Which memory stores running ios configuration

Before we discuss the memories that are used in a Cisco router, let’s understand what basically a Router is.

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In which memory the IOS is stored?
Which memory type stores the running configuration?
Which memory location on a Cisco IOS device stores the startup configuration file?
In which memory the IOS is located in Cisco router?

When we want to connect two or more IP networks or subnetworks together, then a piece of hardware is needed and that hardware is called a Router. It generally serves two purposes:

Enabling the usage of a single Internet connection by several devices.
By forwarding packets to their designated IP addresses, traffic among these networks can be monitored.

PC1 and PC2 are connected to a router that is connected to Internet Service.

How does Router work?

Each data packet has a distinct destination and travels a distinct path to get there and these packets are guided by the router. The router also ensures that each data packet is successfully delivered to its destination without any interference in between the transmission. The best route is selected with the help of routing protocols like RIP(Routing Internet Protocol) /OSPF(Open Shortest Path First).

A routing table is used by the router internally where both the static (these types of addresses are assigned by the ISP(Internet Service Provider) and these types of addresses do not change) as well as dynamic addresses (these types of addresses are assigned using DHCP(Dynamic host configuration Protocol) and these addresses change over time, they are not fixed) are stored.

Different types of Memories in a Router :

1. Random Access Memory (RAM) –
Random Access Memory (RAM) in a router is similar to the RAM that is installed in our PCs, Mobile Phones, and Laptops. The RAM is categorized into two areas:

Main Processor Memory, where information related to running router configuration (commands that we are currently running), routing table, and ARP (Address Resolution Protocol) cache is stored/saved.
Shared I/O Memory, which acts as a temporary storage memory where the data packets that are in queue are stored.

Whenever the Router is rebooted or restarted, all the data, all the information that is stored in the RAM gets removed or deleted. The data gets deleted because all the information in the RAM is temporarily stored in a router. We can permanently store the data in NVRAM.

2. Non-Volatile Random Access Memory (NVRAM) :
NVRAM is used to store the startup configuration file. Startup configuration files are the copies of the Cisco Router Configuration file and they are retained after the router is restarted or rebooted. In NVRAM, if the router is rebooted or if the router is switched off then the data is not lost, it can be easily recovered. If we want to save the running configuration files permanently then we can move these files from RAM to NVRAM.

3. Read-Only Memory (ROM) :
A Cisco router’s boot procedure starts from the ROM memory section. ROM consists of programming instructions like POST (Power-On-Self-Test) and Bootstrap program. POST test is used to verify that whether hardware components like CPU, RAM, and interfaces are functioning properly or not. If they are not working properly then POST sends an error message. After this, the bootstrap application is used to set up the router’s CPU and boot functions. The bootstrap program is in charge of discovering and loading the router’s operating system (IOS). All this information is saved/stored in ROM and the data is retained even if the router is switched off or rebooted.

4. Flash Memory :
This is an Erasable Programmable Read-only Memory chip (EPROM). In flash memory, the operating system of the router I.e. IOS (Internetwork Operating System) is available. Unlike ROM, it is easily upgradable and does not require any hardware changes. In flash memory also, the content is retained whenever the router the switched off or rebooted.

Summarizing different memories that are used in Cisco Router –

RAM stores the currently working tasks.
NVRAM where startup configuration takes place.
ROM where the information of POST and bootstrap program is available.
And Flash Memory where the operating system of Router IOS is present.

It may often seem like nothing more than a sleek metal box on the outside, but the internal memory components of a Cisco router represent where much of the real magic takes place.

Speak of a Cisco router is being really no more than a specialized computer running a custom operating system. In this case, it is a computer optimized to provide routing and related functions. Instead of relying on a hard disk for storage, a Cisco router relies on different types of memory, each with a different purpose. In this article we’ll take a look at each of these different storage areas, and the functions they’re responsible for.

There are four main memory areas within a Cisco router that it’s important to be familiar with, namely Flash, RAM, ROM, and NVRAM.

Flash Memory
Flash memory is implemented (on a Cisco 2500) using two Single Inline Memory Module (SIMM) slots that hold erasable programmable read-only memory (EPROM). Flash memory is used to store and run the Cisco IOS software – the router’s operating system. When a router is powered down, the contents of Flash memory are not lost. However, its contents can be upgraded by “flashing” the chip, not unlike one can do with a BIOS in the PC world. While a router is running, the contents of Flash are set to a read-only mode.

Flash memory for a Cisco 2500 series router ranges in size from a minimum of 4MB up to a maximum of 16MB. You might consider adding additional Flash memory to meet the space requirements of the IOS version that you have chosen to run. For a Cisco 2501, the base IP version of IOS 12.0 requires a minimum of 8MB of Flash memory. So, if you had a Cisco 2501 that shipped with only 4MB of Flash, you would require at least one additional 4MB SIMM. For IOS versions with more advanced feature sets, it is not uncommon to require at least 16MB of Flash.

When installing or upgrading Flash using multiple SIMMs, it is important to note that they must be the same size. For example, if you already have 4MB of Flash and wish to upgrade, you can either replace the 4MB SIMM with an 8MB SIMM, or simply add a second 4MB SIMM. You cannot mix and match SIMMs with different storage capabilities. As such, you cannot have one 4MB and one 8MB SIMM installed at the same time – their storage capabilities must equal.

RAM
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. 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.

ROM
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.

NVRAM
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.

More Related Readings:

Cisco Memory: Types and Functions

Maintaining Router Memory

In which memory the IOS is stored?

The Flash memory is where the IOS is stored at boot time.

Which memory type stores the running configuration?

RAM (Random Access Memory) – the running configuration and routing tables of the device are stored here.

Which memory location on a Cisco IOS device stores the startup configuration file?

NVRAM — Nonvolatile RAM (NVRAM) stores the initial or startup configuration file that is used when the Cisco device is powered on or reloaded.

In which memory the IOS is located in Cisco router?

There are at least two main pieces of non-volatile storage in a Cisco router. The router's configuration information is stored in a device called the Non-Volatile RAM (NVRAM), and the IOS images are stored in a device called the flash (lowercase).