Prior to the introduction of SSH in the Cisco IOS, the only remote login protocol was Telnet. Although quite functional, Telnet is a non-secure protocol in which the entire session, including authentication, is in clear text and thus subject to snooping.
SSH is both a protocol and an application that replaces Telnet and provides an encrypted connection for remote administration of a Cisco network device such as a router, switch, or security appliance.
The Cisco IOS includes both an SSH server and an SSH client. This document is concerned only with the configuration of the SSH server component.