Definition of IOS
IOS (inter-networking Operating Systems ) is the operating system which controls all the functions of the devices such as access lists to queuing and provides the User Interface (UI) for managing the device. Generally, UI is the command line interface (CLI) in Cisco devices which is similar to other popular CLIs such as DOS and C shell in UNIX.
IOS version and structure
Cisco IOS is versioned using three numbers and some letters, in the general form a.b(c.d)e, where:
- a is the major version number.
- b is the minor version number.
- c is the release number, which begins at one and increments as new releases in the same a.b train are released.
- d (omitted from general releases) is the interim build number.
- e (zero, one or two letters) is the release train identifier, such as none (which designates the mainline, see below), T (for Technology), E (for Enterprise), S (for Service provider), XA as a special functionality train, XB as a different special functionality train, etc.
Rebuilds : Often a rebuild is compiled to fix a single specific problem or vulnerability for a given IOS version. For example, 12.1(8)E14 is a Rebuild, the 14 denoting the 14th rebuild of 12.1(8)E. Rebuilds are produced to either quickly repair a defect, or to satisfy customers who do not want to upgrade to a later major revision because they may be running critical infrastructure on their devices, and hence prefer to minimize change and risk.
Interim releases : Are usually produced on a weekly basis, and form a roll-up of current development effort. The Cisco advisory web site may list more than one possible interim to fix an associated issue (the reason for this is unknown to the general public).
Maintenance releases : Rigorously tested releases that are made available and include enhancements and bug fixes. Cisco recommend upgrading to Maintenance releases where possible, over Interim and Rebuild releases.
The file naming convention is done by making use of the version of IOS and then adding all sorts of other specific information. The example, you may see an IOS image like this c4500-js40-120-3t-mz. To better understand this convention, read the steps below.
- The c4500 is for platform and it indicates that this image is for 4500 series router.
- The js40 indicates the feature set, in indicates that this image is for 4500 series router.
- The 120-3t is the version (12.0(3)T)
- M indicates that the software has to be executed from RAM.
- Z indicates that the file is zipped or compressed and it has to be decompressed into executable .bin format before loading into the device. A decompressed file is usually denoted with a 1.
Cisco internetwork Operating System was developed to deliver network services. IOS runs on most Cisco routers and on some Cisco switches such as catalyst 2950.
IOS of Cisco router is responsible for
- Carrying network protocols and
- Prevent unauthorized use of network.
- Providing network reliability for connecting to network resources.
Router Components
Cisco router use ROM and RAM similar to computers. Instead of hard drives, routers use special types of memory known as flash of NVRAM and these memories have unique function. The different types of memory include Random Access Memory, flash memory, Non-volatile memory and Read-Only Memory. Each of these are explained below
Random Access Memory (RAM) :
RAM is the only type of volatile memory in the router. With volatile memory, the contents are lost if the router loses power.
RAM contains a running copy of the Cisco IOS , which is loaded into RAM from the FLASH memory. It also contains the running copy of the configuration file which is loaded into the RAM from the NVRAM at start-up. In addition to this, it stores routing table, ARP tables and other data structures. RAM also provides packet buffering capabilities.
Non-volatile RAM (NVRAM) :
NVRAM (Nonvolatile RAM) is used to store the router’s startup configuration file and configuration register. It doesn’t lose data when power is switched off. So the contents of the start-up configuration file are maintained even when you switch off or restart the router
Flash Memory
Flash memory also holds a copy of IOS image. By flashing a new version on to the chip, this IOS image can be easily upgraded. Flash memory does not loss its contents when the router is switched off.
Read-Only Memory (ROM) :
ROM is used to store the router’s bootstrap start-up program, operating system software, and power-on diagnostic tests programs. ROM contains microcode for basic functions to start and maintain the router. RXBOOT is also contained in ROM. The table describes the major functions of Cisco router components.
Command-Line Interface (CLI IOS Command Modes)
IOS software uses a command-line (CLI as its traditional console environment. The
IOS interface is similar to a UNIX or DOS commands but the main difference is that IOS has a user interface with several command modes. Certain functions such as configuring a network interface can only be performed in a specific command mode. Each command mode permits to configure different configuration components.
User EXEC mode (user mode)
The first mode which is presented upon logging into the router is the user mode. In this mode, only a limited number of commands can be executed and no configuration parameters ca be read or modified. This mode will not help to perform any sort of debug and it does not contain informational (show) commands and also you cannot reconfigure device in any way. User mode helps to view some basic information such as routing table and the status of the components. In this mode, the prompt appears like this
Router> The greater than (>) symbol indicates that you are in user mode
Privileged EXEC Mode (enable mode)
The second mode is the privilege mode which is also known as enable mode. This mode allows users to view the system configuration, restart the system and enter configuration mode. It allows you to access all commands usable by the device and allows all the commands that are available in user mode.
To enter into the enable mode, type enable command which will ask for password (if password is set). This password will be either the enable secret of enable password. The user has to enter correct password or secret password to access privileged mode. Enable secret password will be in encrypted form when it is stored in the configuration and it is safer. Once you enter into the enable mode, the prompt will be changed to # sign it will be as shown below
Router>enable
Password:
Router#
In the privileged mode, you can use any standard commands such as show command and debug command. Debug command are special commands used to perform advanced diagnostics on the system. Few other configuration commands such as clear, clock, reload, copy and erase can be used in privilege mode. To exit the privileged mode, the user has to enter disable command.
To configure parameters of the router, one needs to proceed from privileged EXEC mode to the global configuration mode.
Global Configuration Mode
By typing configure terminal takes user to the global configurations mode from the enable mode which helps to configure the device. This mode allows users to modify the running system configuration. Once the user enter into the configuration mode, the prompt again changes which can be illustrated as shown below
Router # configure terminal
Router(config)#
This is also known as global configuration mode because all the commands used here will have an affect on the entire device.
Global configuration mode consists of sub modes such as interface mode and line mode. As the configuration sub-modes changes depending on what is being configured, the words within the parenthesis also changes for example: when user enters interface configuration sub-mode, the prompt changes to (config-if)# following the router name.
To exit configuration mode, the user can enter end or press Ctrl+Z
Interface Mode
The commands entered in this mode affect the current interface only. Interface command can be used to modify the configuration of a specific interface. In order to perform any change enter into global configuration mode:
Router 1 (config)# interface serial 0
OR
Router 1 (config)# int serial 0
The result will be:
Router 1 (config-if)#
The prompt changes to Router (config-if) # which indicates that you are in interface configuration mode.
Line Mode
Line command helps to configure user-mode passwords. The prompt then changes to Route (config-line)#;
Router (config)# line con 0
OR
Router 1 (config)line vty 0 4
The result will be:
Router 1 (config-line)#
The line console 0 commands are called as major command and it is also known as global command. The commands which are entered from (config-line) prompt are known as global command.
Each mode is represented by a different prompt, as show in the table below:-
Command mode
| Command and mode | Prompt | Command to enter mode |
| User EXEC | Router> | Login |
| Privileged EXEC | Router# | Enable |
| Global Configuration | Router(config)# | Configure terminal |
| Interface Configuration | Router(config-if)# | Interface type number ( from the global configuration mode ) |
| Line Configuration | Router(config-line)# | Line line_type line_number editing_line_number ( from global configuration mode ) |
Enhanced Editing Commands and History Commands
The user interface contains an enhanced editing mode that provides a set of editing key functions. No terminal editing command at the privileged mode helps to disable advanced editing features. You can also re-enable these features bu using terminal editing command. The table below shows some of the editing commands and how it can be helpful while working on router
Editing commands
| Command | Meaning |
| Ctrl+A | Will take cursor to the beginning of the current line |
| Ctrl+E | Moves the cursor to the end of the line |
| Esc+B | Moves back one word |
| Ctrl+B | Moves back one character |
| Ctrl+F | Move forward one character |
| Esc+F | Moves forward one character |
| Ctrl+D | Deleted the single character |
| Backspace | Removes one character to the left of the cursor |
| Ctrl+R | Redisplays a line |
| Ctrl+U | Erases a line |
| Ctrl+W | Erases a word |
| Ctrl+Z | Ends configuration mode and returns to EXEC mode |
Router Command History
Router command history will shown the commands which are typed previously. The history size can be changed and it can be verified by using show terminal command. The table below shows the router command history with the commands:
| Command | Meaning |
| Ctrl+P or up arrow | Shows last command entered |
| Ctrl+N or down arrow | Shows previous command entered |
| show history | Shows last 10 commands typed by default |
| show terminal | Shows terminal configuration and history buffer size |
| terminal history size | Changes buffer size but the maximum size is 256 |
0 comments:
Post a Comment