Chapter 4. Line Commands

What Is a Line?

Cisco routers make a fairly basic distinction between the characteristics of a serial line (which you might want to think of as “physical” characteristics) and the characteristics of the protocols running over the line. The physical characteristics of a line are configured by the line command (and various commands that follow it) and include items such as parity and port speed. The high-level protocol characteristics are configured by the interface command (and the commands that follow it); these characteristics include IP addresses and other properties.

The line command can configure:

  • The router’s console port (CTY)

  • The router’s asynchronous ports (TTYs), used for dial-in and dial-out modem connections

  • The router’s auxiliary port (AUX), used for backup modem connections

  • Telnet and rlogin connections to the router (“virtual terminals” or VTYs)

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