IOS Location Process
The sequence of events the bootstrap code follows to find the image follows.
- Check the boot field of the configuration register
- If the configuration register boot field value is from 0x2 to 0xF the bootstrap code parses any configuration in NVRAM for boot system commands that specify the name and location of the IOS image to load
- If there are no boot system commands in the configuration, the router defaults to loading the first file in Flash memory and running it.
- If no valid file is found in Flash, the router attempts a net boot using the boot field value as a pointer for the name of the file to request to be downloaded.
- If net boot fails, the router will boot the RXBOOT file from ROM.
- If there is no RXBOOT (mini-IOS) file or it is corrupted, the router will boot the ROM monitor (ROMMON) from ROM.
Examining the Configuration Register
The configuration register is a 16-bit register. The lowest four bits of the configuration register (bits 3, 2, 1, and 0) form the boot field. A hexadecimal number is used as the argument to set the value of Configuration Register. When changing the boot field follow these guidelines:
Set the boot field to 0 to enter ROM monitor mode automatically. (This value sets the boot field bits to 0-0-0-0.) The router displays the > or the rommon> prompt in this mode.
Set the boot field to 1 to configure the system to boot automatically from ROM. (This value sets the boot field bits to 0-0-0-1.) The router displays the Router(boot)> prompt in this mode.
Set the boot field to any value from 2 to F to configure the system to use the boot system commands in NVRAM. This is the default. (These values set the boot field bits to 0-0-1-0 through 1-1-1-1.)
You can change the default configuration register setting with the enabled config-mode config-register command.
Use the show version command to verify your changes in the boot field setting.
Examining the IOS Copy Command
The IOS copy commands are used to move configurations from one component or device to another. The syntax of the copy command requires that the first argument indicate the source (from where the configuration is to be copied) and the second argument indicate the destination (to where the configuration is to be copied).
TFTP servers can be used to store configurations in a central place. This allows centralized management and updating. The copy command is used to both store and retrieve a configuration through tftp.
copy running-config tftp
The copy running-config tftp command allows you to upload and save your current configuration to a TFTP server. You are required to supply the TFTP server''s address or name and to specify a filename for your uploaded configuration. In the display the series of exclamation marks are used to show the progress of the upload.
copy tftp running-config
The copy tftp running-config command downloads a configuration file from the TFTP server to running memory.
Viewing the Configurations
The show running-config and show startup-config commands are among the most used Cisco IOS software EXEC commands. They allow an administrator to see the current running configuration in RAM on the router or the startup configuration commands in NVRAM that the router will use on the next restart.
CLI Command Modes
The first method of configuration presented was the setup utility. Setup allows a basic initial configuration to be created. For more complex and specific configurations, we can use the command line interface to enter terminal configuration mode.
Accessing the Modes
From the privileged EXEC mode you can enter global configuration mode with the configure terminal command. From the global configuration mode, you have access to the specific configuration modes, which include:
- Interface - Supports commands that configure operations on a per-interface basis
- Subinterface - Supports commands that configure multiple virtual interfaces on a single physical interface
- Controller - Supports commands that configure controllers (for example E1 and T1 controllers)
- Line - Supports commands that configure the operation of a terminal line
- Router - Supports commands that configure an IP routing protocol
- IPX-router - Supports commands that configure the Novell network-layer protocol
Grouping Configuration Commands
Global Configuration Commands Commands that affect the entire router are called global commands. The hostname and enable password commands are examples of global commands.
Major Process/Interface Commands Commands that point to or indicate a process or interface that will be configured are called major commands. When entered, major commands cause the CLI to enter a specific configuration mode. Major commands have no effect unless they are immediately followed by a subcommand that supplies the configuration entry. For example, the major command interface serial 0 has no effect unless followed by a subcommand that tells what you want to do to that interface.
Saving Configurations
After you have entered the commands you wish to configure the router, you must save your running configuration to NVRAM with the copy running-config startup-config command. If you fail to save your configuration to NVRAM and you reload the router, your configuration will be lost and the router will revert to the last saved configuration.
Exiting the Configuration Modes
If you type exit, the router will back out one level, eventually allowing you to log out. In general, typing exit from one of the specific configuration modes will return you to global configuration mode. Pressing Ctrl-Z leaves configuration mode completely and returns the router to the privileged EXEC mode.
Examining Router Identification Configuration
There are three different identifications that can be configured in the router:
- Router Name
- Message of the Day Banner
- Interface Description
Other Console Line Commands
Other useful console commands include the exec-timeout 0 0 command and the logging synchronous console line command.
- The exec-timeout 0 0 command sets the timeout for the console EXEC session to zero. This prevents the session from timing out and disconnecting your session.
- The logging synchronous console line command is useful whenever there are console messages being displayed at the same time that you are attempting to input EXEC or configuration commands. Instead of the console messages being interspersed with your input, your input is redisplayed on a single line at the finish of each console message that "interrupts" your input. This makes reading your input and the messages much easier.
The Interface Command
Many features are enabled on a per-interface basis. Interface configuration mode commands modify the operation of Ethernet, Token Ring, FDDI, serial, and many other interface types.
A router''s main function is to relay packets from one network device to another. In order to do that, the characteristics of the interfaces through which the packets are received and sent must be defined. These characteristics include, but are not limited to:
- address of the port
- data encapsulation method
- media-type
- bandwidth
- direct memory access buffering parameters
When you issue the interface command, you must define the interface type and number. The number is assigned to each interface at the factory or by the physical location of the interface hardware in the router and is used to identify each interface. This identification is critical when you have multiple interfaces of the same type in a single router.
Disabling / Enabling an Interface
There may be a reason for you to disable an interface. This function is useful when you want to perform hardware maintenance on a specific interface or segment of a network. You might also want to disable an interface if a problem exists on a specific segment of the network and you need to isolate that segment from the rest of the network until the problem is detected or repaired.
The shutdown subcommand administratively turns off an interface. To reinstate the interface, use the no shutdown command.
The show interfaces command displays statistics for the network interfaces on the router.
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