Cisco IOS memory

Cisco IOS memory Types

RAM, also called dynamic RAM (DRAM), has the following characteristics and functions:

• Stores routing tables
• Holds ARP cache
• Holds fast-switching cache
• Performs packet buffering (shared RAM)
• Maintains packet-hold queues
• Provides temporary memory for the configuration file of the router while the router is powered on
• Loses content when router is powered down or restarted

NVRAM has the following characteristics and functions:

• Provides storage for the startup configuration file
• Retains content when router is powered down or restarted

Flash memory has the following characteristics and functions:

• Holds the operating system image (IOS)
• Allows software to be updated without removing and replacing chips on the processor
• Retains content when router is powered down or restarted
• Can store multiple versions of IOS software
• Is a type of electronically erasable, programmable ROM (EEPROM)

Read-only memory (ROM) has the following characteristics and functions:

• Maintains instructions for power-on self test (POST) diagnostics
• Stores bootstrap program and basic operating system software
• Requires replacing pluggable chips on the motherboard for software upgrades

Resources: 

 http://www.cisco.com/web/learning/netacad/demos/CCNA2v3Demo/ch1/1_1_2/index.html

How to calculate memory for software upgrade?

When downloading new software image for your router through Cisco downloads you see the following screen:

The first number is the required DRAM and the second number is the required Flash, here in this example we need 192MB of DRAM and 64MB space on the flash.

In order to check the current DRAM and flash in the router use the ‘show version’ command to get the following output:

The sum of the two numbers marked with no. 1 are the size of the DRAM, here in my example:

[498688+25600]/1024=512

So the total DRAM is 512MB

And the flash size, which marked with no. 2:

253008/1024=247

Hence I have 256MB flash

So my router has 512MB DRAM and 256 Flash and I can use any software image which requires 512/256 or less.

The location of this  components can be showen here: