Managing Devices

The internal components of a cisco router and switch

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

POST (power-on self-test) Also 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 Again, stored in the microcode of the ROM, the ROM monitor is used for manufacturing, testing, and troubleshooting,as well as running a mini-IOS when the IOS in flash fails to load.

Mini-IOS Called the RXBOOT or bootloader 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 access memory) Used to hold packet buffers, ARP cache, routing tables, and also the software and data structures that allow the router to function. Running-config 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 not erased when the router or switch is reloaded. Does not store an IOS. The configuration register is stored in NVRAM.

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.

The router and switch boot sequence

When a Cisco device boots up, it performs a series of steps, called the boot sequence, to test the hardware and load the necessary software. The boot sequence comprises the following steps:

1. The IOS device performs a POST, which tests the hardware to verify that all components of the device are present and operational. The post takes stock of the different interfaces on the switch or router, and it’s stored in and runs from read-only memory (ROM).

2. The bootstrap in ROM then locates and loads the Cisco IOS software by executing programs responsible for finding where each IOS program is located. Once they are found, it then loads the proper files. By default, the IOS software is loaded from flash memory in all Cisco devices. The default order of an IOS loading from a Cisco device begins with flash, then TFTP server, and finally, ROM.

3. The IOS software then looks for a valid configuration file stored in NVRAM. This file is called startup-config and will be present only if an administrator has copied the running-config file into NVRAM.

4. If a startup-config file is found in NVRAM, the router or switch will copy it, place it in RAM, and name the file the running-config. The device will use this file to run, and the router/switch should now be operational. If no startup-config file is found in NVRAM, the router/switch reacts by broadcasting out any interface that detects carrier detect (CD) to locate a TFTP host in its search for a configuration. When that fails, which it typically does, the device will begin the setup mode configuration process. Most people don’t even realize the device has attempted this process!

The default order of an IOS loading from a Cisco device begins with flash, then TFTP server, and finally, ROM.

Booting System

Booting from Flash memory

Booting from tftp server

Device LED

Configuration Register

Managing the 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. In the following sections, I am going to discuss the configuration register settings and how to use these settings to provide password recovery on your routers.

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. This means that bits 13, 8, and 1 are on. Notice that each set of 4 bits (called a nibble) is read in binary with a value of 8, 4, 2, 1.

The boot field, which consists of bits 0–3 in the configuration register (the last 4 bits), controls the router boot sequence and locates the Cisco IOS.

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:

The last information given from this command is the value of the configuration register. In this example, the value is 0x2102—the default setting. The configuration register setting of 0x2102 tells the router to look in NVRAM for the boot sequence.

Register Values and rules