A Cisco Catalyst Switch includes hundreds of commands to fulfill the requirement of network. In this article, I will explain the essential commands. As a instructor I can personally assure you that those individuals who spent time in learning the essential technologies have always had a much higher chance of success, both in exam and on job.
To take full advantage of this article I recommend you to use Packet Tracer software. Packet Tracer is a network simulator software developed by Cisco System for critical hands-on experience. This software includes all necessary devices for the practice of entry level cisco exams. You can download Packet Tracer freely from our site. For this article I assume that you have packet tracer installed on your system.
Create a topology as showing in following image. Alternatively you can download this preconfigured topology.
In this topology
Click Switch1 and click CLI menu item and press Enter Key
Cisco switches run on proprietary OS known as Cisco IOS. IOS is a group of commands used for monitoring, configuring, maintaining cisco devices. For security and easy administration, IOS commands are divided in the set of different command modes. Each command mode has its own set of commands. Which commands are available to use, depend upon the mode you are in. Use following table to navigate between different switch command mode.
|Mode||Purpose||Prompt||Command to enter||Command to exit|
|User EXEC||Allow you to connect with remote devices, perform basic tests, temporary change terminal setting, list system information||Switch >||Default mode after booting. Login with password, if configured.||Use exit command|
|Privileged EXEC||Allow you to set operating parameters. It also includes high level testing and list commands like show, copy, debug.||Switch #||Use enable command from user exec mode||Use exit command|
|Global Configuration||Contain commands those affect the entire system||Switch (config)#||Use configure terminal command from privileged exec mode||Use exit command|
|Interface Configuration||Contain commands those modify the operation of an interface||Switch (config-if)#||Use interface type number command from global configuration mode||Use exit command to return in global configuration mode|
|VLAN configuration||Create new VLAN or configure, modify, delete existing VLAN.||Switch(config-vlan)#||Use vlan vlan-id from Global configuration mode||Use exit to return previous mode. Use end command to return in privileged exec mode.|
|Line configuration||Line configuration commands modify the operation of a terminal line. Line configuration commands always follow a line command, which defines a line number.||Switch(config-line)#||Use line vty line_number [ending_line_number] from global configuration mode||Use exit command to return in global configuration mode.|
Switch provides two types of context sensitive help, word help and command syntax help.
Word help is used to get a list of available commands that begin with a specific letter. For example if we know that our command begin with letter e. We can hit enter key after typing e? at command prompt, it will return with all possible commands that begin with letter e.
We can list all available commands, if we don't know the initials of our command. For example to list all available commands at User exec mode, just type ? at command prompt and hit enter key.
Command syntax help can be used to get the list of keyword, commands, or parameters that are available starting with the keywords that we had already entered. Enter a ? after hitting a space and switch will return with the list of available command options. For example to know the parameters required by show ip command type show ip ? and prompt will return with all associate parameters. If prompt returns with <CR> only as option, that means switch does not need any additional parameters to complete the command. You can execute the command in current condition.
Switch name can be set from global configuration mode. Use hostname [desired hostname] command to set name on switch.
Passwords are used to restrict physical access to switch. Cisco switch supports console line for local login and VTYs for remote login. All supported lines need be secure for User Exec mode. For example if you have secured VTYs line leaving console line insecure. An intruder can take advantage of it to connect with device. Once you are connected with device, all remaining authentication are same. No separate configuration is required for further modes.
Password can be set from their respective line mode. Enter in line mode from global configuration mode.
VTY term stand for virtual terminal such as telnet or SSH. Switch may support from five to thousand of VTYs lines. By default first five (0 - 4) lines are enabled. If we need more lines, we have to enable them manually. 2960 Series switch supports 16 lines. We can set a separate password for each line, for that we have to specify the number of line. In our example we set a common password for all lines.
Above method is good for small companies, where number of network administrators are very few. In above method we have to share password between all administrators. Switch supports both local and remote server authentication. Remote server authentication is a complex process and not included in any entry level exams. For this article I am also skipping remote server method. In local database authentication method switch allows us to set a separate password for each user. Two global configuration commands are used to set local user database.
Switch(config)#username [Username] password[test123] Or Switch(config)#username [Username] secret[test123]
Both commands do same job. Advantage of using secret over password is, in secret password is stored in MD5 encryption format while in password password is stored in plain text format.
Along with User Exec mode we can also secure Privilege Exec mode. Two commands are available for it.
Switch(config)# enable password Privilege_EXEC_password or Switch(config)# enable secret Privilege_EXEC_password
Again as I mentioned earlier password stored with secret command is encrypted while password stored with password command remains in plain text. You only need to use single command. If you would use both commands as I did, enable secret command would automatically replace the enable password command.
During the practice several time we have to reset switch to factory defaults. Make sure you don't run following commands in production environment unless you understand their effects clearly. In production environment we take backup of configuration before removing it. In Lab environment we could skip it. Following commands will erase all configurations from switch( without taking backup, I would include this process with backup in next article).
Switch>enable Switch#delete flash:vlan.dat Delete filename [vlan.dat]? [Press Enter Key] Delete flash:vlan.dat? [confirm] [ Reconfirm by pressing enter key] Switch#erase startup-config Switch#reload
IP address is the address of device in network. Switch allows us to set IP address on interface level. IP address assigned on interface is used to manage that particular interface. To manage entire switch we have to assign IP address to VLAN1( Default VLAN of switch). We also have to set default gateway IP address from global configuration mode. In following example we would assign IP 172.16.10.2 255.255.255.0 to VLAN1 and set default gateway to 172.16.10.1.
Switch>enable Switch#configure terminal Switch(config)#interface vlan1 Switch(config-if)#ip address 172.16.10.2 255.255.255.0 Switch(config-if)#exit Switch(config)#ip default-gateway 172.16.10.1
Switches have several interfaces. Adding description to interface is a good habit. It may help you in finding correct interface. To add description use following commands. In following example we would add description Development VLAN to interface FastEthernet 0/1.
Switch(config)#interface fastethernet 0/1 Switch(config-if)#description Development VLAN
Switch stores MAC addresses in MAC address table. Gradually it could be full. Once it full, switch automatically starts removing old entries. You can also clear these tables manually from privileged exec mode. To delete all entries use following command
switch#clear mac address-table
To delete only dynamic entries use
switch#clear mac address-table dynamic
For security purpose sometime we have to add mac address in CAM table manually. To add static MAC address in CAM table use following command
Switch(config)#mac address-table static aaaa.aaaa.aaaa vlan 1 interface fastethernet 0/1
In above command we entered an entry for static MAC address aaaa.aaaa.aaaa assigned to FastEnternet 0/1 with default VLAN1.
Switch keeps all running configuration in RAM. All data from RAM is erased when we turned off the device. To save running configuration use following command
Switch#copy running-config startup-config
Switch automatically adjust duplex mode depending upon remote device. We could change this mode with any of other supported mode. For example to force switch to use full duplex mode use
Switch(config)# #interface fastethernet 0/1 Switch(config-if)#duplex full
Switch(config)# #interface fastethernet 0/1 Switch(config-if)#duplex half
show version command provides general information about device including its model number, type of interfaces, its software version, configuration settings, location of IOS and configuration files, and available memories.
Switch stores MAC address of devices those are attached with its interfaces in CAM table. We can use show mac-address-table command to list all learned devices. Switch uses this table to make forward decision. See our previous article to learn more about switch functions.
Switch stores IOS image file in flash memory. show flash command will list the content of flash memory. This command is useful to get information about IOS file and available memory space in flash.
Configuration parameter values are created, stored, updated and deleted from running configuration. Running configuration is stored in RAM. We can use show running-config command to view the running configuration.
Any configuration stored in RAM is erased when devices is turned off. We can save running configuration in NVRAM. If we have saved running configuration in NVRAM, it would be automatically loaded back in RAM from NVRAM during the next boot. As switch load this configuration back in RAM in startup of device, at NVRAM it is known as startup-config.
show vlan command will display the VLANs. For administrative purpose, switch automatically create VLAN 1 and assign all its interfaces to it. You can create custom VLANs from global configuration mode and then assign them to interfaces.
show interface command displays information about interfaces. Without argument it would list all interfaces. To get information about specific interface we need to pass its interface number as an argument. For example to view details about FastEthernet 0/1, use show interface fastethernet 0/1.
First line from output provides information about the status of interface.
FastEthernet0/1 is up, line protocol is up ( connected)
The first up indicates the status of the physical layer, and the second up indicates to the status of the data link layer.
Possible interface status
Possible values for physical layer status
Possible values for data link layer status
show ip interface brief
show ip interface brief is a extremely useful command to get quick overview of all interfaces on switch. It lists their status including IP address and protocol.