Cisco Component Description


Bootstrap Stored in the microcode of the ROM, the bootstrap is used to bring a router up during initialization. It will boot the router and then load the IOS.


POST (power-on self-test) Stored in the microcode of the ROM, the POST is used to check the basic functionality of the router hardware and determines which interfaces are present.

ROM monitor

ROM monitor Stored in the microcode of the ROM, the ROM monitor is used for manufacturing, testing, and troubleshooting.


Mini-IOS Called the RXBOOT or boot loader by Cisco, the mini-IOS is a small IOS in ROM that can be used to bring up an interface and load a Cisco IOS into flash memory. The mini-IOS can also perform a few other maintenance operations.


RAM (random Used to hold packet buffers, ARP cache, routing tables, and also access memory) the software and data structures that allow the router to function. Running-configuration is stored in RAM, and most routers expand the IOS from flash into RAM upon boot.


ROM (read-only memory) Used to start and maintain the router. Holds the POST and the bootstrap program, as well as the mini-IOS. Flash memory Stores the Cisco IOS by default. Flash memory is not erased when the router is reloaded. It is EEPROM (electronically erasable programmable read-only memory) created by Intel.


NVRAM (nonvolatile RAM) Used to hold the router and switch configuration. NVRAM is noterased when the router or switch is reloaded. Does not store an IOS. The configuration register is stored in NVRAM.

Configuration register

Configuration register Used to control how the router boots up. This value can be found as the last line of the show version command output and by default is set to 0x2102, which tells the router to load the IOS from flash memory as well as to load the configuration from NVRAM.

  • Router Boot Sequence

    The Router Boot Sequence When a router boots up, it performs a series of steps, called the boot sequence, to test the hard- ware and load the necessary software. The boot sequence consists of the following steps:
    1. The router performs a POST. The POST tests the hardware to verify that all components of the device are operational and present.
    2. The bootstrap then looks for and loads the Cisco IOS software. The bootstrap is a program in ROM that is used to execute programs. The bootstrap program is responsible for finding where each IOS program is located and then loading the file. By default, the IOS software is loaded from flash memory in all Cisco routers. The default order of an IOS loading from a router is Flash, TFTP server, then ROM.
    3. The IOS software looks for a valid configuration file stored in NVRAM. This file is called startup-config and is only there if an administrator copies the running-config file into NVRAM.
    4. If a startup-config file is in NVRAM, the router will copy this file and place it in RAM and call the file running-config. The router will use this file to run the router. The routershould now be operational. If a startup-config file is not in NVRAM, the router will broadcast out any interface that detects carrier detect (CD) for a TFTP host looking for a configuration, and when that fails (typically it will fail—most people won’t even realize the router has attempted this process), it will start the setup mode configuration process.
  • Managing Configuration Register

    All Cisco routers have a 16-bit software register that’s written into NVRAM. By default, the configuration register is set to load the Cisco IOS from flash memory and to look for and load the startup-config file from NVRAM.

    Understanding the Configuration Register Bits

    The 16 bits (2 bytes) of the configuration register are read from 15 to 0, from left to right. The default configuration setting on Cisco routers is 0x2102.

    Checking the Current Configuration Register Value

    You can see the current value of the configuration register by using the show version command (sh version or show ver for short), as demonstrated here:

    Router# sh version

    The show version command will display system hardware configuration information, software version, and the names of the boot images on a router.
  • Changing the Configuration Register

    You can change the configuration register value to modify how the router boots and runs.

    These are the main reasons you would want to change the configuration register:

    To force the system into the ROM monitor mode to select a boot source and default boot filename.
    1. To enable or disable the Break function.
    2. To control broadcast addresses.
    3. To set the console terminal baud rate
    4. To load operating software from ROM
    5. To enable booting from a Trivial File Transfer Protocol (TFTP) server Before you change the configuration register, make sure you know the current configuration register value
    You can change the configuration register by using the config-register command.
    Router (config) # config-register 0x2101
    Router (config) # ^Z
    Router# shver
    Configuration register is 0x2102 (will be 0x2101 at next reload)

    Here is our router after setting the configuration register to 0x2101 and reloading:

    Router(boot)# shver

    Configuration register is 0x2101 At this point, if you typed Show flash, you’d still see the IOS in flash memory ready to go.

    But we told our router to load from ROM, which is why the host name shows up with (boot).

    Router(boot)# sh flash

    So even though we have our full IOS in flash, we changed the default loading of the router’s software by changing the configuration register. If you want to set the configuration register back to the default, just type this:
    Router(boot)#config t
    Router(boot)(config)# config-register 0x2102
    Router(boot)(config)# ^Z
    Router(boot)# reload
  • Summarization:-

    Summarization, also called route aggregation, allows routing protocols to advertise many networks as one address. The purpose of this is to reduce the size of routing tables on routers to save memory, which also shortens the amount of time for IP to parse the routing table and find the path to a remote network.
  • Recovering Passwords

    If you’re locked out of a router because you forgot the password, you can change the configuration register. The default configuration register value is 0x2102, meaning that bit 6 is off. With the default setting, the router will look for and load a router configuration stored in NVRAM (startup-config). To recover a password, you need to turn on bit 6. Doing this will tell the router to ignore the NVRAM contents. The configuration register value to turn on bit 6 is 0x2142.

    Password recovery steps:
    1. Boot the router and interrupt the boot sequence by performing a break, which will takethe router into ROM monitor mode.
    2. Change the configuration register to turn on bit 6 (with the value 0x2142).
    3. Reload the router.
    4. Enter privileged mode.
    5. Copy the startup-config file to running-config.
    6. Change the password.
    7. Reset the configuration register to the default value.
    8. Save the router configuration.
    9. Reload the router (optional).
  • Interrupting the Router Boot Sequence

    Your first step is to boot the router and perform a break. This is usually done by pressing the Ctrl+Break key combination when using HyperTerminal (personally, I use SecureCRT) whilethe router first reboots. Rommon 1 >

    Notice the line monitor: command “boot” aborted due to user interrupt. At this point, you will be at the rommon 1> prompt, which is called ROM monitor mode.

    Changing the Configuration Register

    Change the configuration register by using the config-register command. To turn on bit 6, use the configuration register value 0x2142.

    Remember that if you change the configuration register to 0x2142, the startup-config will be bypassed and the router will load into setup mode.
    Rommon 1> prompt:
    Rommon 1 >confreg 0x2142
    You must reset or power cycle for new config to take effect
    Rommon 2 >reset
    Viewing and Changing the Configuration

    Now you’re past the point where you would need to enter the user-mode and privileged-mode passwords in a router. Copy the startup-config file to the running-config file:

    copy startup-config running-config

    Or use the shortcut: copy start run
  • Backing Up the Cisco IOS

    To back up the Cisco IOS to a TFTP server, you use the copy flash tftp command. It’s a straight forward command that requires only the source filename and the IP address of the TFTP server.

    The key to success in this backup routine is to make sure you’ve got good, solid connectivity to the TFTP server. Check this by pinging the TFTP device from the router console prompt like this:

    Router# ping

    The Packet Internet Groper (Ping) utility is used to test network connectivity,after you ping the TFTP server to make sure that IP is working, you can use the copy flash tftp command to copy the IOS to the TFTP server as shown next:
    Router# copy flash tftp
    Source filename []?
    Address or name of remote host []?
    Destination filename [c2800nm-advsecurityk9-mz.124-12.bin]?
    21710744 bytes copied in 60.724 secs (357532 bytes/sec)

    Restoring or Upgrading the Cisco Router IOS

    What happens if you need to restore the Cisco IOS to flash memory to replace an original file that has been damaged or if you want to upgrade the IOS? You can download the file from a TFTP server to flash memory by using the copy tftp flash command. This command requires the IP address of the TFTP host and the name of the file you want to download.But before you begin, make sure the file you want to place in flash memory is in the default TFTP directory on your host. When you issue the command, TFTP won’t ask you where the file is, so if the file you want to use isn’t in the default directory of the TFTP host, this just won’t work.
    Router# copy tftp flash
    Address or name of remote host []?
    Source filename []?
    Destination filename [c2800nm-advsecurityk9-mz.124-12.bin]?
    %Warning:There is a file already existing with this name
    Do you want to over write? [Confirm]
    Accessing tftp://
    Loading c2800nm-advsecurityk9-mz.124-12.bin from (via
    [OK - 21710744 bytes]
    21710744 bytes copied in 82.880 secs (261954 bytes/sec)
  • Gathering Neighbor Information

    The show cdp neighbor command (shcdpnei for short) delivers information about directly connected devices. It’s important to remember that CDP packets aren’t passed through a Cisco switch and that you only see what’s directly attached. So this means that if your router is connected to a switch, you won’t see any of the devices hooked up to that switch.
    Corp# shcdp neighbors 
    Corp# shcdp neighbors detail

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