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Router Bootup Process

There are four major phases to the bootup process:

1. Performing the POST

2. Loading the bootstrap program

3. Locating and loading the Cisco IOS software

4. Locating and loading the startup configuration file or entering setup mode

1. Performing the POST

The Power-On Self Test (POST) is a common process that occurs on almost every computer during bootup. The POST process is used to test the router hardware. When the router is powered on, software on the ROM chip conducts the POST. During this self-test, the router executes diagnostics from ROM on several hardware components including the CPU, RAM, and NVRAM. After the POST has been completed, the router executes the bootstrap program.

2. Loading the Bootstrap Program

After the POST, the bootstrap program is copied from ROM into RAM. Once in RAM, the CPU executes the instructions in the bootstrap program. The main task of the bootstrap program is to locate the Cisco IOS and load it into RAM.

Note: At this point, if you have a console connection to the router, you will begin to see output on the screen.

3. Locating and Loading Cisco IOS

Locating the Cisco IOS software. The IOS is typically stored in flash memory, but can also be stored in other places such as a TFTP (Trivial File Transfer Protocol) server.

If a full IOS image can not be located, a scaled-down version of the IOS is copied from ROM into RAM. This version of IOS is used to help diagnose any problems and can be used to load a complete version of the IOS into RAM.

Note: A TFTP server is usually used as a backup server for IOS but it can also be used as a central point for storing and loading the IOS. IOS management and using the TFTP server is discussed in a later course.

Loading the IOS. Some of the older Cisco routers ran the IOS directly from flash, but current models copy the IOS into RAM for execution by the CPU.

Note: Once the IOS begins to load, you may see a string of pounds signs (#), as shown in the figure, while the image decompresses.

4. Locating and Loading the Configuration File

Locating the Startup Configuration File. After the IOS is loaded, the bootstrap program searches for the startup configuration file, known as startup-config, in NVRAM. This file has the previously saved configuration commands and parameters including:
interface addresses
routing information
any other configurations saved by the network administrator

If the startup configuration file, startup-config, is located in NVRAM, it is copied into RAM as the running configuration file, running-config.

Note: If the startup configuration file does not exist in NVRAM, the router may search for a TFTP server. If the router detects that it has an active link to another configured router, it sends a broadcast searching for a configuration file across the active link. This condition will cause the router to pause, but you will eventually see a console message like the following one:

%Error opening tftp:// (Timed out)
%Error opening tftp:// (Timed out)

Executing the Configuration File. If a startup configuration file is found in NVRAM, the IOS loads it into RAM as the running-config and executes the commands in the file, one line at a time. The running-config file contains interface addresses, starts routing processes, configures router passwords and defines other characteristics of the router.

Enter Setup Mode (Optional). If the startup configuration file can not be located, the router prompts the user to enter setup mode. Setup mode is a series of questions prompting the user for basic configuration information. Setup mode is not intended to be used to enter complex router configurations, and it is not commonly used by network administrators.

When booting a router that does not contain a startup configuration file, you will see the following question after the IOS has been loaded:

Would you like to enter the initial configuration dialog? [yes/no]: no

Setup mode will not be used in this course to configure the router. When prompted to enter setup mode, always answer no. If you answer yes and enter setup mode, you can press Ctrl-C at any time to terminate the setup process.

When setup mode is not used, the IOS creates a default running-config. The default running-config is a basic configuration file that includes the router interfaces, management interfaces, and certain default information. The default running-config does not contain any interface addresses, routing information, passwords, or other specific configuration information.

Command Line Interface

Depending on the platform and IOS, the router may ask the following question before displaying the prompt:

Would you like to terminate autoinstall? [yes]:
Press the Enter key to accept the default answer.

Note: If a startup configuration file was found, the running-config may contain a hostname and the prompt will display the hostname of the router.

Once the prompt displays, the router is now running the IOS with the current running configuration file. The network administrator can now begin using IOS commands on this router.

Note: The bootup process is discussed in more detail in a later course.
Verifying Router Bootup Process

The show version command can be used to help verify and troubleshoot some of the basic hardware and software components of the router. The show version command displays information about the version of the Cisco IOS software currently running on the router, the version of the bootstrap program, and information about the hardware configuration, including the amount of system memory.

The output from the show version command includes:

IOS version

Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)

This is the version of the Cisco IOS software in RAM and that is being used by the router.

ROM Bootstrap Program

ROM: System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)

This shows the version of the system bootstrap software, stored in ROM memory, that was initially used to boot up the router.

Location of IOS

System image file is "flash:c2600-i-mz.122-28.bin"

This shows where the boostrap program is located and loaded the Cisco IOS, and the complete filename of the IOS image.

CPU and Amount of RAM

cisco 2621 (MPC860) processor (revision 0x200) with 60416K/5120K bytes of memory

The first part of this line displays the type of CPU on this router. The last part of this line displays the amount of DRAM. Some series of routers, like the 2600, use a fraction of DRAM as packet memory. Packet memory is used for buffering packets.

To determine the total amount of DRAM on the router, add both numbers. In this example, the Cisco 2621 router has 60,416 KB (kilobytes) of free DRAM used for temporarily storing the Cisco IOS and other system processes. The other 5,120 KB is dedicated for packet memory. The sum of these numbers is 65,536K, or 64 megabytes (MB) of total DRAM.

Note: It may be necessary to upgrade the amount of RAM when upgrading the IOS.


2 FastEthernet/IEEE 802.3 interface(s)
2 Low-speed serial(sync/async) network interface(s)

This section of the output displays the physical interfaces on the router. In this example, the Cisco 2621 router has two FastEthernet interfaces and two low-speed serial interfaces.

Amount of NVRAM

32K bytes of non-volatile configuration memory.

This is the amount of NVRAM on the router. NVRAM is used to store the startup-config file.

Amount of Flash

16384K bytes of processor board System flash (Read/Write)

This is the amount of flash memory on the router. Flash is used to permanently store the Cisco IOS.

Note: It may be necessary to upgrade the amount of flash when upgrading the IOS.

Configuration Register

Configuration register is 0x2102

The last line of the show version command displays the current configured value of the software configuration register in hexadecimal. If there is a second value displayed in parentheses, it denotes the configuration register value that will be used during the next reload.

The configuration register has several uses, including password recovery. The factory default setting for the configuration register is 0x2102. This value indicates that the router will attempt to load a Cisco IOS software image from flash memory and load the startup configuration file from NVRAM.

Note: The configuration register is discussed in more detail in a later course.
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Netgear router support said...

It seems to be a complete guide for the router setup and configuration issues.

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