ROM, RAM, NVRAM and Flash Memory on Cisco Routers

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Sometimes people get confused when it comes to telling how much memory the router has and which memory does what. There are 4 memory terms you should be familiar with, and 2 of these should be checked before upgrading the router’s IOS. These are the following:

ROM: ROM stands for Read Only Memory. It stores the System Bootsrap.
NVRAM: This is where the configuration is saved when you type copy run start.
RAM: RAM stores the running config, CDP information, ARP cache, routing table, etc. Basically everything the router needs for it’s operation. Before you upgrade the IOS you should check if the router has sufficient memory to run that particular IOS!
FLASH: This is where the IOS is stored. Before upgrading you should check if the flash is big enough to hold the old and the new images or that you have to delete the old one before downloading the new one.

To check the memory type in show version

AS#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-I-L), Version 12.1(3)T,  RELEASE SOFTWARE 
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Wed 19-Jul-00 06:05 by ccai
Image text-base: 0x03042200, data-base: 0x00001000

ROM: System Bootstrap, Version 11.0(10c)XB1, PLATFORM SPECIFIC RELEASE 
BOOTFLASH: 3000 Bootstrap Software (IGS-BOOT-R), Version 11.0(10c)XB1, 
TWARE (fc1)

AS uptime is 30 minutes
System returned to ROM by reload
System image file is "flash:c2500-i-l.121-3.t.bin"

cisco AS2511-RJ (68030) processor (revision K) with 6144K/2048K bytes of
Processor board ID 17142046, with hardware revision 00000000
Bridging software.
X.25 software, Version 3.0.0.
1 Ethernet/IEEE 802.3 interface(s)
1 Serial network interface(s)
16 terminal line(s)
32K bytes of non-volatile configuration memory.
8192K bytes of processor board System flash (Read ONLY)

Configuration register is 0x2102


The highlighted values:

6144K/2048K: This is the amount of installed memory (RAM or DRAM). DRAM is divided logically so you need to combine the two numbers to determine the total memory. 6144+2048=8192K.
32K: NVRAM to save the configuration file. It appears to be very small but because the config is just a text file it can hold a huge amount of data. Even this tiny 32K can store about a 1000-1500 lines of code! It’s handy to know thought that if you need more space to store your startup configuration you can configure the router to store it on the flash or boot from a TFTP server.
8192K: This is the size of the flash memory where the IOS is stored.


Let’s say we want to upgrade this Access Server. It is currently running c2500-i-l.121-3.t and we want to upgrade to c2500-is-l.122-8.T. First we have to check the IOS requirements on the Cisco Feature Navigator site to see if we have enough RAM and if the image would fit on the flash.


We can see that both, the RAM and the flash have to be 16MB/16MB in order to be able to use this IOS. We have only 8MB of RAM and 8MB of flash so the upgrade is going to have to happen after a quick shopping on ebay! :)

Sisko Warrior

5 Responses

  1. Please what is a DCE/DTE cable and how can I get it. Does it mean I can’t just crimp straight through cables and connect two routers with their fast ethernet ports.

    What also is a CSU/DSU device, is it a wireless router or device provided by an ISP that connects 1 to the internet? Thanks in anticipation of your help

  2. This is an excellent review for a student to become familiar with the type and size of RAM on a Cisco device. Good Stuff
    Bryan McGann

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