CISCO IOS in a Nutshell

Asynchronous Interfaces

Asynchronous interfaces are almost always used to provide dial-in PPP access. The hardware configuration of the asynchronous line (parity, baud rate, etc.) is determined by the corresponding TTY line; for more information about configuring TTY lines, see Chapter 4. When you configure an async interface, you specify logical aspects of the connection. Here is a configuration for an async port with dial-in access. Each command is preceded by a comment describing the command’s function.

interface Async1
 ! Assign IP address for the router's end of the link 
 ! by assigning it the ethernet0 port
 ip unnumbered ethernet0
 ! Set the serial encapsulation to PPP
 encapsulation ppp
 ! Set the interface mode to dedicated since this is a dial-up connection
 async mode dedicated
 ! Assign the IP address for the remote connection 
 peer default ip address 10.10.1.20

Notice that the asynchronous interface doesn’t need its own IP address; it can borrow the IP address of the ethernet0 interface using the unnumbered command. However, we do need to assign an address to the PPP peer that connects through this port. That address is assigned by the peer default ip address command; in this configuration, the peer is given the address 10.10.1.20. It is also worth mentioning that the IP address of the ethernet0 interface should be on the same network as the 10.10.1.20 address.

Using the group-async Command

Here’s a basic fact of life: async lines usually occur in large numbers. If you’re ...