You want to configure a sync/async interface in asynchronous mode.
Cisco has a class of serial modules that can support either synchronous or asynchronous communications, as required. You can use the physical-layer async command to change the interface from the default synchronous to asynchronous mode:
Router3#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router3(config)#interface Serial1/7 Router3(config-if)#physical-layer async Router3(config-if)#encapsulation ppp Router3(config-if)#exit Router3(config)#line 40 Router3(config-line)#speed 115200 Router3(config-line)#exit Router3(config)#end Router3#
As soon as you configure the physical-layer async command, the router will wipe out any configuration that you previously might have had on this interface for synchronous communications.
The only real trick in this configuration is that you need to apply many of the important configuration commands to a terminal line rather than the interface itself. In this example, the only command we have included in the line section is the speed command, but there could be others. We discuss the line configuration commands for connecting to asynchronous modems in Chapter 13.
The line number in this configuration is not arbitrary. In fact, after you enable the physical-layer async command on the serial interface, you should break out of configuration mode and use the show line command to see which line the router has decided to associate with this serial interface:
Router3#show line Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int 0 CTY - - - - - 0 0 0/0 - 40 TTY 9600/9600 - - - - - 0 0 0/0 Se1/7 65 AUX 2400/2400 F - - - - 0 0 0/0 - * 66 VTY - - - - - 5 0 0/0 - 67 VTY - - - - - 0 0 0/0 - 68 VTY - - - - - 0 0 0/0 - 69 VTY - - - - - 0 0 0/0 - 70 VTY - - - - - 0 0 0/0 - Line(s) not in async mode -or- with no hardware support: 1-39,41-64 Router3#
Here you can see that the router has assigned our interface, Serial1/7, with line number 40. Note also that it has set it to the default speed of 9600 baud. So we have increased this speed in the example.
Router3(config)#line 40 Router3(config-line)#speed 115200
Recipe 16.2; Chapter 13