Which command is used to enable CHAP authentication, with PAP as the fallback method, on a serial interface?

Which command is used to enable CHAP authentication, with PAP as the fallback method, on a serial interface?
A . Router(config-if)# ppp authentication chap fallback ppp
B . Router(config-if)# ppp authentication chap pap
C . Router(config-if)# authentication ppp chap fallback ppp
D . Router(config-if)# authentication ppp chap pap

View Answer

Answer: B

Explanation:

The command “ppp authentication chap pap” command indicates the CHAP authentication is used first. If it fails or is rejected by other side then uses PAP instead. If you want to use PAP first (then CHAP) you can use the “ppp authentication pap chap” command

Reference: http://www.cisco.com/en/US/docs/ios/12_2/security/configuration/guide/scfathen.html