单项选择题
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
A.CDBA B.DBAC C.ACDB D.BADC