Type of Memory in Router
1.
2.
3.
4.
ROM
FLASH
RAM
NV-RAM
Boot Sequence of Router
ROM has a bootstrap program called ROM Monitor. When the router is turned on, it
performs the bootstrap program hardware diagnostic called Power On Self Test. If this
diagnostic is successful, then the bootstrap program loads and loads the IOS from the flash
memory. Flash memory can be rewritten. This gives Cisco router the ability to upgrade IOS.
If the bootstrap is not found in IOS Flash Memory, then the ROM is loaded with the
temporary version of the IOS. ROM cannot be rewritten or erased. You need to upgrade the
device, then you can change the ROM.
If IOS is found in flash memory then it is loaded into RAM. After that bootstrap finds the
Startup-Configuration file in NV-RAM. NV-RAM is non-volatile RAM, so its contents are
not erased.
If IOS does not find the startup configuration file, it tries to load the configuration file from
the TFTP server. If TFTP server also does not respond, then the IOS is entered in the initial
configuration mode. In this mode, users are asked questions which allow the router to be
quickly configured.
If IOS gets a startup configuration file in NV-RAM, then it is loaded into RAM and it
becomes a running configuration file.
Every Cisco passes through a booting process, this process is being explained below:
1.
2.
3.
4.
The power of the router is switched on.
Bootstrap program is loaded from the ROM.
Bootstrap runs program POST (Power On Self Test).
Bootstrap tries to load IOS from flash memory a. If IOS is not available in flash memory, then loads basic IOS from bootstrap
ROM.
b. If IOS is found in flash memory it is loaded in RAM.
5. IOV NV-RAM tries to load startup config filea. If the startup configuration file is not found in NV-RAM, then IOS tries to
load a configuration file from the TFTP server.
b. If TFTP server does not respond then the router enters the initial configuration
mode.
c. If the startup config file is found in NVRAM then it is loaded into RAM.
6. Running configuration is created in the startup configuration RAM.
1) Memory
There are 4 types of memory in Cisco IOS, which are being given below:




ROM
This is read-only memory. It is hardwired in the router. Inside the ROM is a bootstrap
program that performs power on self-test. This is called a ROM monitor mode. When
the router can not find IOS, it boots from ROM itself.
FLASH
By default First, the router only searches for flash memory for IOS and boots the IOS.
This is the electronically erasable programmable read-only memory.
RAM
It is also called dynamic random access memory. RAM is the working area of the
Cisco router CPU. In this memory, running configuration file and routing tables are
stored.
NV-RAM
It is called non-volatile random access memory. In NV-RAM the startup
configuration file is stored, which is used to start the system.
Boot Sequence of PC


Prior to boot sequence is the power-on self-test (POST), which is the initial diagnostic
test performed by a computer when it is switched on. When POST is finished, the
boot sequence begins. If there are problems during POST, the user is alerted by beep
codes, POST codes or on-screen POST error messages.
Unless programmed otherwise, the BIOS looks for the OS on drive A first, then looks
for the drive C. It is possible to modify the boot sequence from BIOS settings.