Content Overview A Cisco router cannot
operate without the Cisco Internetworking Operating System
(IOS). Each Cisco router has a predetermined boot-up sequence
for locating and loading the IOS. This module will describe the
stages and importance of this boot up procedure. Cisco
internetworking devices operate using several different files,
including Cisco Internetwork Operating System (IOS) images and
configuration files. A network administrator who wishes to keep
the network running smoothly and reliably must carefully manage
these files to ensure that the proper versions are used and
that necessary backups are performed. This module also
describes the Cisco file system and provides the tools to
manage it effectively. Students completing this module should
be able to: - Identify the stages of the router boot
sequence
- Determine how a Cisco device locates and
loads the Cisco IOS
- Use the boot system command
- Identify the configuration register values
- Briefly describe the files used by the Cisco IOS and their
functions
- List the locations on the router of the
different file types
- Briefly describe the parts of
the IOS name
- Save and restore configuration files
using TFTP and copy-and-paste
- Load an IOS image using
TFTP
- Load an IOS image using XModem
- Verify
the file system using show commands
Content
5.1 Router Boot Sequence and Verification
5.1.1 Stages of the router power-on boot sequence
The goal of the startup routines for Cisco IOS software is to
start the router operations. The router must deliver reliable
performance in its job of connecting any configured networks.
To do this, the startup routines must do the following:
- Test the router hardware.
- Find and load the Cisco
IOS software.
- Find and apply configuration
statements, including protocol functions and interface
addresses.
Figure illustrates the sequence and
services used to initialize the router. Interactive Media
Activity Drag and Drop: Router Boot Sequence When the
student has completed this activity, the student will be able
to understand the sequence in which a router boots up. Web
Links Router Setup and Startup
http://csc.colstate.edu/summers/ NOTES/CS458/chap11/
tsld004.htm
Content 5.1 Router Boot
Sequence and Verification 5.1.2 How a Cisco
device locates and loads IOS The default source for Cisco
IOS software depends on the hardware platform, but most
commonly the router looks to the boot system commands saved in
NVRAM. Cisco IOS software allows several alternatives to be
used. Other sources can be specified for the software, or the
router can use its own fallback sequence to load the software.
The settings in the configuration register enable the following
alternatives: - Global configuration mode boot system
commands can be specified to enter fallback sources for the
router to use in sequence. The router will use these commands
as needed, in sequence, when it restarts.
- If NVRAM
lacks boot system commands that the router can use, the system
by default uses the Cisco IOS software in flash memory.
- If flash memory is empty, the router then attempts to use
TFTP to load an IOS image from the network. The router uses the
configuration register value to form a filename from which to
boot a default system image stored on a network server.
Interactive Media Activity Drag and Drop: How A
Cisco Device Locates And Loads IOS When the student has
completed this activity, the student will be able to identify
the process a Cisco device goes through in order to find IOS
during boot-up. Web Links Cisco IOS
http://csc.colstate.edu/summers/ NOTES/CS458/
chap11/tsld006.htm
Content 5.1 Router
Boot Sequence and Verification 5.1.3 Using the
boot system command The following examples show the use of
multiple boot system commands to specify the fallback sequence
for booting Cisco IOS software. The three examples show boot
system entries which specify that a Cisco IOS software image
will load first from flash memory, then from a network server,
and finally from ROM: - Flash memory – A system
image from flash memory can be loaded. The advantage is that
information stored in flash memory is not vulnerable to network
failures that can occur when loading system images from TFTP
servers.
- Network server – In case flash memory
becomes corrupted, a system image can be loaded from a TFTP
server.
- ROM – If flash memory is corrupted and
the network server fails to load the image, booting from ROM is
the final bootstrap option in software. However, the system
image in ROM will likely be a subset of the Cisco IOS that
lacks the protocols, features and configurations of the full
Cisco IOS. Also, if the software has been updated since the
router was purchased, the router may have an older version
stored in ROM.
The command copy running-config
startup-config saves the commands in NVRAM. The router will
execute the boot system commands as needed in the order in
which they were originally entered into configuration mode.
Lab Activity Lab Exercise: Using the Boot System
Command In this lab, the students will display information
about the Cisco IOS Image that runs on the router. Lab
Activity e-Lab Activity: Preparing for Boot System
Commands In this lab, the students will display information
relating to the router boot procedure, using the appropriate
show commands. Web Links boot system Global
Configuration Commands http://www.cisco.com/en/US/products/sw/
iosswrel/ps1835/products_command_reference_
chapter09186a00800ca743.html#1017706
Content
5.1 Router Boot Sequence and
Verification 5.1.4 Configuration register
The order in which the router looks for system bootstrap
information depends on the boot field setting in the
configuration register. The default configuration register
setting can be changed with the global configuration mode
command config-register. Use a hexadecimal number as
the argument for this command. The configuration register is a
16-bit register in NVRAM. The lowest four bits of the
configuration register form the boot field. To ensure that the
upper 12 bits are not changed, first retrieve the current
values of the configuration register using the show
version command. Then use the config-register
command, changing only the value of the last hexadecimal digit.
To change the boot field in the configuration register, follow
these guidelines: - To enter the ROM monitor mode, set
the configuration register value to 0xnnn0, where
nnn represents the previous value of the non-boot field
digits. This value sets the boot field bits to 0000 binary.
From ROM monitor, boot the operating system manually by using
the b command at the ROM monitor prompt.
- To
configure the system to boot automatically from ROM, set the
configuration register to 0xnnn1, where nnn
represents the previous value of the non-boot field digits.
This value sets the boot field bits to 0001 binary.
- To configure the system to use the boot system commands in
NVRAM, set the configuration register to any value from
0xnnn2 to 0xnnnF, where nnn represents the
previous value of the non-boot field digits. These values set
the boot field bits to a value between 0010 and 1111 binary.
Using boot system commands in NVRAM is the default.
Lab Activity e-Lab Activity: Configuration
Register In this lab, the students will learn to change the
boot procedure of the router. Web Links config-register
Global Configuration Command http://www.cisco.com/en/US/products/sw/