Cisco Router Practice Software

[Heberto Calderon]

The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.

This document describes the information to help you secure your Cisco IOS system devices, which increases the overall security of your network. Structured around the three planes into which functions of a network device can be categorized, this document provides an overview of each included feature and references to related documentation.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

The coverage of security features in this document often provides enough detail for you to configure the feature. However, in cases where it does not, the feature is explained in such a way that you can evaluate whether additional attention to the feature is required. Where possible and appropriate, this document contains recommendations that, if implemented, help secure a network.

Secure network operations is a substantial topic. Although most of this document is devoted to the secure configuration of a Cisco IOS device, configurations alone do not completely secure a network. The operational procedures in use on the network contribute as much to security as the configuration of the underlying devices.

The Cisco Product Security Incident Response Team (PSIRT) creates and maintains publications, commonly referred to as PSIRT Advisories, for security-related issues in Cisco products. The method used for communication of less severe issues is the Cisco Security Response. Security advisories and responses are available at

In order to maintain a secure network, you need to be aware of the Cisco security advisories and responses that have been released. You need to have knowledge of a vulnerability before the threat it can pose to a network can be evaluated. Refer to Risk Triage for Security Vulnerability Announcements for assistance this evaluation process.

The Authentication, Authorization, and Accounting (AAA) framework is vital to secure network devices. The AAA framework provides authentication of management sessions and can also limit users to specific, administrator-defined commands and log all commands entered by all users. See the Authentication, Authorization, and Accounting section of this document for more information about how to leverage AAA.

In order to gain knowledge about existing, emerging, and historic events related to security incidents, your organization must have a unified strategy for event logging and correlation. This strategy must leverage logging from all network devices and use pre-packaged and customizable correlation capabilities.

After centralized logging is implemented, you must develop a structured approach to log analysis and incident tracking. Based on the needs of your organization, this approach can range from a simple diligent review of log data to advanced rule-based analysis.

Many protocols are used in order to carry sensitive network management data. You must use secure protocols whenever possible. A secure protocol choice includes the use of SSH instead of Telnet so that both authentication data and management information are encrypted. In addition, you must use secure file transfer protocols when you copy configuration data. An example is the use of the Secure Copy Protocol (SCP) in place of FTP or TFTP.

NetFlow enables you to monitor traffic flows in the network. Originally intended to export traffic information to network management applications, NetFlow can also be used in order to show flow information on a router. This capability allows you to see what traffic traverses the network in real time. Regardless of whether flow information is exported to a remote collector, you are advised to configure network devices for NetFlow so that it can be used reactively if needed.

Configuration management is a process by which configuration changes are proposed, reviewed, approved, and deployed. Within the context of a Cisco IOS device configuration, two additional aspects of configuration management are critical: configuration archival and security.

You can use configuration archives to roll back changes that are made to network devices. In a security context, configuration archives can also be used in order to determine which security changes were made and when these changes occurred. In conjunction with AAA log data, this information can assist in the security auditing of network devices.

The configuration of a Cisco IOS device contains many sensitive details. Usernames, passwords, and the contents of access control lists are examples of this type of information. The repository that you use in order to archive Cisco IOS device configurations needs to be secured. Insecure access to this information can undermine the security of the entire network.

The management plane consists of functions that achieve the management goals of the network. This includes interactive management sessions that use SSH, as well as statistics-gathering with SNMP or NetFlow. When you consider the security of a network device, it is critical that the management plane be protected. If a security incident is able to undermine the functions of the management plane, it can be impossible for you to recover or stabilize the network.

The management plane is used in order to access, configure, and manage a device, as well as monitor its operations and the network on which it is deployed. The management plane is the plane that receives and sends traffic for operations of these functions. You must secure both the management plane and control plane of a device, because operations of the control plane directly affect operations of the management plane. This list of protocols is used by the management plane:

Passwords control access to resources or devices. This is accomplished through the definition a password or secret that is used in order to authenticate requests. When a request is received for access to a resource or device, the request is challenged for verification of the password and identity, and access can be granted, denied, or limited based on the result. As a security best practice, passwords must be managed with a TACACS+ or RADIUS authentication server. However, note that a locally configured password for privileged access is still needed in the event of failure of the TACACS+ or RADIUS services. A device can also have other password information present within its configuration, such as an NTP key, SNMP community string, or Routing Protocol key.

The enable secret command is used in order to set the password that grants privileged administrative access to the Cisco IOS system. The enable secret command must be used, rather than the older enable password command. The enable password command uses a weak encryption algorithm.

If no enable secret is set and a password is configured for the console tty line, the console password can be used in order to receive privileged access, even from a remote virtual tty (vty) session. This action is almost certainly unwanted and is another reason to ensure configuration of an enable secret.

The service password-encryption global configuration command directs the Cisco IOS software to encrypt the passwords, Challenge Handshake Authentication Protocol (CHAP) secrets, and similar data that are saved in its configuration file. Such encryption is useful in order to prevent casual observers from reading passwords, such as when they look at the screen over the muster of an administrator. However, the algorithm used by the service password-encryption command is a simple Vigen re cipher. The algorithm is not designed to protect configuration files against serious analysis by even slightly sophisticated attackers and must not be used for this purpose. Any Cisco IOS configuration file that contains encrypted passwords must be treated with the same care that is used for a cleartext list of those same passwords.

While this weak encryption algorithm is not used by the enable secret command, it is used by the enable password global configuration command, as well as the password line configuration command. Passwords of this type must be eliminated and the enable secret command or the Enhanced Password Security feature needs to be used.

The enable secret command and the Enhanced Password Security feature use Message Digest 5 (MD5) for password hashing. This algorithm has had considerable public review and is not known to be reversible. However, the algorithm is subject to dictionary attacks. In a dictionary attack, an attacker tries every word in a dictionary or other list of candidate passwords in order to find a match. Therefore, configuration files must be securely stored and only shared with trusted individuals.

The feature Enhanced Password Security, introduced in Cisco IOS Software Release 12.2(8)T, allows an administrator to configure MD5 hashing of passwords for the username command. Prior to this feature, there were two types of passwords: Type 0, which is a cleartext password, and Type 7, which uses the algorithm from the Vigen re cipher. The Enhanced Password Security feature cannot be used with protocols that require the cleartext password to be retrievable, such as CHAP.

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