• RAM
  • Stores packet buffers, ARP cache, routing table, software code and data structures necessary for router operation. Running configuration and decompressed IOS code is stored in RAM
• ROM
  • Contains basic software for hardware testing and initialization
• Flash
  • Stores IOS and backup configuration files. Not volatile
• NVRAM (non-volatile RAM)
  • Saves router configuration

• POST
  • Power-on Self-Test. Stored in ROM. Checks basic router functions

    • Bootstrap
      • In ROM. Initiates router and loads IOS
    • ROM Monitor
      • In ROM. Used for tests and troubleshooting. Basic interface for troubleshooting low-level issues
    • IOS (Internetwork Operating System)
      • Provides all of the higher-level router functionalities

• Controls various low-level settings
  • Tells router to load or ignore NVRAM configuration at startup
  • Terminal behaviour (e.g. serial speed)
• Current value can be seen with
  • show version
• Most common settings are:
  • 0x2102 - normal
  • 0x2142 - ignore configuration

• Router always has two configurations
  • running-config
    • In RAM. Shows which parameters are currently in use.
    • show running-config
    • Modified with configure terminal command
  • startup-config
    • In NVRAM. Loaded by router in next reboot
    • This is where the running-config is saved
    • show startup-config

• You can store configuration in other places
  • In router’s Flash memory
  • In a server, via TFTP
• Can be copied around with copy command
  • copy running-config startup-config
    • or write memory (old style)
  • copy running-config tftp
  • copy startup-config tftp
  • copy startup-config flash:saved-config
  • copy flash:saved-config startup-config

• It asks Would you like to enter the initial configuration dialog? [yes/no]:
• Just say n (no)
  • otherwise you get a tedious series of questions to answer

• User EXEC
  • Limited access. Show interface state, etc.
  • Router>
• Privileged EXEC (enabled mode)
  • Detailed examination, manipulate configuration and files, run tests, debugging, etc.
  • Router#
• ROM Monitor
  • Password recovery and IOS installation
  • rommon 1>

• Console
  • Direct access via serial port
• Auxiliary Port
  • Access via Modem
• Virtual Terminals (VTY)
  • Telnet/SSH

Commands are activated immediately. So be careful when typing!

When working on serial console or via Telnet or SSH, commands can be copied from a text file and pasted into the terminal

router>
router>enable
... type password ...
router#
router# configure terminal
router(config)#
... type commands ...
router(config)# end
router# write memory

Router> - USER EXEC
Router# -  PRVILEDGED EXEC
Router(config) - Global configuration
Router(config-if) - Interface configuration
Router(config-subif) - Sub-interface configuration
Router(config-route-map) - Route-map configuration
Router(config-router) - Routing protocol configuration
Router(config-line) - Line configuration
rommon 1> - ROM Monitor

Use “?” to obtain a list of commands available in your current configuration mode

Router(config)#?
Configure commands:
  aaa                         Authentication, Authorization and Accounting.
  aal2-profile                Configure AAL2 profile
  access-list                 Add an access list entry
  alarm-interface             Configure a specific Alarm Interface Card
  alias                       Create command alias
  appfw                       Configure the Application Firewall policy
  application                 Define application
  archive                     Archive the configuration
  arp                         Set a static ARP entry
  ...

Use “?” also to see all possible parameters to an incomplete command:

Router(config)#username ?
  WORD  User name
Router#show ?
  aaa                       Show AAA value
  aal2                      Show commands for AAL2
  access-expression         List access expression
  access-lists              List access lists
  accounting                Accounting data for active sessions
  ...

Use the Tab key to complete a command

router(config)#int<TAB>
router(config)#interface et<TAB>
router(config)#interface ethernet 0
router(config-if)#ip add<TAB>
router(config-if)#ip address n.n.n.n m.m.m.m

You can just type the abbreviated form if it's unambiguous

router(config)#int e 0
router(config-if)#ip add n.n.n.n m.m.m.m

Move within command history:

• [Cursor Up] - Previous command
• [Cursor Down] - Next command

Line editing:

• [Cursor Left] and [Right] - move within the line
• Ctrl-a : move to beginning of line
• Ctrl-e : move to end of line
• Ctrl-k : delete until end of line

show running-config
show run interface f0/0
show ip int brief
debug ip ospf hello / events / adj
show log
show version