CISCO IOS in a Nutshell

Asynchronous Ports (TTYs)

TTYs are asynchronous connections between the router’s async interfaces and serial devices (modems). If you are connecting modems to your router or access server for dial-up or dial-out connections, you will need to configure the TTY ports.

The TTY ports correspond directly to async interfaces. Therefore, whenever you configure a TTY line, you will probably also configure the corresponding interface. If you plugged a modem into async port 1, you would use TTY1 to configure all the hardware aspects of the connection between the router and the modem, and the interface Async1 would configure the protocol. (The interface commands are defined in Chapter 5.) Figure 4-3 demonstrates the possible modem configuration on a router or terminal server.

Figure 4-3. TTY connections to modems

Here is an example of a modem configuration on TTY port 3:

! Select line 3
line tty 3
  ! Tell the router to use its local username list
  login local
  ! This line is for dial-in access only
  modem dialin
  ! The speed of the serial connection is 115200 bps
  speed 115200
  ! Use hardware flow control 
  flowcontrol hardware
  ! The type of modem is autoconfigured by the router
  modem autoconfigure discovery

The configuration isn’t difficult to read. The router, which is some sort of terminal server, maintains its own list of usernames and passwords (login local); the modem is used only for dial-in; the serial connection between the modem and the router is set to 115200 baud; hardware flow control is used; and the modem is configured by the router.