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The Innards of a Cisco Router

Networking Topics > Layer 3 Routing > The Innards of a Cisco Router

Types of Memory

Cisco routers have four types of memory, RAM (Random Access Memory), ROM (Read Only Memory), Flash memory and NVRAM (Non-Volatile RAM). Routers use these types of memory for different purposes.

RAM (Random Access Memory)

RAM (Random Access Memory) is used to store the routers running configuration, routing tables, ARP cache and packet buffers. RAM is a fast type of memory enabling fast reads and writes, however RAM requires constant power. When the router is rebooted or powered down the data stored is lost.

NVRAM (Non-volatile RAM)

NVRAM (Non-volatile RAM) as the name suggests is a type of RAM which is non volatile, it does not lose the data stored when powered down. NVRAM however does not provide fast reads and writes like RAM and therefore is used only to store the start-up configuration.

When the router boots the start-up configuration is copied to RAM and becomes the running config.

NVRAM also contains the config register, this controls where the bootstrap will look for the operating system (IOS).

ROM (Read Only Memory)

ROM (Read Only Memory) is used within the router mainly for booting up, it is read only and therefore the contents of ROM cannot be altered.

ROM contains the routers bootstrap, which is used to initialise the router and load the IOS. It is also used to hold the POST code. POST (Power On Self Test) is a small program designed to test the basic functionality of the router on booting.

ROM also contains a cut down IOS and troubleshooting code used during problem solving.

Router Boot Sequence

1. Run POST
2. Bootstrap loads IOS
3. IOS loads configuration file (If no config file is present it will enter setup mode)

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