Cisco Router Boot Sequence Tutorial

This tutorial looks at the main memory components inside a Cisco router (or switch) and walks through what happens, step by step, from power-on to a usable prompt.

Types of memory

A Cisco router or switch generally works with four kinds of memory:

Read-Only Memory (ROM): holds the bootstrap startup program, a copy of the operating system software, and the power-on diagnostic routines (POST).

Flash memory: usually just called “flash,” this is where the IOS image itself lives. It’s erasable, reprogrammable ROM, and its contents survive a reload.

Random-Access Memory (RAM): holds operational data such as the routing table and the running configuration. RAM is cleared whenever the device is powered off or reloaded.

Non-Volatile RAM (NVRAM): holds the startup configuration file. Unlike RAM, its contents survive a power-off or reload.

A few comparisons worth keeping straight:

• RAM is volatile and loses its contents on reload; NVRAM and flash do not.
• NVRAM stores the startup configuration; RAM stores the running configuration.
• ROM holds a bootstrap program called ROM Monitor (ROMmon). On power-up, this bootstrap runs a hardware diagnostic called POST (Power-On Self Test).