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All contents copyright © 2003 Cisco Systems, Inc. All rights reserved. | |
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Overview |
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5.1 |
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Router Boot Sequence and Verification | |
| 5.1.1 |
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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:
Figure
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5.1 |
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Router Boot Sequence and Verification | |
| 5.1.2 |
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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:
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5.1 |
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Router Boot Sequence and Verification | |
| 5.1.3 |
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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:
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.
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5.1 |
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Router Boot Sequence and Verification | |
| 5.1.4 |
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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. To change the boot field in the configuration
register, follow these guidelines:
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5.1 |
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Router Boot Sequence and Verification | |
| 5.1.5 |
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Troubleshooting IOS boot failure |
In the event that the router does not boot
properly, there are several things that could be wrong:
When the router boots, it looks in the
configuration file for a boot
system statement. This
boot system
statement can force the router to boot from another image instead of the IOS
in flash. To identify the boot image source, type the
show version
command and look for the line that identifies the image boot source.
Use the show running-config command and look for a boot system statement near the top of the configuration. If the boot system statement points to an incorrect IOS image, delete the statement using the “no” version of the command. An incorrect configuration register setting will prevent the IOS from loading from flash. The value in the configuration register tells the router where to get the IOS. This can be confirmed by using the show version command and looking at the last line for the configuration register. The correct value varies from hardware platform to hardware platform. A part of the documentation of the internetwork should be a printed copy of the show version output. If that documentation is not available, there are resources on the Cisco documentation CD or Cisco website to identify the correct configuration register value. Correct this by changing the configuration register in the configuration and saving this as the start-up configuration. If there is still a problem, the router may have a corrupted flash image file. If this is the case, an error message should be displayed during boot. That message may take one of several forms. Some examples are:
If the flash image is corrupt, a new IOS should be uploaded into the router. If none of the above appears to be the problem, the router could have a hardware failure. If this is the case, the Cisco Technical Assistance (TAC) center should be contacted. Although hardware failures are rare, they do occur.
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5.2 |
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Managing the Cisco File System | |
| 5.2.1 |
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IOS file system overview |
| Routers and switches depend on software for
their operation. The two types of software required are operating systems
and configuration.
The operating system used in almost all Cisco devices is the Cisco Internetwork Operating System (IOS). The Cisco IOS® is the software that allows the hardware to function as a router or switch. The IOS file is several megabytes. The software a router or switch uses is referred to as the configuration file or the config. The configuration contains the “instructions” that define how the device is to route or switch. A network administrator creates a configuration that defines the desired functionality of the Cisco device. The functions that can be specified by the configuation are the IP addresses of the interfaces, routing protocols, and networks to be advertised. The configuration file typically is a few hundred to a few thousand bytes. Each of the software components is stored in
memory as a separate file. These files are also stored in different types of
memory.
The IOS is stored in a memory area called flash. Flash memory provides non-volatile storage of an IOS that can be used as an operating system at startup. The flash allows the IOS to be upgraded or stores multiple IOS files. In many router architectures, the IOS is copied into and run from Random Access Memory (RAM). A copy of the configuration file is stored in Non-volatile RAM (NVRAM) to be used as a configuration during startup. This is referred to as the “startup config”. The startup configuration is copied into RAM at boot time. This configuration in RAM is the configuration used to operate the router. It is referred to as the “running config”. Beginning with version 12 of the IOS, a
single interface to all the file systems that a router uses is provided.
This is referred to as the Cisco IOS File System (IFS). The IFS provides a
single method to perform all the file system management that a router uses.
This would include the flash memory file systems, the network file systems (TFTP,
rcp, and FTP), and reading or writing data (such as NVRAM, the running
configuration, ROM). The IFS uses a common set of prefixes to specify file
system devices.
The IFS uses the URL convention to specify
files on network devices and the network. The URL convention identifies the
location of the configuration files following the colon as
[[[//location]/directory]/filename].
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5.2 |
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Managing the Cisco File System | |
| 5.2.2 |
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IOS naming conventions |
| Cisco develops many different versions of
the IOS. The IOS supports varied hardware platforms and features. Cisco is
also continuously developing and releasing new versions of the IOS.
To identify the different versions, Cisco has
a naming convention for IOS files. This IOS naming convention uses different
fields in the name. Among the fields are the hardware platform
identification, the feature set identification, and the numerical release.
The first part of the Cisco IOS file name identifies the hardware platform for which this image is designed. The second part of the IOS file name identifies the various features that this file contains. There are many different features to choose from. These features are packaged in "software images". Each feature set contains a specific subset of Cisco IOS features. Examples of feature-set categories are:
The third part of the file name indicates the file format. It specifies if the IOS is stored in flash in a compressed format and whether the IOS is relocatable. If the flash image is compressed, the IOS must be expanded during boot as it is copied to RAM. A relocatable image is copied from flash into RAM to run. A non-relocatable image is run directly from flash. The fourth part of the file name identifies the release of the IOS. As Cisco develops newer versions of the IOS, the numerical version number increases.
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5.2 |
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Managing the Cisco File System | |
| 5.2.3 |
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Managing configuration files using TFTP |
In a Cisco router or switch, the active
configuration is in RAM and the default location for the startup
configuration is NVRAM. In the event the configuration is lost, this startup
configuration should be backed up. One of these backup copies of the
configuration can be stored on a TFTP server. The
copy running-config tftp
command can be used to do this.
The steps for this process are listed below:
Loading the backup configuration file from a
TFTP server can restore the router configuration.
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5.2 |
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Managing the Cisco File System | |
| 5.2.4 |
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Managing configuration files using copy and paste |
Another way to create a backup copy of the
configuration is to capture the output of the
show running-config
command. This can be done from the terminal session by copying the output,
pasting to a text file, then saving the text file. This file will need some
editing before it can be used to restore configuration to the router.
 Perform the following to capture the configuration using the text displayed on the HyperTerminal screen to a text file:
After the capture is complete, the configuration file needs to be edited to remove extra text. To create this in a form to be “pasted” back into the router, remove any unnecessary information from the captured configuration. Comments may also be added to explain the various parts of the configuration. A comment is added by beginning a line with an exclamation mark “!”. The configuration file can be edited from a text editor such as Notepad. To edit the file from Notepad click on File > Open. Find the captured file and select it. Click Open. The lines that need to be deleted contain:
At the end of each of the interface sections add the no shutdown command. Clicking File > Save will save the clean version of the configuration. The backup configuration can be restored from a HyperTerminal session. Before the configuration is restored any remaining configuration should be removed from the router. This can be done by entering the command erase startup-config at the privileged EXEC router prompt and then restarting the router by entering the reload command. HyperTerminal can be used to restore a configuration. The clean backup of the configuration can be copied into the router.
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5.2 |
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Managing the Cisco File System | |
| 5.2.5 |
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Managing IOS images using TFTP |
| Occasionally the router will need to have
the IOS upgraded or restored. When a router first arrives, the IOS should be
backed up. This IOS image can be stored in a central server with other IOS
images. These images can be used to restore or upgrade the IOS on the
routers and switches in the internetwork.
This server should have a TFTP service running.
The IOS backup can be initiated from the privileged EXEC mode with the
copy flash tftp
command.
The router will prompt the user to enter the IP address of the TFTP server. When prompted for the filename of the IOS image on the server, the router may then prompt to erase flash. This often happens if there is not sufficient flash available for the new image. As the image is erased from flash, a series of “e’s” will appear to show the erase process. As each datagram of the IOS image file is downloaded, an “!” will be displayed. This IOS image is several megabytes and may take some time. The new flash image will be verified after it is downloaded. The router is now ready to be reloaded to use the new IOS image.
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5.2 |
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Managing the Cisco File System | |
| 5.2.6 |
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Managing IOS images using Xmodem |
| If the IOS image in flash has been erased or
corrupted, the IOS may need to be restored from the ROM monitor mode (ROMmon).
In many of the Cisco hardware architectures, the ROMmon mode is identified
from the rommon 1 >
prompt. This first step in this process is to identify why the IOS image did not load from flash. This could be due to a corrupt or missing image. The flash should be examined with the dir flash: command. If an image is located that appears to be valid, an attempt should be made to boot from that image. This is done using boot flash: command. For example if the image name was “c2600-is-mz.121-5”, the command would be:
If the router properly boots, then there are a couple of items that need to be examined to determine why the router booted to the ROMmon instead of using IOS from flash. First, use the show version command to check the configuration register to ensure that it is configured for the default boot sequence. If the configuration register value is correct, use the show startup-config command to see if there is a boot system command instructing the router to use the IOS for ROM monitor. If the router will not properly boot from the image or there is no IOS image, a new IOS will need to be downloaded. The IOS file may be recovered using either Xmodem to restore the image through the console, or downloading the image using TFTP from the ROMmon mode. Download using Xmodem from ROMmon When prompted “change console baud rate? y/n [n]:” selecting y will provide a prompt to select the new speed. After changing the console speed and restarting the router into the ROMmon mode, the terminal session (at 9600) should be terminated and a new session started at 115200 bps to match the console speed. The Xmodem command can be used from the ROMmon mode to restore the IOS software image from the PC. The format of the command is xmodem -c image_file_name. For example, to restore an IOS image file with the name “c2600-is-mz.122-10a.bin”, type the command:
The -c instructs the Xmodem process to use Cyclic Redundancy Check (CRC) for error checking during the download. The router will prompt not to begin the transfer and present a warning message. The warning message informs that the bootflash will be erased and asks to confirm continuing. When the process is continued, the router will then prompt to start the transfer. Now the Xmodem transfer needs to be started
from the terminal emulator. In HyperTerminal, select Transfer >
Send File. Then in the Send File popup specify the image
name/location, select Xmodem as the protocol, and start the transfer. During
the transfer, the Sending File popup will display the status of the
transfer.
Once the transfer is complete a message appears indicating that flash is being erased. This is followed by the “Download Complete!” message. Before restarting the router, the console speed needs to be set back to 9600 and the config register back to 0x2102. Enter the command config-register 0x2102 at the privileged EXEC prompt. While the router is rebooting, the 115200 bps terminal session needs to be ended and a 9600 bps session begun.
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5.2 |
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Managing the Cisco File System | |
| 5.2.7 |
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Environment variables |
| The IOS can also be restored from a TFTP
session. Downloading the image using TFTP from ROMmon is the fastest way to
restore an IOS image to the router. This is done by setting environmental
variables and then using the
tftpdnld command.
Since the ROMmon has very limited functions, no
configuration file is loaded during boot. The router therefore has no IP or
interface configuration. The environmental variables provide a minimal
configuration to allow for the TFTP of the IOS. The ROMmon TFTP transfer
works only on the first LAN port so a simple set of IP parameters are set
for this interface. To set a ROMmon environment variable, the variable name
is typed, then the equal sign (=), and the value for the variable (VARIABLE_NAME=value).
For example, to set the IP address to 10.0.0.1, type IP_ADDRESS=10.0.0.1 at
the ROMmon prompt.
The minimum variables required to use tftpdnld are:
To check the ROMmon environment variables,
the set
command can be used.
Once the variables are set for the IOS
download, the tftpdnld
command is entered with no arguments. The ROMmon will echo the variables and
then a confirmation prompt will appear with a warning that this will erase
the flash.
As each datagram of the IOS file is received, a “!” will be displayed. When the complete IOS file has been received, the flash will be erased and the new IOS image file written. Appropriate messages will be displayed as the process is completed. When the new image is written into flash and the ROMmon prompt is displayed, the router can be restarted by typing i. The router should now boot from the new IOS image in flash.
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5.2 |
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Managing the Cisco File System | |
| 5.2.8 |
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File system verification |
| There are several commands that can be used
to verify the router file system. One of those is the
show version
command.
The show version
command can be used to check the current image and the total amount of
flash. It also verifies two other items concerning loading the IOS. It
identifies the source of the IOS image that the router used to boot and
displays the configuration register. The boot field setting of the
configuration register can be examined to determine from where the router is
to load the IOS. If these do not agree, there may be a corrupt or missing
IOS image in flash or there may be boot system commands in the startup
configuration. The
show flash
command can also be used to verify the file system.
As previously mentioned, the configuration file may contain boot system commands. These commands can be used to identify the source of the desired IOS boot image. Multiple boot system commands may be used to create a fallback sequence to discover and load an IOS. These boot system commands will be processed in the order of their appearance in the configuration file.
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Summary |
An understanding of the following key points
should have been achieved:
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Web Links
The IFS also supports FTP file transfer. 
Web Links