Chapter 1: Overview of Network Security

Cisco Press

At the same time networks are growing exponentially, they are becoming complex and mission critical, bringing new challenges to those who run and manage them. The need for integrated network infrastructure comprising voice, video, and data (all-in-one) services is evident, but these rapidly growing technologies introduce fresh security concerns. Therefore, as network managers struggle to include the latest technology in their network infrastructure, network security has become a pivotal function in building and maintaining today's modern high-growth networks.

This chapter presents a broad description of network security in the context of today's rapidly changing network environments. The security paradigm is changing, and security solutions today are solution driven and designed to meet the requirements of business. To help you face the complexities of managing a modern network, this chapter discusses the core principles of security—the CIA triad: confidentiality, integrity, and availability.

In addition to discussing CIA, this chapter discusses security policies that are the heart of all network security implementations. The discussion covers the following aspects of security policies: standards, procedures, baselines, guidelines, and various security models.

The chapter takes a closer look at the perimeter security issue and the multilayered perimeter approach. The chapter concludes with the Cisco security wheel paradigm involving five cyclical steps.

Fundamental Questions for Network Security

When you are planning, designing, or implementing a network or are assigned to operate and manage one, it is useful to ask yourself the following questions:

  1. What are you trying to protect or maintain?

  2. What are your business objectives?

  3. What do you need to accomplish these objectives?

  4. What technologies or solutions are required to support these objectives?

  5. Are your objectives compatible with your security infrastructure, operations, and tools?

  6. What risks are associated with inadequate security?

  7. What are the implications of not implementing security?

  8. Will you introduce new risks not covered by your current security solutions or policy?

  9. How do you reduce that risk?

  10. What is your tolerance for risk?

You can use these questions to pose and answer some of the basic questions that underlie fundamental requirements for establishing a secure network. Network security technologies reduce risk and provide a foundation for expanding businesses with intranet, extranet, and electronic commerce applications.

Solutions also protect sensitive data and corporate resources from intrusion and corruption.

Advanced technologies now offer opportunities for small and medium-sized businesses (SMB), as well as enterprise and large-scale networks to grow and compete; they also highlight a need to protect computer systems against a wide range of security threats.

The challenge of keeping your network infrastructure secure has never been greater or more crucial to your business. Despite considerable investments in information security, organizations continue to be afflicted by cyber incidents. At the same time, management aims for greater results with fewer resources. Hence, improving security effectiveness remains vital, if not essential, while enhancement of both effectiveness and flexibility has also become a primary objective.

Without proper safeguards, every part of a network is vulnerable to a security breach or unauthorized activity from intruders, competitors, or even employees. Many of the organizations that manage their own internal network security and use the Internet for more than just sending/receiving e-mails experience a network attack—and more than half of these companies do not even know they were attacked. Smaller companies are often complacent, having gained a false sense of security. They usually react to the last virus or the most recent defacing of their website. But they are trapped in a situation where they do not have the necessary time and resources to spend on security.

To cope with these problems, Cisco has developed the SAFE Blueprint, a comprehensive security plan that recommends and explains specific security solutions for different elements of networks.

Cisco also offers the integrated security solution, which delivers services above and beyond the "one size fits all" model. In addition, Cisco services are designed to deliver value throughout the entire network life cycle that includes the stages of prepare, plan, design, implement, operate, and optimize (PPDIOO). the Cisco PPDIOO model, as shown in Figure 1-1, encompasses all the steps from network vision to optimization, enabling Cisco to provide a broader portfolio of support and end-to-end solutions to its customers.

Figure 1-1

The Cisco PPDIOO Model

Transformation of the Security Paradigm

As the size of networks continues to grow and attacks to those networks become increasingly sophisticated, the way we think about security changes. Here are some of the major factors that are changing the security paradigm:

  • Security is no longer about "products": Security solutions must be chosen with business objectives in mind and integrated with operational procedures and tools.

  • Scalability demands are increasing: With the increasing number of vulnerabilities and security threats, solutions must scale to thousands of hosts in large enterprises.

  • Legacy endpoint security Total Cost of Ownership (TCO) is a challenge: Reactive products force deployment and renewal of multiple agents and management paradigms.

  • Day zero damage: Rapidly propagating attacks (Slammer, Nimda, MyDoom) happen too fast for reactive products to control. Therefore, an automated, proactive security system is needed to combat the dynamic array of modern-day viruses and worms.

With modern-day distributed networks, security cannot be enforced only at the network edge or perimeter. We will discuss perimeter security in more detail later in this chapter.

Zero-day attacks or new and unknown viruses continue to plague enterprises and service provider networks.

To attempt to establish protection against attacks, enterprises try to patch systems as vulnerabilities become known. This clearly cannot scale in large networks, and this situation can be addressed only with real-time proactive-based systems.

Security now is about management and reduction of risk in a rapidly evolving environment. Maximum risk reduction is achieved with an integrated solution built on a flexible and intelligent infrastructure and effective operations and management tools. Business objectives should drive security decisions. Today, we are in the new era that forces us to rethink security and outbreak prevention.

Principles of Security—The CIA Model

A simple but widely applicable security model is the confidentiality, integrity, and availability (CIA) triad. These three key principles should guide all secure systems. CIA also provides a measurement tool for security implementations. These principles are applicable across the entire spectrum of security analysis—from access, to a user's Internet history, to the security of encrypted data across the Internet. A breach of any of these three principles can have serious consequences for all parties concerned.

Figure 1-2

The CIA Triad


Confidentiality prevents unauthorized disclosure of sensitive information. It is the capability to ensure that the necessary level of secrecy is enforced and that information is concealed from unauthorized users. When it comes to security, confidentiality is perhaps the most obvious aspect of the CIA triad, and it is the aspect of security most often attacked. Cryptography and encryption methods are examples of attempts to ensure the confidentiality of data transferred from one computer to another. For example, when performing an online banking transaction, the user wants to protect the privacy of the account details, such as passwords and card numbers. Cryptography provides a secure transmission protecting the sensitive data traversing across the shared medium.


Integrity prevents unauthorized modification of data, systems, and information, thereby providing assurance of the accuracy of information and systems. If your data has integrity, you can be sure that it is an accurate and unchanged representation of the original secure information. A common type of a security attack is man-in-the-middle. In this type of attack, an intruder intercepts data in transfer and makes changes to it.


Availability is the prevention of loss of access to resources and information to ensure that information is available for use when it is needed. It is imperative to make sure that information requested is readily accessible to the authorized users at all times. Denial of service (DoS) is one of several types of security attacks that attempts to deny access to the appropriate user, often for the sake of disruption of service.

Policies, Standards, Procedures, Baselines, Guidelines

A security model is a multilayered framework made of many integrated entities and logical and physical protection mechanisms, all working together to provide a secure system that complies with industry best practices and regulations.

Security Policy

A security policy is a set of rules, practices, and procedures dictating how sensitive information is managed, protected, and distributed. In the network security realm, policies are usually point specific, which means they cover a single area. A security policy is a document that expresses exactly what the security level should be by setting the goals of what the security mechanisms are to accomplish. Security policy is written by higher management and is intended to describe the "whats" of information security. The next section gives a few examples of security policies. Procedures, standards, baselines, and guidelines are the "hows" for implementation of the policy. Information security policies underline the security and well-being of information resources; they are the foundation of information security within an organization.

Trust is one of the main themes in many policies. Some companies do not have policies because they trust in their people and trust that everyone will do the right thing. But, that is not always the case, as we all know. Therefore, most organizations need policies to ensure that everyone complies with the same set of rules.

In my experience, policies tend to elevate people's apprehension because people do not want to be bound by rules and regulations. Instead, people want freedom and non-accountability. A policy should define the level of control users must observe and balance that with productivity goals. An overly strict policy will be hard to implement because compliance will be minimal or ignored. On the contrary, a loosely defined policy can be evaded and does not ensure accountability and responsibility. A good policy has to have the right balance.

Examples of Security Policies

Depending on the size of the organization, potentially dozens of security policy topics may be appropriate. For some organizations, one large document covers all facets; at other organizations, several smaller, individually focused documents are needed. The sample list that follows covers some common policies that an organization should consider.

  • Acceptable use: This policy outlines the acceptable use of computer equipment. The rules are established to protect the employee and the organization. Inappropriate use exposes the company to risks including virus attacks, compromise of network systems and services, and legal issues.

  • Ethics: This policy emphasizes the employee's and consumer's expectations to be subject to fair business practices. It establishes a culture of openness, trust, and integrity in business practices. This policy can guide business behavior to ensure ethical conduct.

  • Information sensitivity: This policy is intended to help employees determine what information can be disclosed to nonemployees, as well as the relative sensitivity of information that should not be disclosed outside an organization without proper authorization. The information covered in these guidelines includes but is not limited to information that is either stored or shared via any means. This includes electronic information, information on paper, and information shared orally or visually (such as by telephone, video conferencing, and teleconferencing).

  • E-mail: This policy covers appropriate use of any e-mail sent from an organization's e-mail address and applies to all employees, vendors, and agents operating on behalf of the company.

  • Password: The purpose of this policy is to establish a standard for creation of strong passwords, the protection of those passwords, and the frequency of change.

  • Risk assessment: This policy is used to empower the Information Security (InfoSec) group to perform periodic information security risk assessments (RA) for the purpose of determining areas of vulnerability and to initiate appropriate remediation.

Tip - Examples of policies listed previously and other templates can be found at the SANS website:

Note - Policies need to be concise, to the point, and easy to read and understand. Most policies listed previously are on average two to three pages.


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