You can use Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) for authentication with PointtoPoint Protocol (PPP). PPP relies on an authentication protocol to provide authentication. PPP is a wide area network (WAN) protocol used to provide remote connectivity. PPP is easy to configure and can transport multiple Layer 3 protocols, such as IP, Internetwork Packet Exchange (IPX), and AppleTalk. To configure an interface for PPP, you should issue the encapsulation ppp command in interface configuration mode.

When PAP is used, the user name and the password of the originating router are sent over the link in plain text. By contrast, when CHAP is used, a hash of the user name and password combination, as well as a random number, is sent to the destination router; the user name and password are not sent across the link. Thus CHAP is more secure than PAP for authentication.

To configure a router interface to use CHAP authentication, you should issue the ppp authentication chap command from interface configuration mode. To configure a router interface to use PAP authentication, you should issue the ppp authentication papcommand. To configure a router interface to use both CHAP and PAP, you should issue the ppp authentication chap pap command or the ppp authentication pap chap command? the authentication methods will be used in the order they are listed.

You can also use Microsoft CHAP (MSCHAP), MSCHAP version 2 (MSCHAP v2), and Extensible Authentication Protocol (EAP) with PPP. To configure a router interface to use MSCHAP, you should issue the ppp authentication mschap command. To configure a router interface to use MSCHAP v2, you should issue the ppp authentication mschapv2 command. To configure a router interface to use EAP, you should issue the ppp authentication eap command.

You cannot use High-Level Data Link Control (HDLC) for authentication. Similar to PPP, HDLC is a WAN encapsulation protocol, and it can be used with multiple Layer 3 protocols. However, HDLC does not support authentication.

Like PPP, the older Serial Line Internet Protocol (SLIP) can also be used for remote connectivity and relies on an authentication mechanism to provide authentication. Unlike PPP and HDLC, SLIP can transport only IP traffic. To configure an interface for SLIP, you should issue the encapsulation slip command in interface configuration mode.