Lines on Cisco routers

Lines on Cisco routers are physical async serial ports on the router (such as a terminal or modem), a virtual network connection, or another type of serial line on the router. To see which lines you have on your router, use the show line command. Example: show line serial 0/0

Use the show line summary command to get a nice overview.

The console port doesn't need any introduction. The CTY port is, of course, where you configure the router when it’s brand-new — before it has any IP address configuration. The console port is a serial port, so you must have a PC/laptop with a serial interface and connect to the console with a rolled cable, it should be included with your newly arrived devices :)

Once you’ve used the console port to configure the router’s network configuration, it isn’t common to have to use it again. However, it’s good to know that it’s there if anything ever goes wrong. In addition, you should secure the console port to keep someone from connecting to it when you aren’t around.

While not all routers these days have an AUX port, the AUX port is the auxiliary. Think of it as a secondary console port. The AUX ports don’t get a lot of use except to access the router if locked out of the console port. In the past, network admins would connect modems to the AUX ports so they could dial into their routers. Like the console port, the AUX port is a serial port, and you should also take steps to secure it.

VTY ports are virtual TTY ports, used to Telnet or SSH into the router over the network. You can use them to connect to the router to make configuration changes or check the status. Most routers have five VTY ports, numbered 0 to 4. That means you can have up to five concurrent network admins configuring the router at one time.
 

Remember that you can always use the clear line command to clear out a connection on a router line if you run into a problem.