Memory of cisco routers

Your router has many different locations where it can store images, configuration files, and microcode. Refer to your hardware documentation for details on which types of memory your routing device contains, where files can be stored (saved), and where images and boot images are located by default. This section provides information on the following memory types:


  • RAM
  • ROM


Random Access Memory (RAM) represents the non-permanent or volatile working area of memory on a Cisco router. When the router is powered down, the contents of RAM are lost.

By default, RAM is broken up into two main areas – Main Processor Memory, and Shared I/O Memory. Main Processor Memory is where the routing table, ARP tables, and running configuration are stored. Shared I/O Memory is used as a buffer location for temporarily storing packets prior to processing. Most Cisco 2500 routers will have 2MB of RAM soldered to the system board (this amount, however, depends on the revision number of the router), along with one SIMM slot to add additional RAM. The maximum amount of RAM that can be added to a Cisco 2500 is 16MB. If 16MB is added, that provides a maximum of 18MB of available RAM. On the Cisco 3600 series routers, you can use the memory-size iomem command to configure the proportion of DRAM devoted to main memory and to shared memory.

In cases where a RAM SIMM is installed, its capacity will be used as Main Processor Memory, while the onboard RAM (2MB) will be used as Shared I/O memory. If no SIMM chip is present, that 2MB of on-board RAM will be split between both areas, providing each with 1MB of working space. This should be avoided for performance reasons.

DRAM often comes on dual in-line memory modules (DIMMs).


In older Cisco router models, Read-Only Memory (ROM) chips were used to store the IOS software. In newer models, this is no longer the case. As mentioned previously, the IOS image is now stored in Flash memory (it can also be stored on a TFTP server, as I’ll discuss in the next chapter). ROM is now used as the memory area from which a Cisco router begins the boot process, and is made up of a number of elements. These elements are implemented via microcode, a set of programming instructions that are contained in ROM.

  • Power-on Self Test (POST). When the router is powered up, microcode stored in ROM performs a POST sequence. This is used to ensure that elements such as the CPU, memory, and interfaces are capable of functioning correctly. 
  • Bootstrap Program. The bootstrap program is used to initialize the CPU and boot functions of the router. The bootstrap program is responsible for locating and loading the router’s IOS.
  • ROM Monitor. A special diagnostic environment used for the purpose of troubleshooting or special configuration. For example, this mode can be used to transfer an IOS image over a console connection.
  • RxBoot. When a valid IOS image cannot be found in Flash or on a TFTP server, this limited IOS version is loaded for the purpose of installing a new IOS image into Flash. It is also sometimes referred to as the boot loader, boot image, or helper image. The command set provided is only a subset of normal IOS commands.

On Cisco 2500 series routers, ROM is 2MB in size. In cases where ROM needs to be upgraded (which is rare), the actual chips needs to be replaced on the router’s motherboard. When a router is powered down, the contents of ROM are always retained.


Non-Volatile Random Access Memory (NVRAM) is used as the storage location for the router’s startup configuration file. After the router loads its IOS image, the settings found in the startup configuration are applied. When changes are made to a router’s running configuration, they should always be saved to the startup configuration (stored in NVRAM) or they will be lost when the router shuts down. Remember that the running configuration is stored in RAM, which is erased when the router is powered down. On a Cisco 2500 series router, NVRAM is a relatively tiny 32KB in size. Knowing what’s going on where is an important part of not only understanding how a Cisco router operates, but will also help to determine the source of problems or issues, should the need arise. Remember – just because something can’t be seen, that doesn’t mean it’s not important.


Flash memory stores the Cisco IOS software image. On most platforms, it can store boot-images and/or configuration files. Depending on the hardware platform, Flash memory might be available as EPROM, single in-line memory modules (SIMMs), dual in-line memory modules (DIMMs), or Flash memory cards. Check the appropriate hardware installation and maintenance guide for information about types of Flash memory available on a specific platform.

Depending on the platform, Flash memory is available in the following forms:

Internal Flash memory

  • Internal Flash memory often contains the system image.
  • Some platforms have two or more banks of Flash memory on one in-line memory module (in other words, on one SIMM). If the SIMM has two banks, it is sometimes referred to as dual-bank Flash memory. The banks can be partitioned into separate logical devices.


  • Bootflash often contains the boot image.
  • Bootflash sometimes contains the ROM Monitor.

Flash memory PC cards or PCMCIA cards

  • A Flash memory card that is inserted in to a Personal Computer Memory Card International Association (PCMCIA) slot. This card is used to store system images, boot images, and configuration files.

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