Author: Dan DiNicolo

In order to help ensure the success of any network design project, the implementation process for the network needs to be carefully planned. The main element of this step involves the creation of documentation that includes detailed instructions to be followed by the engineers that will ultimately handle the implementation process.

The methods used to document a network implementation plan are often different based on the preferences of both the customer and the network designer. In some cases, documents are created for specific purposes and use tables or stepped instructions that will ultimately walk the engineer through every step of a certain process. Another method involves the creation of a master document that contains cross-references that engineers can refer to for more information. For example, if a design project involves the implementation of three PIX firewalls with different configurations, the implementation plan might include one set of generic configuration settings to be applied to each system, and then specific sub-sections that provide the unique settings for each individual system. Regardless of the specific task to be carried out, it is essential that the network designer not assume anything about what the engineers should be familiar with; good implementation documentation provides instructions that even the most junior engineer should be able to follow.

In order to ensure that the implementation plan includes adequate levels of detail, the following information should always be included in the documentation:

• Detailed descriptions of each step, in order to reduce any issues associated with misinterpretation on the part of engineers.
• References to other parts of the design document for more information as required.
• Implementation guidelines that outline issues such as any other configuration settings that a particular step is dependent upon. For example, the details of configuring OSPF on a router might depend upon basic router settings having been already completed.
• Estimated time required for each step, so that the implementation can be scheduled effectively.
• Detailed fallback instructions that can be carried out in cases where the steps result in a serious network problem or failure.

After the individual modules of the new or upgraded network are defined, the specific topologies, technologies, protocols, and network services should be designed. Examples of these elements include:

Network topology design. The design of the new network topology needs to consider both the LANs to be implemented in any offices, as well as the WAN links interconnecting locations. Both the physical and logical elements of the proposed network should be documented, with appropriate diagrams for each. For equipment, a physical diagram would show the interconnection of network elements, while a logical diagram would display the layout of elements like VLANs, network modules, and so on.

Network technology design. The technologies associated with the new network also need to be planned at diagrams. For example, the design may specify that Fast Ethernet is to be used for client connection at the access layer, while the links connecting different layers use redundant fiber-optic Gigabit Ethernet in a redundant design. For WAN links, the technologies to be used should be specified, along with information about key elements such as link speeds.

Network protocol design. The network protocols to be implemented on the new network needs to be specified, along with their scope. For example, the entire network may run TCP/IP as its primary protocol, but certain portions of the network may require the use of other protocols such as IPX/SPX or NetBEUI.

Logical addressing design. The network design needs to include information about the design of logical addressing on the network. For example, a network may use public or private IP addresses internally, along with different hierarchical addressing schemes like VLSM or CIDR. In the case of private IP addressing, connectivity through the use of services like network address translation (NAT) or proxy servers must also be addressed.

Routing infrastructure and protocol design. The routing infrastructure of the network also needs to be considered when developing the network design. For example, will the network use traditional routers only, or some combination that includes Layer 3 switching? Outside of the physical equipment to be implemented, the routing protocols to be used needs to be specified and documented. In a network design using OSPF as its only routing protocol, information about elements such as areas and autonomous systems need to be included. Similarly, the proposed hardware that will be used to implement the design, such as specific switch and router models and specifications, must be fully documented

Security design. The security design of the network can encompass a variety of hardware and software features. The need for and placement of elements like firewalls and intrusion detection systems much be documented, as should the particular hardware and software that will carry out these functions.

Advanced features design. Any advanced features to be implemented on the new or upgraded network need to be carefully documented, ensuring that any proposed equipment provides the capabilities that these features require. For example, a new network might require QoS features for a VoIP implementation, or need to include IP Multicasting capabilities in order to support a streaming media application.

Although most of the network applications, features, and equipment required in a network design fall into one of the categories listed above, these are not the only possibilities that exist. The general rule of thumb to be followed is to be sure that any new or upgraded network element that falls into the scope of the project should be fully documented and diagrammed as appropriate. This information will ultimately be provided to the customer as part of the network design document that will be developed as part of step 6 in the structured network design process.

Once the customer requirement have been finalized, all goals and constraints defined, and the state of the current network assessed, it’s time to begin designing the new or upgraded network. Earlier in this chapter the two major approaches to network design were discussed, and you should already be aware that the top-down approach is preferable in almost all circumstances. Using this method, a network is designed with the specific application and service requirements of the customer, as well as a very high-level concept of the network infrastructure that will be needed. In particular, the top-down approach should begin by considering the following general requirements:

Functionality. The new design must meet all of the functional requirements for different applications and services as defined by the customer, and augmented by your analysis. All of the functional requirements of the network, such as the need to support VoIP or IP Multicasting, should be defined and well understood at this point.

Performance. The performance requirements for the new or upgraded network must also be defined and understood as part of the design process. All of the proposed applications and services must be understood with respect to the impact that they will have both individually and together on the new network. Other examples of factors that will influence the performance of a network include the number of users that the network needs to support, the speed of the network, and so forth.

Scalability. Although current needs usually have the biggest impact on network design, it is imperative that planned future expansion also be considered. For example, a company may be planning to implement new applications or services in the next 12 months, or be planning to merge with another organization. The design must factor in these elements, including how the new design will allow the network to scale to meet these needs as necessary.

Availability. The availability requirements of a network also have a tremendous impact on any network design project. If a customer’s network or certain applications and services are considered to be mission-critical in nature, the new design must take factors like redundancy into consideration. This may ultimately involve connections to multiple ISPs, the need for redundant switching paths, and so forth. At this stage in the process, a designer is not usually worried about the specific technologies or methods to be used to ensure this availability, but instead concentrates on defining where redundancy needs to be considered.

Security. The security of a new network is a critical component of any design. In the top-down approach, the security needs of an organization should be considered at the beginning of the design process rather than as an afterthought. Again, elements like specific firewalls or intrusion detection systems are not the focus at this point. Instead, the network designer should be focused on the specific network applications and services that need to be secured, and how these requirements will impact the rest of the design process.

Budget. As mentioned in the business constraints section earlier, any network design project needs to consider any budgetary limitations that exist. The designer must be aware of and consider these limitations at each stage of designing an appropriate solution, based on the importance that the customer has associated with various planned applications and services. Ultimately, the budget associated with the project will impact everything from the equipment chosen through to project staffing.

After considering the general network requirements listed above, a network designer using the top-down approach would continue by organizing the network into what are referred to as “modules”. A module is a distinct portion of a network that is responsible for a specific and particular role. For example, the WAN module would be concerned with the equipment and links that would make communication with other locations possible. The network management module would be focused on the applications and equipment required to manage and monitor a network, such as a network management system (NMS) like CiscoWorks.

The model used by Cisco to segment a network into distinct modules is known as the Enterprise Composite Model. This model, along with the functional areas and modules it is comprised of, will be looked at in more detail in future articles.

Once information about the existing network has been gathered using the various methods and tools outlined in the section, it is typically compiled into a document that summarizes the health of the existing network. This document includes information about the status of current network equipment, links, and applications, while also documenting key elements like the physical and logical topology of the network.

The main purpose of this summary report is to provide the customer with a frame of reference that identifies some of the key issues that need to be considered as part of the network design project. For example, the summary report may provide information about some of the limitations of existing hardware, such as routers that will require an IOS upgrade, more resources (like RAM), or even outright replacement. Along the same lines, this document is used to pinpoint and ultimately address the limitations of the current network, ensuring that the customer has the information necessary to make critical decisions about the scope of the project and how it will ultimately move forward.

A variety of different tools and utilities exist for the purpose of network traffic analysis. While some of these tools were developed by Cisco, others are popular third-party alternatives. Each of the bullet points listed below outlines a particular tool or utility, along with examples of the types of information that the tool provides.

• Network-Based Application Recognition (NBAR). NBAR is a network traffic classification engine that is able to recognize the traffic flows associated with a wide variety of popular network applications. NBAR is not a separate network utility, but instead a feature found in versions of Cisco’s IOS software.
• Cisco IOS NetFlow. NetFlow is one of the switching methods available on Cisco routers and switches that allows data to be gathered about the specific traffic flows that traverse the device. This information can be viewed locally on the router, or collected centrally by an application like Cisco FlowCollector and Data Analyzer. Ultimately, NetFlow data can be used to identify network applications, as well as network utilization information.
• Third-party tools. Third-party hardware and software tools are also commonly used to analyze network traffic. Examples include protocol analyzers like Sniffer, EtherPeek, Ethereal, and Microsoft’s Network Monitor utility. Another tool commonly used to analyze network traffic patterns and utilization is MRTG, which was mentioned in the network auditing section.

Aside from auditing a customer’s current network using various network management tools, utilities, and Cisco IOS commands, analyzing existing applications and network traffic also provides accurate sources of information to characterize a network.

As part of most network design projects, the customer will usually provide a list of existing network-related applications, either verbally or through various forms of documentation. In order to ensure that all applications are properly accounted for and their impact on the network understood, a network designer must perform a careful analysis that includes a combination of documentation, prioritization, and traffic analysis. Each of these elements is looked at in more detail below:

• Documentation. Although a customer will generally provide a network designer with a list of the network-related applications in use on the current network, it is important to investigate whether this list is complete and current. One effective way to summarize information about the current applications in use is through the use of a decision matrix, similar to the one looked at earlier in Table 12-2.
• Prioritization and features. After the application decision matrix has been developed, it should be provided to the customer in order to determine the priority or level of importance associated with each application. Similarly, any particular features associated with the application should be documented. For example, certain application may have very specific security requirements.
• Traffic analysis. In order to determine the impact of various applications on the network, and to be sure that the documented list of applications is complete, a network designer should typically conduct network traffic analysis. In a manner similar to auditing a network, various tools and utilities can be used to gather accurate traffic analysis information.

The following article takes a look at some of the network traffic analysis tools, utilities, and IOS commands typically used to gather accurate information about the current network.