Cisco IOS Introduction

What is Cisco IOS you ask?  It is the operating system that runs Cisco's network devices.  This operating system allows the hardware to become flexible and malleable with respect to your network application.  It was once told to me that Cisco considers itself more of a software vendor than a hardware vendor - this is pretty much true when you consider that just plugging things into a switch or router with an idea in mind does nothing except establish physical connectivity; the true magic of bringing networks to life and securing them happens within the IOS.

Cisco IOS comes in many flavors for all of its devices.  It is not uncommon to run into scenarios where certain commands, protocols, or functions are not available simply because of the IOS version that is being used, likewise, it is also not uncommon to find versions of the IOS for other versions of the device that have additional capabilities.

Cisco recommends giving four points important consideration when considering upgrading or implementing an IOS on your device(s).

1. Check to be sure your hardware is supported
• Use the Cisco Product Documentation section of your Documentation CD
• Use the Cisco Feature Navigator Tool
2. Check Feature Support
• Run #show version and record the version of software you are using
• Use the Output Interpreter Tool to find potential issues and fixes
• I RECOMMEND running all of your IOS versions at least once a year to determine if there are upgrades you should be using to close any "flaws" or potential problems, security or otherwise, with the IOS version you are using.
3. Choose Cisco IOS Release Version
• Verify the one you are looking for supports the features you want, your hardware, and is compatible with the memory of your device
• Release format is:  A,B(C)D
• A, B, C are numbers and D (if shown) is a letter
• A,B are major releases
• C is a maintenance release/version (bug fixes)
• D is not a major release, but an extension of the major release it is attached to (contains new features usually)
4. Memory Requirements
• Make sure your device meets the memory requirements for the particular image you are looking at - DRAM, Packet Memory, Flash Memory

Your devices have to be registered to access the IOS downloads.  IOS downloads typically come as a .bin file.

For more detailed basic IOS information you can reference Cisco's page here