CCENT (Cisco Certified Entry Network Technician) Certification Exam Notes

E. Cisco IOS

1. Cisco router loads the IOS image based on the boot field and the boot system commands in the configuration. Various boot system commands are:

1. boot system flash <IOS_filename> ;IOS image is loaded from flash memory
2. boot system tftp <IOS_filename> <tftp_ip_address> ;Boots the system image from the TFTP server.
3. boot system rom ; IOS is loaded from rom
4. The command to save the running configuration to NVRAM is:
copy running-config startup-config (write memory)

2. At times it may be necessary to upgrade the IOS software. You cannot load the router from Flash, and upgrade the flash at the same time. You need to load from other sources such as tftp server or ROM. To load the image from ROM, there are two ways:
· BY using config-register 0X2101 command. This will enable the router to load the IOS from the ROM and Flash image is not used. This way we can load the upgrade to flash, and set back the configuration register to load from flash as usual after the upgrade is completed.
· Another way is by adding the command BOOT SYSTEM ROM to the startup configuration file.

3. Boot system - This is a global command that allows you to specify the source of the IOS software image to load. If you configure more than one source, attempts are made to load the IOS from the first command in the configuration to the last successively. If the first fails, the second boot command is used.
· Boot system rom - Loads IOS from ROM.
· Boot system flash - Loads the first file from flash memory.
· Boot system tftp <file name> < tftp_address > - Loads IOS with a filename <file name> from a TFTP server.
Both "copy star run" and "reload" will load Cisco router configuration into RAM.

4. Router Memory Components: 

·ROM (Read Only Memory); Memory containing micro-code for basic functions to start and maintain the router. ROM is not typically used after the IOS is loaded. RXBOOT is located here.
· RAM/DRAM: stores the running configuration, routing tables, and packet buffers. Some routers, such as the 2500 series, run IOS from Flash, not RAM.
· NVRAM (Non-Volatile Ram): Memory that does not lose information when power is lost. Stores the system’s configuration file and the configuration register. NVRAM uses a battery to maintain the data when power is turned off.
· Flash Memory: Stores the compressed IOS (IOS stands for Cisco Internetwork Operating System) image. Flash memory is either EEPROM or PCMCIA card. Flash memory enables you to copy multiple versions of IOS software. This allows you to load a new lever of the operating system in every router in your network and then, to upgrade the whole network to that version at a convenient time.

5. When you first switch on a router, you enter Setup mode. Setup mode is different from configuration mode in that setup mode appears when there is no configuration file present. Upon entering setup mode, you can supply some basic configuration parameters to Cisco router.

6. Router modes of operation:

1. User EXEC mode:- This is the LOWEST level of access. This allows examination of router status, see routing tables, and do some diagnostics. However, you cannot change the router configuration, view the configuration files, or control the router in any way. The prompt in this mode is "Router>".
2. Privileged (enable) EXEC mode:- This mode allows you to have all the privileges of EXEC (user) mode plus commands that enable you to view configuration files, change the router configuration, perform troubleshooting that could potentially disrupt traffic. The default prompt for this mode is "Router#".
When you are working in the privileged mode (at # prompt), you can get back to user mode by typing "disable" at the "#" prompt.

7. The Cisco router can be configured from many locations.

1. Console port: During the initial installation, you configure the router from a console terminal connected to the "Console port" of the router.
2. Virtual Terminals (vty): A virtual terminal (vty) is typically accessed through Telnet. A router can be accessed through vty after it the initial installation in the network. There are five virtual terminals, namely, vty0, vty1, vty2, vty3, vty4.
3. Auxiliary Port: you can configure a router through auxiliary port. Typically, a modem is used to configure the modem through aux port.
4. TFTP Server: Configuration information can be downloaded from a TFTP server over the network.
5. NMS (Network Management Station): You can also manage router configuration through NMS such as CiscoWorks or HP OpenView.

8. There may be times that you have lost the passwords for accessing the router and need to recover from the lost passwords. ROM Monitor (rommon), stored ROM is used for password recovery. By hitting special Break key within the first 60 seconds of the router power-on, it is possible to recover from lost passwords. Broadly, the sequence of operations is as below (For a 2501 router):

Typical sequence of steps involved in password recovery (on 2501 router) is:
1. Power cycle the router by turning it OFF and ON.
2. Press Break key (key configuration depends on the router type) within 60 seconds of power on.
3. Use the rommon command o/r 0X2142
4. Use rommon command INITIALIZE to load IOS
5. Skip the setup mode (by choosing NO to configuration commands).
6. Enter privileged mode
7. After entering the privileged mode, you can view the passwords by issuing appropriate config commands.

9. During password recovery, the config register and NVRAM are modified. ROM hold the bootstrap code to start up the router and Flash contain the IO image.
Each exclamation mark (!) represents that one UDP segment has been successfully transferred. While erasing, you get 'e' displayed on screen.

10. To change the enable password, you use "enable password password".
To change the enable secret, you use "enable secret password".

11. The following are some important commands that can be used to edit and review command history buffer. It will be useful to practice these commands.
· <ctrl> A : Move to the beginning of the command line
<ctrl> E : Move to the end of the command line
<ctrl> F : Move forward one character, same as using "Right Arrow".
<ctrl> B : Move backward one character, same as using "Left Arrow".
<ctrl> P : Repeat Previous command, same as using "Up Arrow".
<ctrl> N : Repeat Next (more recent) command, same as using "Down Arrow".
<esc> B : Moves to beginning of previous word.
<esc> F : Moves to beginning of next word.
<ctrl>R : Creates new command prompt, followed by all the characters typed at the last one.

12. To configure your router to do look-up for domain name server, the correct syntax is:
· ip name-server <ipaddress_of_server>
Here the ipaddress_of_server is the ip address of the DNS server.

13. To enable the Cisco IOS to forward packets destined for obscure subnets of directly connected networks onto the best route, you use "ip classless" command.

14. By default, Cisco routers support 5 simultaneous telnet sessions. This number can be configured using ios commands.

i. CDP

1. CDP, short for Cisco Discovery Protocol runs over Layer 2 (the data link layer) on all Cisco routers, bridges, access servers, and switches. CDP allows network management applications to discover Cisco devices that are neighbors of already known devices. CDP runs on all LAN and WAN media that support SubNetwork Access Protocol (SNAP).

2. The “no cdp enable” instruction has to be given at the interface configuration mode, where CDP needs to be disabled. (in this case S0/0).

3. The command "show CDP neighbors" displays all the neighboring devices connected and their capability. Several capability codes are:
· R - Router H - Host
· T - Trans Bridge I - IGMP
· B - Source Route Bridge r - repeater
· S - Switch

This command displays the following:
1. Neighbor Device ID : The name of the neighbor device;
2. Local Interface : The interface to which this neighbor is heard
3. Capability: Capability of this neighboring device - R for router, S for switch, H for Host etc.
4. Platform: Which type of device the neighbor is. (2500 router or anything else)
5. Port ID: The interface of the remote neighbor you receive CDP information
6. Holdtime: Decremental hold time in seconds

4. The command "terminal no editing" stops all advance editing features.

5. Important CDP Commands:
· The command “cdp run” in global configuration mode enables CDP globally on the router.
· The command “cdp enable” is used to enable CDP on a particular interface/port.
· The command used to disable CDP is “no cdp enable”.
· The command “ip default-gateway” in global configuration mode can be used to sets the default gateway on a Cisco switch/router.

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