10 commands you should master when working with the Cisco IOS

[Sayfullah]

The Cisco IOS provides thousands of commands, and configuring it can be challenging. Here are 10 commands you need to know, inside and out, when using the Cisco IOS.

#1: The "?"
It may seem entirely too obvious that you should know how to type ? to ask for help when using the Cisco IOS. However, the Cisco IOS is completely different from other operating systems when it comes to using the question mark (help key). As the IOS is a command-line operating system with thousands of possible commands and parameters, using the ? can save your day.

You can use the command in many ways. First, use it when you don't know what command to type. For example, type ? at the command line for a list of all possible commands. You can also use ? when you don't know what a command's next parameter should be. For example, you might type show ip ? If the router requires no other parameters for the command, the router will offer CR as the only option. Finally, use ? to see all commands that start with a particular letter. For example, show c? will return a list of commands that start with the letter c.

#2: show running-configuration
The show running-config command shows the router, switch, or firewall's current configuration. The running-configuration is the config that is in the router's memory. You change this config when you make changes to the router. Keep in mind that config is not saved until you do a copy running-configuration startup-configuration. This command can be abbreviated sh run.

#3: copy running-configuration startup-configuration
This command will save the configuration that is currently being modified (in RAM), also known as the running-configuration, to the nonvolatile RAM (NVRAM). If the power is lost, the NVRAM will preserve this configuration. In other words, if you edit the router's configuration, don't use this command and reboot the router—those changes will be lost. This command can be abbreviated copy run start. The copy command can also be used to copy the running or startup configuration from the router to a TFTP server in case something happens to the router.

#4: show interface
The show interface command displays the status of the router's interfaces. Among other things, this output provides the following:

Interface status (up/down)
Protocol status on the interface
Utilization
Errors
MTU
This command is essential for troubleshooting a router or switch. It can also be used by specifying a certain interface, like shint fa0/0.

#5: show ip interface
Even more popular than show interface are show ip interface and show ip interface brief. The show ip interface command provides tons of useful information about the configuration and status of the IP protocol and its services, on all interfaces. The show ip interface brief command provides a quick status of the interfaces on the router, including their IP address, Layer 2 status, and Layer 3 status.

#6: config terminal, enable, interface, and router
Cisco routers have different modes where only certain things can be shown or certain things can be changed. Being able to move between these modes is critical to successfully configuring the router.

For example, when logging in, you start off at the user mode (where the prompt looks like >). From there, you type enable to move to privileged mode (where the prompt looks like #). In privileged mode, you can show anything but not make changes. Next, type config terminal (or config t) to go to global configuration mode (where the prompt looks like router(config)# ). From here, you can change global parameters. To change a parameter on an interface (like the IP address), go to interface configuration mode with the interface command (where the prompt looks like router(config-if)#). Also from the global configuration mode, you can go into router configuration using the router {protocol} command. To exit from a mode, type exit.

#7: no shutdown
The no shutdown command enables an interface (brings it up). This command must be used in interface configuration mode. It is useful for new interfaces and for troubleshooting. When you're having trouble with an interface, you may want to try a shut and no shut. Of course, to bring the interface down, reverse the command and just say shutdown. This command can be abbreviated no shut.

#8: show ip route
The show ip route command is used to show the router's routing table. This is the list of all networks that the router can reach, their metric (the router's preference for them), and how to get there. This command can be abbreviated shipro and can have parameters after it, like shiproospf for all OSPF routers. To clear the routing table of all routes, you do clear ip route *. To clear it of just one route, do clear ip route 1.1.1.1 for clearing out that particular network.

#9: show version
The show version command gives you the router's configuration register (essentially, the router's firmware settings for booting up), the last time the router was booted, the version of the IOS, the name of the IOS file, the model of the router, and the router's amount of RAM and Flash. This command can be abbreviated shver.

#10: debug
The debug command has many options and does not work by itself. It provides detailed debugging output on a certain application, protocol, or service. For example, debug ip route will tell you every time a router is added to or removed from the router. Reference: techrepublic.com

[Sayfullah]

Nine Switch Commands Every Cisco Network Engineer Needs to Know


Overview
To be considered experts, network engineers need experience with a wide variety of commands used with network technology. At the Cisco Certified Network Associate (CCNA) level, Cisco has indicated a number of commands that should be known initially for Cisco network switches. This article covers these commands, explaining what they do and how they alter the behavior and/or use of a Cisco switch.

hostname
Syntax: hostname hostname

One of the most basic network commands, hostname configures the hostname used for a device. This hostname identifies the device to other locally connected devices for protocols such as the Cisco Discovery Protocol (CDP), which helps in the identification of devices attached directly to the network. Although it is not case-sensitive, the hostname must follow certain rules: It must begin with a letter and end in a letter or digit, and interior characters must be letters, digits, or hyphens (-).

ip default-gateway
Syntax: ip default-gateway gateway

The ip default-gateway command configures the default gateway for a switch when IP routing is not enabled (with the ip routing global configuration command), which is typical when lower-level Layer 2 switches are being configured. The easiest way to determine whether IP routing has been enabled is to run the show ip route command. When IP routing has not been enabled, the output will look similar to the following example:

SW1#show ip route
Default gateway is 10.10.10.1

Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty
SW1#
When IP routing is enabled, the output looks similar to the output displayed on a router:

SW1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        10.10.10.0/24 is directly connected, Vlan1
L        10.10.10.10/32 is directly connected, Vlan1
SW1#
NOTE
The configuration entered with the ip default-gateway command has no effect when IP routing is enabled.

username
Syntax: username username {password | secret} password

The username command configures a username and associates a password with it. Using the password or secret version of this command is a matter of security:

The password version of this command will do one of two things with the configured password:

Place the password into the configuration in plaintext (if the service password-encryption command is not enabled).
Put the password through a Cisco-proprietary encryption algorithm before placing it into the configuration. (Note that this encryption is easily reversed.)
The secret version of this command will create an MD5 hash with the configured password and then place it into the configuration. This reconfigured password is much harder to crack than the encrypted version created with the password version of this command.
This username/password can be used for a number of different features, including Telnet and SSH.

enable
Syntax: enable {password | secret} password

The enable command configures the password that will be used to access a switch's privileged configuration mode. Because all configuration of a Cisco IOS switch requires privileged configuration mode, keeping this password private is very important. As with the username command, this command has two options: password and secret. The differences between these two options are the same as those for the username command in the preceding section. The enable secret version of the command should be used in all production environments.

Console and Terminal Login Commands
Five commands are used to configure login via the control and virtual terminal (VTY) lines of a switch:

password
login
exec-timeout
service password-encryption
copy running-config startup-config
The following sections describe these individual commands.

password
Syntax: password password

When entered in line-configuration mode (console or terminal), the password command is used to configure the password that will be used to access a switch from that specific line, depending on the line mode (console or terminal). However, the password configured with this command is used only if the login command is used (which is the default).

login
Syntax: login [local]

The login command is used to enable password checking on an interface. If this command is used without any parameters, the system will check the password entered with the login against the one entered with the password command discussed in the preceding section. If used with the local parameter, both username and password will be prompted, and the entries will then be checked against the local username database that was created with the username command discussed previously.

exec-timeout
Syntax: exec-timeout minutes [seconds]

The exec-timeout command is used to configure the amount of time that can pass before a device considers the connection idle and disconnects. By default, timeout is set to 10 minutes. This timeout can be disabled with the no exec-timeout command. (This command is a shortcut and actually enters the exec-timeout 0 0 command into the configuration.)

service password-encryption
Syntax: service password-encryption

The service password-encryption command is used to enable the encryption of configured passwords on a device. The passwords referenced with this command are the ones configured with a command's password parameter, such as username password and enable password. The passwords encrypted with this command are not highly encrypted and can be broken relatively easily. By and large this command is deprecated, as most network engineers will use the secret version of the appropriate commands; however, even weak protection is better than nothing.

copy running-config startup-config
Syntax: copy running-config startup-config

The copy running-config startup-config command (popularly shortened to copy run start) is one of the most fundamental commands learned by new Cisco network engineers. It copies the active configuration (running-config) on a device to non-volatile memory (NVRAM) (startup-config), which maintains a configuration across a reload. Without this command, a configuration can be lost when a device is reloaded or powered off. The copy command can also be extended to save configuration and IOS images to and from a local device, as well as to and from different locations on the local device.

Summary
Network engineers must learn many Cisco OS commands in the process of becoming a CCNA (and beyond), and understanding these basic management commands is where the process starts. Without the knowledge of how to access devices, the complex commands are useless. You must understand when learning these concepts that they are intended to be stacked on top of each other. Lack of knowledge of a few base concepts undermines learning other, more advanced concepts that build on top of those basics. Ref: pearsonitcertification.com