Telnet protocol enables users to remotely connect to Cisco devices and it is enabled by default on most Cisco device. However it is not very secure to enable Telnet on your Cisco device as the login information and commands are sent in clear text and can be easily hacked. It is recommended to use SSH for remote access as this gives you secure, encrypted connection to your Cisco device.
This article demonstrates how to disable Telnet Protocol on a Cisco Router ( the same applies to all Cisco IOS devices)
To disable Telnet and enable only SSH connections:
First login to the Cisco Switch or Router and enter configuration mode
Router# config t Router1(config)# line vty 0 4 Router1(config-line)# transport input ssh
This disables telnet and enables ssh on all the five VTYs (Virtual Terminal Lines)
On earlier platforms five simultaneous remote connections are allowed and these are vty 0 to 4. On IOS 12.2 and up 16 simultaneous connections are allowed. On these
Router1(config)# line vty 0 15 Router1(config-line)# transport input ssh
This disables telnet and enables ssh on all the 16 VTYs.
Sometimes you want to disable all forms of remote access (Telnet and SSH). For this
Router1(config)# line vty 0 4 Router1(config-line)# transport input none
Or
Router1(config)# line vty 0 15 Router1(config-line)# transport input none
After this you can only connect to your Cisco device by direct console connection.
To enable Telnet and SSH back
Router1(config)# line vty 0 4 Router1(config-line)# transport input telnet ssh
(Note: The default for this command is transport input all which allows all protocols)
