Introduction

In this article I am going to write about components of a Cisco router and the process by which router IOS boots.

Ø RAM is a volatile memory in this contents are lost on reload, where NVRAM and Flash contents are not.

Ø NVRAM holds the startup configuration file, where RAM holds the running configuration file.

Ø ROM contains a bootstrap program called ROM Monitor (or ROM-on). When a router is powered on, the bootstrap runs a hardware diagnostic called POST (Power-On Self-Test).

Process

Generally Cisco routers (and switches) contain four types of memory:

Read-Only Memory (ROM): ROM stores the router’s bootstrap startup program, operating system software, and power-on diagnostic test programs (POST).

Flash Memory: Generally referred to simply as “flash”, the IOS images are held here. Flash is erasable and re programmable ROM. Flash memory content is retained by the router on reload.

Random-Access Memory (RAM): Stores operational information such as routing tables and the running configuration file. RAM contents are lost when the router is powered down or reloaded. By default, routers look here first for an Inter-network Operating System (IOS) file during boot.

Non-volatile RAM (NVRAM): NVRAM holds the router’s startup configuration file. NVRAM contents are not lost when the router is powered down or reloaded.

Ø The router is powered on.

Ø The router first runs Power-On Self-Test (POST)

Ø The bootstrap checks the Configuration Register value to specify where to load the IOS. By default (the default value of Configuration Register is 2102, in hexadecimal), the router first looks for “boot system” commands in startup-config file.

If the command is find, it will run boot system commands in order they appear in startup-config to locate the IOS.

If not, the IOS image is loaded from Flash.

If the IOS is not found in Flash, the bootstrap can try to load the IOS from TFTP server or from ROM (mini-IOS)

Ø After the IOS is found, it is loaded into RAM.

Ø The IOS attempts to load the configuration file (startup-config) from NVRAM to RAM. If the startup-config is not found in NVRAM, the IOS attempts to load a configuration file from TFTP. If no TFTP server responds, the router enters Setup Mode (Initial Configuration Mode).

conclusion

I hope you understand the basic things behind router IOS Boot process .Please refer the below link for in depth details Router IOS Boot Process Explained very simple & easy to understand

For queries

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