Bringing Up a Switch
When you first bring up a Cisco IOS device, it will run a power-on self- test—a POST. Upon passing that, the machine will look for and then
load the Cisco IOS from flash memory if an IOS file is present, then expand it into RAM. As you probably know, flash memory is
electronically erasable programmable read-only memory—an EEPROM. The next step is for the IOS to locate and load a valid configuration known as the startup-config that will be stored in
nonvolatile RAM (NVRAM) .
Once the IOS is loaded and up and running, the startup-config will be copied from NVRAM into RAM and from then on referred to as the
But if a valid startup-config isn’t found in NVRAM, your switch will enter setup mode, giving you a step-by-step dialog to help configure
some basic parameters on it.
You can also enter setup mode at any time from the command line by typing the command setup from privileged mode, which I’ll get to in a minute. Setup mode only covers some basic commands and generally
isn’t really all that helpful. Here’s an example:
Would you like to enter the initial configuration dialog?
At any point you may enter a question mark '?' for help.
Use ctrl-c to abort configuration dialog at any prompt.
Default settings are in square brackets ''.
Basic management setup configures only enough connectivity
for management of the system, extended setup will ask you
to configure each interface on the system
Would you like to enter basic management setup? [yes/no]: y
Configuring global parameters:
Enter host name [Switch]: Ctrl+C
Configuration aborted, no changes made.
You can exit setup mode at any time by pressing Ctrl+C.
I highly recommend going through setup mode once, then never again because you should always use the CLI instead!Command-Line Interface (CLI)
I sometimes refer to the CLI as “cash line interface” because the ability to create advanced configurations on Cisco routers and switches using the CLI will earn you some decent cash!
Entering the CLI
After the interface status messages appear and you press Enter, the Switch> prompt will pop up. This is called user exec mode , or user mode for short, and although it’s mostly used to view statistics, it is also a stepping stone along the way to logging in to privileged exec
mode , called privileged mode for short.
You can view and change the configuration of a Cisco router only while in privileged mode, and you enter it via the enable command like this:
The Switch# prompt signals you’re in privileged mode where you can both view and change the switch configuration. You can go back from privileged mode into user mode by using the disable command:
You can type logout from either mode to exit the console:
Switch con0 is now available
Press RETURN to get started.
Next, I’ll show how to perform some basic administrative configurations.
Overview of Router Modes
To configure from a CLI, you can make global changes to the router by typing configure terminal or just config t . This will get you into global configuration mode where you can make changes to therunning-config. Commands run from global configuration mode are predictably referred to as global commands, and they are typically set
only once and affect the entire router.
Type config from the privileged-mode prompt and then press Enter to opt for the default of terminal like this:
Configuring from terminal, memory, or network [terminal]? [ press
Enter configuration commands, one per line. End with CNTL/Z.
At this point, you make changes that affect the router as a whole (globally), hence the term global configuration mode . For instance, to
change the running-config—the current configuration running in dynamic RAM (DRAM)—use the configure terminal command, as I just demonstrated.