RAM NVRAM Flash ROM
Temporary storage for router configuration files
RAM content is lost on power down or restart
Fast switching cache
Stores backup/startup configuration files
Content is not lost when router is powered down or
EEPROM (Electronically Erasable Programmable
Holds the Cisco IOS
(Internetworking Operating System)
Allows updating of software without replacing the
Multiple versions of IOS can be stored
Retained on power down
Contains POST (Power On Self Test)
A bootstrap program
(loads the Cisco IOS)
And mini-operating system software:
Backup, trimmed down version of the IOS
Upgrades require installing new chip set
Network connections through which packets enter
and exit the router
Attached to the motherboard or as separate modules
on “modular” routers.
As the router is booted, the following is
loaded into RAM’s Working Storage.
Programs include the bootstrap program that
• tests the equipment (POST)
• and locates the IOS.
Once the IOS is located by the bootstrap
program, it is loaded in to RAM.
Part of the IOS is the Command EXEC. This is the
program that translates the commands you type
into a language that the IOS can understand.
Once the IOS is initialized, it looks for an active
configurations file (usually in NVRAM) and loads
it. This file tells the router specifically what to do.
The tables are loaded from the configuration file
• ARP Tables
• Routing Tables
Finally, all leftover memory in RAM is used as the
buffer for processing incoming and outgoing
Router Status Commands
“Show” commands yield status information about
various router components, as shown on the next
Example Show Commands
configuration of the system hardware
names and sources of configuration files
the current setting of the configuration register
(used in password recovery)
displays the active configuration file loaded in RAM
displays the backup configuration file stored in NVRAM
# copy run start
displays the status of all configured Layer 3 protocols
displays the status of all interfaces
show ip route
displays the routes to all networks known to the router
displays information about each interface controller.
Key: shows whether serial interface is DTE or DCE
(setting determined by the connected cable).
Important mainly in Cisco lab, where some routers are configured