Introduction to Router’s Internal Components

Internal Components

RAM NVRAM Flash ROM

InterfacesConsoleAuxiliary

RAM

Temporary storage for router configuration files

RAM content is lost on power down or restartStores...

Routing tables ARP cache Fast switching cache Packet buffering

NVRAM

Non-volatile RAMStores backup/startup configuration filesContent is not lost when router is powered

down or restarted.

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 storedRetained on power down

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

Interfaces

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.

RAM Specifics

CommandExecutive

InternetworkOperating

System(IOS)

ProgramsActiveConfig

FileTables Buffers

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

RAM Specifics

RAM Specifics

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

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.

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.

RAM Specifics

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

RAM Specifics

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

Router Status Commands

“Show” commands yield status information about various router components, as shown on the next slides...

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)

show running-config displays the active configuration file loaded in

RAM

show startup-config displays the backup configuration file stored in

NVRAM # copy run start

show protocols 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

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.