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Basic Cisco Terminal Access

June 02, 2009
Jason Wyatte

Cisco Switches and Routers running the Internet Operating System (IOS) have many things in common. Configuring these devices of course, is a skill that is sharpened the more you touch the device. During this post, our discussion will primarily focus on the basic commands associated with console and telnet access to routers and switches for out-of-band and in-band management.

First the routers and switches can be configured with an out-of-band interface called the console port. (I’m pretty sure many of you are familiar with the console port and that you have to connect a rollover cable with a DB 9 or 25 pin connector, to that interface to gain access.) The console port is important because system messages are displayed by default and access to this interface is always up (though it may be password protected). The Auxiliary port is another type of out-of-band management interface but it interfaces with a modem. Console and Auxiliary ports are a special type of EIA 232 (RS 232 is the older term for this) that normally uses an RJ 45 connector. However the auxiliary port uses RTS and CTS for flow Control with the modem. These pins (1 and 8) on the console port are disabled.

Configuring the console port and auxiliary ports are normally a basic procedure. The global commands for displaying the default for these ports are line con 0, line aux 0. On these lines it’s possible to add a simple layer of security by adding a line password to these ports. As shown in the example, you can configure a simple password.

Example 1

example 1

The example also show the exec-timeout 0 0 command. This means that on the console port will never timeout. The default for this command is exec-timeout 10 0 which means that the console line will logoff when inactivity at this line reaches 10 minutes.

Other things commands shown with the command above help the display of the terminal access when someone is logged in. Logging synchronous means that system messages will never interrupt your terminal display. This command prevents you from being distracted when many system messages pop up when interfaces come on or off line, or router neighbor relationships are built or torn down. Also the length 40 means that 40 lines will be displayed at a time when a show command is issued. The example below displays the terminal length 5 command which is another form of the length command. Here you can see it only displays five lines at a time. If the terminal length is set to zero, then every show command will be displays from beginning to end without stopping.


example 2

Second, configuration can be done through the network via a web based interface like Cisco SDM or though Telnet or SSH. The HTTP and HTTPS must be enabled for SDM to work on a router. For telnet and SSH to access the router or switch must be reachable via and ipv4 or ipv6 address and you must configure the VTY (Virtual Teletype) lines. When configuring line vty 0 4, Login and password commands are necessary for telnet. Trying to configure these commands without the other can result in these messages when trying to telnet or configuring. (In a future post SSH and SDM will be discussed.)


example 3


example 4


example 5

Above you can see that Example 3 and 4 both display a message that says the login process will be disabled until the password is set. Example 5 shows what will happen when someone tries to telnet to that router and it is connection being closed. It is also possible to configure more VTY lines than the default, though this is normally done in environments where terminal access is the overall goal.

These are basic terminal commands and settings. For the CCNA these skills must be well understood and under your belt. Using these commands can help any network administer when managing routers and switches.

Author: Jason Wyatte