Cisco Router Components

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

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)

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 the hardware carrying out the OS instructions for routing and switching.  The CPU is 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 used for most Cisco devices, such as routers, network switches, access points, and others. Before Cisco IOS switches ran CatOS, IOS was 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, allows the router to load the IOS and the configuration file. This is a volatile memory in Cisco routers, just like in computers: this memory stores applications, 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, it is a volatile memory that requires power to keep the data accessible. If the router is turned off, all data in RAM is lost has offered the following primary function:

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

The amount of RAM your router requires depends on the size of the IOS image and configuration file. The smaller routers (up to the 1600 series) likely work 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 run from the system’s RAM, so the router requires more RAM for larger and more complex routing tables.

Read Only Memory (ROM)

The ROM is used to start and keep up the router. It is a volatile memory containing some code, like Bootstrap and POST. The codes help the router do basic tests and boot up when powered on or reloaded. It has included firmware software on the integrated circuit inside the router, which can only alter Cisco. We cannot modify any code in this memory. It must be 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 been deleted or corrupted, the limited IOS restores full-featured IOS.

NVRAM

This is non-volatile RAM (NVRAM). The NVRAM is where the router stores its configuration. It is the router’s permanent memory storage. When you configure a router and then save the configuration, it is stored in the NVRAM.

This memory is not significant 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 the 2900 and 3900 series have a memory of 256 KB. Usually, 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 is not erased when the router is reloaded or switched off.

Flash

Flash memory is non-volatile and 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 was copied from Flash to RAM during the router reboot. RAM is an EEPROM (Electrical Erasable Programmable Read Only Memory) card. It usually fits into a particular slot 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 uses a limited version of Cisco IOS. It allows configuring the router when a valid IOS image cannot be found on a TFTP server or in flash memory. Using RXBoot 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 change router behavior in several ways, such as how the router boots, options while booting, and the console speed.