Cisco Router Components

I have already discussed the general introduction in the articleNetwork Devices”. In this article, we will briefly discuss Cisco Router components. The router is similar to a computer. Regardless of their function, size or complexity, all router models are basically computers.

Cisco Router Components

  • Central processing units (CPU)
  • Internetwork Operating System (IOS)
  • Random-Access Memory (RAM), including Read-Only Memory (ROM), non-volatile random-access memory (NVRAM), and flash memory.
  • Interfaces
  • RXBoot Image
  • Configuration Register


Central Processing Unit (CPU)

Just like computers, tablets, gaming consoles, and smart devices, Cisco devices need a CPU to execute OS instructions, such as system initialization, routing, and switching functions. The central processing unit (CPU) is hardware that carries out the instructions of the OS to perform routing and switching.  The CPU sometimes called the processor for short. The CPU generates interrupts (IRQ) to communicate with other electronic components in the router.

Internetwork Operating System (IOS)

The Cisco Internetwork Operating System (IOS) is the operating system used for most Cisco devices like Cisco routers, Cisco network switches, Cisco access points and many other. Before Cisco IOS switches run CatOS.  IOS is a package of routing, switching, internetworking and telecommunications functions integrated into a multitasking operating system.

The IOS is loaded ahead of the router’s bootup process. The size of the Cisco IOs It usually around 2 to 5MB, but can be a lot larger depending on the router series. The IOS is the most significant parts of the router, without IOS the router is pretty much useless.

Random Access Memory (RAM)

The RAM, or Random Access Memory, is hardware allow to the router to loads the IOS and the configuration file. This is a volatile memory in Cisco routers just like computers. 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 offered the following main function:

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

The amount of RAM your router requires subject to the size of the IOS image and configuration file in the router. The smaller routers (up to the 1600 series) are likely working with 12 to 16 MB while the bigger routers with larger IOS images would require around 32 to 64 MB of memory. Routing tables also running from the system’s RAM, So for larger and complex routing tables, router required more RAM.

Read Only Memory (ROM)

The ROM is used to start and keep up the router. It is a volatile memory containing some code, like the Bootstrap and POST. The codes help the router do some basic tests and boot up when it powered on or reloaded. It has included firmware software on the 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:

  • 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. On the Cisco 1600 series, the size of NVRAM is only 8 KB while on bigger routers, like the 2600 series, it is 32 KB and 2900 and 3900 series have the memory of 256KB. Normally, when a router starts up after it loads the IOS image it will look into the NVRAM and load the configuration file in order to configure the router. The NVRAM is not erased when the router is 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 from 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.

The RXBoot Image

RXBoot mode is the configuration mode that has a limited version of the Cisco IOS allowing configuring the router when a valid IOS image cannot found on a TFTP server or in flash memory. Using the RXBood mode we can perform minor maintenance operations and bring various interfaces up or down.

Configuration Register

The Configuration Register determines the router boot option in Cisco routers. It can be used to change router behaviour in several ways, such as how the router boots, options while booting and the console speed.