1. Introduction to Router’s Internal
Components

2. Internal Components
RAM NVRAM Flash ROM
Interfaces
Console
Auxiliary

3. RAM
 Temporary storage for router configuration files
 RAM content is lost on power down or restart
 Stores...
 Routing tables
 ARP cache
 Fast switching cache
 Packet buffering

4. NVRAM
 Non-volatile RAM
 Stores backup/startup configuration files
 Content is not lost when router is powered down or
restarted.

5. Flash
 EEPROM (Electronically Erasable Programmable
Read-Only Memory)
 Holds the Cisco IOS
(Internetworking Operating System)
 Allows updating of software without replacing the
Flash chip
 Multiple versions of IOS can be stored
 Retained on power down

6. ROM
 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

7. Interfaces
 Network connections through which packets enter
and exit the router
 Attached to the motherboard or as separate modules
on “modular” routers.

8. As the router is booted, the following is
loaded into RAM’s Working Storage.
RAM Specifics
Command
Executive
Internetwork
Operating
System
(IOS)
Programs
Active
Config
File
Tables Buffers

9. Programs include the bootstrap program that
• tests the equipment (POST)
• and locates the IOS.
RAM Specifics

10. RAM Specifics
Once the IOS is located by the bootstrap
program, it is loaded in to RAM.

11. RAM Specifics
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.

12. RAM Specifics
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.

13. RAM Specifics
The tables are loaded from the configuration file
including...
• ARP Tables
• Routing Tables

14. RAM Specifics
Finally, all leftover memory in RAM is used as the
buffer for processing incoming and outgoing
packets.

15. Router Status Commands
 “Show” commands yield status information about
various router components, as shown on the next
slides...

16. Example Show Commands
show version
 displays:
 configuration of the system hardware
 IOS version
 names and sources of configuration files
 the current setting of the configuration register
(used in password recovery)

17. show running-config
 displays the active configuration file loaded in RAM

18. show startup-config
 displays the backup configuration file stored in NVRAM
 # copy run start

19. show protocols
 displays the status of all configured Layer 3 protocols
 displays the status of all interfaces

20. show ip route
 displays the routes to all networks known to the router

21. show controller
 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
DCE.