====== Cisco IOS ======

Cisco's **I**nternetwork **O**peration **S**ystem is the operating system that runs on a majority of Cisco hardware. In the case of CCNA R&S it'll be running on routers and switches.

===== Command Modes =====
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IOS operates in 3 main command modes to accomplish config tasks.

== User EXEC Mode ==
The first mode, also known as ''**guest mode**''. This is the mode you'll be in if you fresh boot a device or remote in with telnet / SSH. The User EXEC Mode prompt is a single ''**Router>**''

== Privileged EXEC Mode ==
The second mode, also known as ''**enable mode**''. To enter Priveleged EXEC Mode issue the ''**enable**'' command from User EXEC. This mode hosts a majority of ''**show**'' commands in IOS and is where you perform restarts and save configurations to ''**NVRAM**''. The prompt for this mode is an octothorpe: ''**Router#**''

== Global Configuration Mode ==
The third mode. To enter Global Configuration Mode issue the ''**configure terminal**'' command from Privileged EXEC Mode. This is the mode you'll be in for doing most configuration tasks. It's from this mode you can branch off into it's submodes to configure specific interfaces and settings. Config Mode prompt is ''config'' enclosed in parens followed by an octo: ''**Router(config)#**''

===== Basics =====
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Issuing ''**exit**'' will take you to the previous mode. Example:
  Router(config)# exit
  Router#

''**CTRL-Z**'' will immediately throw you back into User EXEC mode from any config mode.

''**?**'' is basic help. Entering ''**?**'' at any point during a command or even at a blank prompt will give you a list of available commands. Use it often.
  Router(config)#interface fastEthernet ?
    <0-9>  FastEthernet interface number
Tab completion is available and short commands are allowed:
  Router(config)# interface fa<TAB>
  Router(config)# interface fastEthernet

  Router(config)# int fa0/1
  Router(config-if)#

While in Global Configuration Mode or deeper, prepend ''**do**'' to run Privileged EXEC commands:
  Router(config)# show run         # Doesn't work!
  Router(config)# do show run      # Will work!

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********** ADD OUTPUT MODIFIERS