PPP Authentication

PAP

R6 requires authentication:

interface Multilink1
 ip address 69.69.69.6 255.255.255.0
 ppp authentication pap
 ppp multilink
 ppp multilink group 1
end
R6(config)#username R9 password cisco

R9 presents login and credential:

interface Multilink1
 ip address 69.69.69.9 255.255.255.0
 ppp pap sent-username R9 password 0 cisco
 ppp multilink
 ppp multilink group 1
end

CHAP

R6 requires authentication:

interface Multilink1
 ip address 69.69.69.6 255.255.255.0
 ppp authentication chap
 ppp multilink
 ppp multilink group 1
end
R6(config)#username R9 password cisco

R9 responses R6 CHAP challenge, the account much exists on both challenger and responder, the username is peer hostname and the password much be same. 

interface Multilink1
 ip address 69.69.69.9 255.255.255.0
  ppp multilink
 ppp multilink group 1
end

R9(config)#username R6 password cisco

Or configure CHAP responder at interface level:

interface Multilink1
 ip address 69.69.69.9 255.255.255.0
  ppp multilink
 ppp multilink group 1
 ppp chap hostname R9
 ppp chap password cisco
end