This section looks at a router with a clean configuration. First we walk through and erase the startup-config file stored in NVRAM. After that, we look at the default configuration of a router.
Critical Information

If you use the write erase or erase startup-config commands and then either reload your router or power it off and on, you’ll see the system configuration dialog screen. You can also type setup within privileged mode to get the screen at any time, which can be helpful in configuring your router.

The setup command can be useful, but you won’t necessarily use it in a production environment. It may prompt you for commands that you don’t use in your internetwork, which can be tedious. When going through this section, notice the difference between configuring your router using the initial setup and the setup command, including which management prompts you are using.

If you erase the startup-config file and reboot your router, you will see the following upon bootup:

Notice: NVRAM invalid, possibly due to write erase.
— System Configuration Dialog —
Enter a question mark ‘?’ for help.
Use CTRL-c to abort configuration dialog at any time.
Default settings are in square brackets ‘[]‘.
Would you like to enter the initial configuration Æ’dialog? [yes][Enter]

Press Enter at the [yes] prompt to continue with the configuration and see how the router responds:

First, would you like to see the current interface Æ’summary? [yes][Enter]
Any interface listed with OK? value “NO” does not have a Æ’valid configuration
Interface IP-Address OK? Method Status Protocol
Ethernet0 unassigned NO not set up down
Serial0 unassigned NO not set up down
Serial1 unassigned NO not set up down
Configuring global parameters:

By pressing Enter at the [yes] prompt to view the current interface summary, you’re assured that the power-on self-test (POST) has found all the interfaces.

In this case, you have two serial ports and one Ethernet port. They aren’t okay (see the NO not set next to them?), because they aren’t set up yet. They’re designated as unassigned. The router now wants the name of the router you’re trying to configure, which is RouterA in this example. Type that now and then press Enter. The router responds with this dialog (we’ll show the passwords from here on, but remember that they won’t be seen on-screen):

Enter hostname [Router]:RouterA
The enable secret is a one-way cryptographic secret Æ’used instead of the enable password when it exists.
Enter enable secret:sean
The enable password is used when there is no enable Æ’secret and when using older software and some boot Æ’images.
Enter enable password:seanx
Enter virtual terminal password:seanvty
Configure SNMP Network Management? [yes]:n
Configure IP? [yes][Enter]
Configure IP? [yes][Enter]
Configure IGRP routing? [yes]:n
Configure RIP routing? [no]:[Enter]

Configuring interface parameters:

Configuring interface Ethernet0:
Is this interface in use? [yes][Enter]
Configure IP on this interface? [yes][Enter]
IP address for this interface:172.16.10.1
Number of bits in subnet field [0]:8
Class B network is 172.16.0.0, 8 subnet bits;
Æ’mask is 255.255.255.0

Configuring interface Serial0:

Is this interface in use? [yes]:[Enter]
Configure IP on this interface? [yes]:[Enter]
Configure IP unnumbered on this interface?
Æ’[no]:[Enter]
IP address for this interface:172.16.20.1
Number of bits in subnet field [8]:[Enter]
Class B network is 172.16.0.0, 8 subnet bits;
Æ’mask is 255.255.255.0
Configuring interface Serial1:
Is this interface in use? [yes]:n

Notice that we had you reply no to the routing commands like RIP and IGRP. Also notice that for the subnet mask, you were to enter 8 for the number of bits. The router doesn’t count the default masks in the number of bits, so even though you set the number of bits to 8, the subnet mask will still be realized as 255.255.255.0. The third byte is the only one used for subnetting.

After you answer no to configuring Serial1, the router shows the configuration it created. It then asks whether you want to save the configuration:
Use this configuration? [yes/no]:y

Type y for yes, and then press Enter. The router saves the configuration to nonvolatile RAM

Related posts:

1. Router Configuration using Setup Command
2. Manage Router Configuration Files from the Privileged EXEC Mode
3. Control Router Passwords, Identification and Banner
4. Log in to a Router in Both User and Privileged Modes
5. Control Router Passwords
6. Examine Router Elements
7. Configure IP Addresses
8. Command History and Editing Features
9. Configure IP addresses Exams Points
10. Commands to Monitor Frame Relay Operation in the Router