Routers CPU, OS and Memory

In chapter 1 we have already studied the general introduction of the router here we will briefly discuss routers. It is similar to a computer. Regardless of their function, size or complexity, all router models are basically computers. Just like computers, tablets, and smart devices, It also requires:

  • Central processing units (CPU)
  • Operating systems (OS)
  • Memory consisting of random-access memory (RAM), read-only memory (ROM), non-volatile random-access memory (NVRAM), and flash.

Here in this section, we will discuss Cisco routers. Like all computers, tablets, gaming consoles, and smart devices, Cisco devices need a CPU to execute OS instructions, such as system initialization, routing functions, and switching functions.

Central Processing Unit (CPU)

The central processing unit (CPU) of a router is a hardware that carries out the instructions of the OS to perform routing and switching.  The CPU is sometimes also called the central processor unit or processor for short. The CPU generates interrupts (IRQ) to communicate with the other electronic components in the router.

Routers Operating System

The Cisco Internetwork Operating System (IOS) is the operating system used for most Cisco devices. Cisco IOS is a family of software used on most Cisco routers, Cisco network switches, Cisco access points and many other devices. Earlier switches run CatOS.  IOS is a package of routing, switching, internetworking and telecommunications functions integrated into a multitasking operating system. Routers Memory-Cisco router uses four types of memory which are the following:

Random Access Memory(RAM)

RAM is a hardware device allows information to store and retrieve a router. This is a volatile memory in Cisco routers. This memory store application, processes, and data needed to be executed by the CPU. Cisco routers use a fast type of RAM called synchronous dynamic random access memory (SDRAM).  As we know that it is a volatile memory and requires power to keep the data accessible. If the router turned off, all data in RAM has lost.RAM has after the main function:

  • Store routing table.
  • Running IOS.
  • Store ARP Table
  • Packet buffer
  • Store running configuration file.


The ROM used to start and keep up the router. This is a volatile memory of the router. It has some code, like the Bootstrap and POST, which helps the router do some basic tests and boot up when it powered on or reloaded. ROM is firmware software on integrated circuit inside the router which can only alter Cisco. We cannot alter any code in this memory. It must set from the factory and is Read Only. ROM stores the following:cisco routers

  • Boot up information that provides the startup information.
  • Power-on self-test (POST)
  • Limited IOS to give a backup version of the IOS. When full feature IOS has deleted or corrupted, the limited IOS restore full featured IOS.


This is non-volatile RAM. The NVRAM is a place where the router holds its configuration. This is the permanent memory storage of the router. When you configure a router and then save the configuration, it stored in the NVRAM. This memory is not big at all when compared to the system’s RAM. When a router starts up after it loads the IOS image it will look into the NVRAM and load the configuration file to configure the router. The NVRAM not erased when the router reloaded or even switched off.


Flash memory is the non-volatile memory and it is permanent storage for the IOS and other systems related files such as log files, voice configuration files, HTML files, backup configurations, and much more. The IOS has copied fromcisco routers flash to RAM during router reboot. RAM is an EEPROM (Electrical Erasable Programmable Read Only Memory) card. It fits into a special slot normally at the back of the router and has nothing more than the IOS image. Usually, it comes in sizes of 4MB for the smaller routers and goes up from there depending on the router model.

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