis been completed.

After the alternative solutions have been developed, the optimal solution must be chosen. A systematic approach works bestall the options should be listed along with how well they meet (or don't meet) the design requirements and constraints. If no clear winner exists, the importance of the requirements (as determined in the requirements-gathering process) should be considered to select the optimal design.

Completing the Final Design Development

Developing the final design involves producing detailed drawings, configuration specifications, costing, addressing plans, and any other information required for implementation.

Key Point

You can verify the design by implementing a prototype network, separate from the existing network. Alternatively, a pilot network can be implemented within a portion of the existing network to verify that the design is feasible.

Deploying the Network

Deployment of the network must start with a plan and a schedule. Deployment planning starts in the PDIOO Design phase and continues into the Implement phase.

The deployment plan must include details of what is to be done and how it is to be done. For example, if new cabling is required, the procedure to run the cable and the location where it is needed must be fully documented. Scheduling is important, not only to identify when things will be done but also to determine who will do them, and what impact the deployment will have on the existing network. For example, if current applications must still work while the new network is being implemented, the schedule must show any times during which the applications will be unavailable.

Contingency plans, that is, plans for what happens if a problem occurs during the implementation, should also be included. These contingency plans should address how the network will be returned to a known working state, if possible. Testing should be incorporated into the deployment plan to ensure that the functions are working as they are implemented.

Any training required for personnel should be planned during this time. For example, if you are deploying a Voice over IP (VoIP) solution, the network administrators might require some instruction on the technology before they can implement and maintain it.

Any contracts required should be negotiated during this time. Examples include outsourcing, Internet connectivity, maintenance, and so forth.

When the plans, schedules, contracts, and so on are in place, the network can be implemented. Any problems found in the design during this phase must be corrected and documented.

Key Point

Implementation is the final verification of the design.

Monitoring and Redesigning

After the network is operating, baseline operational statistics should be gathered so that values for a working network are known. The network should then be monitored for anomalies and problems. If problems that require redesign occur, or if requirements change or are added, the appropriate design changes must be made and the entire design process should be repeated for that portion of the network. Monitoring and redesign take place in the PDIOO Operate and Optimize phases, and can lead back into the Plan and Design phases.

Maintaining Design Documentation

The design should be documented throughout the process. Documentation should include the following items:

• All the agreed-to requirements and constraints

• The state of the existing network, if any

• Preliminary design options and a brief review of why the final design was chosen

• Final design details

• Results of any pilot or prototype testing

• Deployment plans, schedules, and other implementation details

• Monitoring requirements

• Any other pertinent information

Documentation should be started in the PDIOO Design phase but might not be complete until well into the Implement phase. Updates to the design documentation can be made in the Operate and Optimize phases if redesign is required.