Initial startup of Cisco routers

Initial startup of Cisco routers
2.2.1 This page will explain the startup process for Cisco routers.


A router initializes by loading the bootstrap, the operating system, and a configuration file. If the router cannot find a configuration file, it enters setup mode. Upon completion of the setup mode, a backup copy of the configuration file may be saved to NVRAM.

The goal of the startup routines for Cisco IOS software is to start the router operations. To do this, the startup routines must accomplish the following:

• Verify that the router hardware is tested and functional.

• Find and load the Cisco IOS software.

• Find and apply the startup configuration file or enter the setup mode.

When a Cisco router powers up, it performs a power-on self test (POST). During this self test, the router executes diagnostics from ROM on all hardware modules. These diagnostics verify the basic operation of the CPU, memory, and network interface ports. After verifying the hardware functions, the router proceeds with software initialization.

After the POST, the following events occur as the router initializes:

1. The generic bootstrap loader in ROM executes. A bootstrap is a simple set of instructions that tests hardware and initializes the IOS for operation.

2. The IOS can be found in several places. The boot field of the configuration register determines the location that is used to load the IOS. If the boot field indicates a flash or network load, boot system commands in the configuration file indicate the exact name and location of the image.

3. The operating system image is loaded. When the IOS is loaded and operational, a listing of the available hardware and software components is sent to the console terminal screen.

4. The configuration file saved in NVRAM is loaded into main memory and executed one line at a time. The configuration commands start routing processes, supply addresses for interfaces, and define other operating characteristics of the router.

5. If no valid configuration file exists in NVRAM, the operating system searches for an available TFTP server. If no TFTP server is found, the setup dialog is initiated.

Setup mode is not intended to be used to enter complex protocol features in a router. The purpose of the setup mode is to permit administrators to install a basic configuration for routers when a configuration cannot be obtained from another source.

In the setup mode, default answers appear in square brackets [ ] following the question. Press the Enter key to use these defaults. During the setup process, Ctrl-C can be pressed at any time to terminate the process. When Ctrl-C is used to terminate setup, all interfaces are administratively shut down.

When the configuration process is completed in setup mode, the following options will be displayed:

[0] Go to the IOS command prompt without saving this config.
[1] Return back to the setup without saving this config.
[2] Save this configuration to nvram and exit. Enter your selection [2]:

The next page will discuss router LED indicators.