Managing a Cisco Internetwork

The Internal Components of a Cisco Router

To configure and troubleshoot a Cisco internetwork, you need to know the major components
of Cisco routers and understand what each one does.

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

The Cisco Internetwork Operating System (IOS) is the kernel of Cisco routers and most switches. In case you didn’t know, a kernel is the basic, indispensable part of an operating system that allocates resources and manages things such as low-level hardware interfaces and security.

Cisco Router IOS

The Cisco IOS is a proprietary kernel that provides routing, switching, internetworking, and telecommunications features. The first IOS was written by William Yeager in 1986, and it enabled networked
applications. It runs on most Cisco routers as well as an ever-increasing number of Cisco Catalyst switches, like the Catalyst 2950/2960 and 3550/3560 series switches.