d autho- rized. In this section, I will cover only authentication, as it is the most commonly used feature offered by AAA. To use AAA authentication on a switch or router, you must perform the following steps: Enable AAA by entering the aaa new-model command. Configure security server information, if using a security server. Configuring TACACS+ and RADIUS information is included in this step. Create method lists by using the aaa authentication command. Apply the method lists to interfaces or lines as needed. Enabling AAA To use the AAA features discussed here, you'll need to issue the command aaa new-model: Router aaa new-model AAA Authentication If you don't execute this command, the AAA commands discussed in this section will not be available. Be careful when configuring AAA for the first time. You can easily lock yourself out of the router by enabling AAA authentication without configuring any users. Configuring Security Server Information One of the benefits of using AAA is the ability to use an external server for authenti- cation, authorization, and accounting. When an external server is used, all user information is stored externally to the networking device. Administration of user security is therefore centralized. This allows indivual users to access many devices, while also allowing the administrator to limit the users' access. RADIUS and TACACS+ are two protocols used for authentication and authoriza- tion applications. Each is used to authenticate users, though they can also be used for various other features (logging command usage, call detail records, and SO on). Both are widely used, and at some point, you'll need to decide which one to choose. Here's a quick rundown: RADIUS Livingston Enterprises (now Lucent Technologies) originally developed the Remote Authentication Dial-In User Service (RADIUS) for its PortMaster series of network access servers. These devices were widely used by ISPs in the days when 33.6-Kbps modems were the norm. RADIUS was later described in RFCs 2058 and 2059. It is now av