Networking World

ROM (Read-Only Memory) ­

Hosts the basic commands of the router and sometimes a limited version of the IOS (Internet Operating System). ROM is non- volatile, meaning it is hard-coded and does not change. Contains power-on diagnostics, a bootstrap program, and operating system software.

RAM (Random Access Memory)
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Contains the running version of the IOS and the current running configuration. This is extremely volatile; when the router is shutdown, anything in RAM is lost. Stores routing tables, ARP cache, fast-switching cache, packet buffering (shared RAM), and packet hold queues.

NVRAM (Non-Volatile Random Access Memory)

As the name implies, files can be written to this memory and will not be lost when the system is powered down. This is where the startup version of the router configuration is stored.

Flash memory (EEPROM ­ Electronic Erasable Programmable Read Only Memory) ­

This is where the IOS version for the router is stored. It is important when determining what version of IOS to load on a router, that you ascertain how much flash is installed. Different versions of IOS require more flash to be loaded. Flash memory holds the operating system image and microcode.