The default encapsulation type on Serial interfaces is HDLC, PPP generally provides you with more options which will be discussed below. The Packet Tracer save file is also available at the bottom of this post.

You can see the encapsulation type set on a interface by running the following.

 This version of HDLC is Cisco proprietary and only Cisco routers run this encapsulation, the encapsulation type must match on each end of the point to point link. To set encapsulation on an interface go to interface configuration mode and run the following command.

Other vendors routers tend to run PPP and this is the more generally configured encapsulation type. PPP also offers more features than HDLC such as:

• Authentication through CHAP, PAP & MS-CHAP
• Compression
• PPP Multilink (allows you to combine multiple physical channels into a single channel)
• Support for error detection and error recovery features
• Encapsulation for multiple protocols including IP, IPX and Appletalk
• PPP Callback (allows dial-in users to call the router and then be called back)

To configure PPP on a serial interface set the encapsulation type to PPP by using the following command in interface configuration mode on each router.

To configure PPP to use PAP and CHAP you need to configure a username and password using the router database to configure this run the following command in global configuration mode on R1 (the password can be anything you like BUT MUST MATCH AND BE THE SAME CASE ON EACH ROUTER!)

Now we need to configure the authentication to be used on the interface i.e. CHAP, PAP or MS-CHAP to do this go to interface configuration mode and type the following.

The line protocol will go down because we are now using CHAP authentication on R1 s0/0 and not on R2 s0/0 so we have to go to R2 and run the command above on the interface.

 You can configure more than one authentication type on an interface. This is done by typing the following in interface configuration mode

This means that CHAP will be tried as the authentication protocol first then PAP. Remember that when using PAP the username and password is sent in clear text so it is not as secure as CHAP which hashes this information.

The Packet Tracer file for this post is available for download below.

PPP Authentication