A.Layer 3 B.Layer 4 C.Layer 2 D.Layer 5
A. L2TP B. IPsec C. PPTP D.RSA
A.(config-if)#authentication ppp chap fallback ppp B.(config-if)#authentication ppp chap pap C.(config-if)#ppp authentication chap pap D.(config-if)#ppp authentication chap fallback ppp