Configuring PPP on a Serial Line (Optional Commands): Authentication

Router(config)#username routerb password cisco	Sets a username of routerb and a password of cisco for authentication from the other side of the PPP serial link. This is used by the local router to authenticate the PPP peer
Router(config)#int s 0/0
Router(config-if)#ppp authentication pap	Turns on PAP authentication only
Router(config-if)#ppp authentication chap	Turns on CHAP authentication only
Router(config-if)#ppp authentication pap chap	Defines that the link will use PAP authentication, but will try CHAP if PAP fails or is rejected by other side
Router(config-if)#ppp authentication chap pap	Defines that the link will use CHAP authentication, but will try PAP if CHAP fails or is rejected by other side
Router(config-if)#ppp pap sent-username routerb password cisco	This command must be set if using PAP in Cisco IOS Software Release 11.1 or later

Tip:

When setting authentication, make sure that your usernames match the name of the router on the other side of the link, and that the passwords on each router match the other. Usernames and passwords are case sensitive. Consider the following example:

Edmonton(config)#username Calgary password cisco	Calgary(config)#username Edmonton password cisco
Edmonton(config)#int s 0/0	Calgary(config)#int s 0/0
Edmonton(config-if)#encapsulation ppp	Calgary(config-if)#encapsulation ppp
Edmonton(config-if)#ppp authentication chap	Calgary(config-if)#ppp authentication chap

Note:

Because Password Authentication Protocol (PAP) does not encrypt its password as it is sent across the link, recommended practice is that you use Challenge Handshake Authentication Protocol (CHAP) as your authentication method.