We start with 3 basic layers:
- Physical – (Cables, radio waves, binary 1′s and 0′s, etc).
- Data Link – (Switch/Bridge devices/MAC address)
- Network – (Routers)
Routers are the devices which are given the most focus. Here is the back of an example router:
The look of a router will vary greatly, but it’s function and purpose is essentially the same: Route IP traffic, as well as other management stuff we’ll discuss later.
Within the router is a Command Line Interface called the Cisco IOS. The traditional method used to access this operating system is to attach a rollover cable to the routers console port and the other end to a serial port on a PC. Then, using a terminal emulator like Hyper Terminal (included with Windows for free) to establish a serial connection. These settings need to be correct in order for the connection to work:
- Baud – 9600
- Databits – 8
- Parity – None
- Stop Bits – 1
The Aux port on a router can be setup to accept incoming connections from external devices like a phone modem, allowing for remote administration. Virtual Terminal ports can also be configured, allowing the administrator to set up any of the Ethernet ports on the router to accept incoming remote administrator telnet or SSH sessions.
When you establish a console session with a router, you begin in “user mode”, and you are very limited on what commands the router can accept from you. To escalate your privileges, you type “enable” and press enter. A password prompt can be configured later to ask for a password when this command is entered. A password can also be placed over the user mode as well, to prevent access by unauthorized users.
Tab-Completion of commands is supported, as well as ? inquires. If you type “con?” and press enter, the possible commands that begin with “con” will be displayed. If you type “config ?” sub commands that can be sent to config will be shown. This is handy if you forget your commands.
When a router first powers on, it goes through a Power On Self Test by loading a small ROM chip. This mode seeks out an IOS image file, which is the actual OS. The OS is usually stored on Flash memory.
To configure the router, you have to type “enable” to switch into Privlaged mode, then type “config term”. Then you can begin to configure other interfaces (more on that later).
Once you’re done with changing your configuration, you can save it to your startup config file by typing “copy run start”. You can also erase your config by typing “erase start”.
That’s all for now. More on the way.
Friday, March 21st, 2008