Step 1 Analyze the Requirements

This step details the process of determining expectations and then converting those expectations into a real network.

What Do You Know?

Going into Step 1, you know that an OSPF network is required, but you do not know what it will need to accomplish for your users or how you will physically design the network.

The needs of users are always changing. Nevertheless, as the engineer involved in the design of the network, you must still objectively listen and determine the users' needs. In the end, they are going to be the network's customers. You must also take into consideration what the future might hold for them. Therefore, always ask the users what needs they foresee. It is your responsibility to take their response and turn it into the requirements of the network.

A corporate vision is always important. For example, do the long-range corporate plans include having a website? If so, what will it be doing? Will the company be running voice over the network? Is videoconferencing going to be required?

Additional information that you should gather includes the current organization structure, organization locations, and flow of information within the organization as well as internal or external resources that are available to you. Armed with this information, your networks need analysis; you should then begin conducting a cost and benefit analysis. In many cases, you will not be able to get all the equipment or bandwidth that you think is necessary. Therefore, create a risk assessment that details the potential problems or areas of concern regarding the network design.

Consider the following issues when working through the network design process. I find that whenever I am called upon to design a network or respond to an Request for Proposal (RFP) that rereading this section benefits me greatly. Most of this information is common knowledge but a simple refresher will help you be more successful:

• Reliability—When designing networks, reliability is usually the most important goal, as the WAN is often the backbone of any network. There are several levels of redundancy, for example, physical redundancy via the presence of backup routers or logical redundancy via extra circuits between key sites. The resiliency and thus the reliability of a network is related, so plan according to your budget and customer expectations.

• Latency—At certain times, network access requests can take a long time to be granted. Users should be notified about a latency problem in the network whenever possible. When implementing your network, test its latency to ensure that this is not a problem.

Investigate which applications are to be used. This information determines how much latency is acceptable and helps you design the network appropriately.

Cost of WAN resources—WAN resources are expensive, and as such, frequently involve a trade-off between cost efficiency and full network redundancy. Cost efficiency is usually preferred when compared to price, so adjust the network design to reflect the most efficient solution.

Amount of traffic—You must be able to accurately determine the amount of traffic that will be on the network to properly size its various components. As you implement the network, develop a baseline that can be used to project future growth. To do so, investigate the current network and determine the traffic levels and types.

Multiple protocols on the WAN—The simplicity of IP is of great benefit to any network. For example, by allowing only IP-based protocols on the network, you avoid the unique addressing and configuration issues that are related to other protocols. Therefore, you should not allow multiple non-IP protocols in a network—especially in the backbone.

Compatibility with standards or legacy systems—Compatibility will always be an issue with your network throughout its life. As a network designer, keep this in mind as you proceed. Business considerations are likely to force compatibility issues on you and the network.

Simplicity and ease of configuration—You might be involved in only the design and implementation of the network and not the management of it. In that case, the knowledge that you develop must be passed on to those who will manage the network. Consider simplicity and ease of configuration while you develop your design documents for the network.

Support for remote offices and telecommuters—In today's telecommunications environment, satellite offices are becoming commonplace and require network connectivity, so plan accordingly. Furthermore, the number of telecommuters is on the rise. Remember this as you determine the placement of network components to ensure that they can handle this requirement when it becomes a priority for your organization.

OSPF Deployment

As you determine the network requirements, some important questions exist regarding the requirements of OSPF. The answers to the following questions can help you further define the requirements of your OSPF network:

• How should the OSPF AS be delineated? How many areas should it have, and what should the boundaries be?

• Does your network and its data require built-in security? For example, is the encryption of data or authentication of routes required?

• What information from other autonomous systems should be imported into your network?

• Which sites will have links that should be preferred (lower cost)?

• Which sites will have links that should be avoided (higher cost)?

170 Chapter 4: Design Fundamentals

Load Balancing with OSPF

As you determine the network requirements, note the load-balancing feature of OSPF. In the Cisco implementation of OSPF, any router can, by default, support up to four equal-cost routes to a destination. When a failure to the destination is recognized, OSPF immediately switches to the remaining paths. OSPF can support a maximum of six equal-cost paths if it has been configured to do so.

OSPF automatically performs load balancing across or over equal-cost paths to a given destination. The cost associated is determined (by default) by the interface bandwidth statement, unless OSPF is configured to maximize multiple-path routing (that is, to use six paths instead of the default of four).

Cisco IOS Software calculates cost for OSPF by dividing 100 million by the configured bandwidth of the interface, as illustrated in Figure 4-3.

Figure 4-3 OSPF Costs

OSPF Cost is calculated as follows: 10s divided by bandwidth



Token Ring

Ethernet and Fast Ethernet

Gigabit Ethernet






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