Router A is unable to reach Router B.Both routers are running ios version 12.0.After reviewing the command output and graphic,what is the most likely cause of the problem?（）

单项选择题Which encapsulation type is a Frame Relay encapsulation type that is supported by Cisco routers?（）

A.IETF
B.HDLC
C.Q9333-A Annex A
D.ANSI Annex D

单项选择题Which layer 2 protocol encapsulation type supports synchronous and asynchronous circuis and has built-in security mechanisms?（）

A.Frame Relay
B.HDLC
C.x.25
D.PPP

多项选择题Which two options are valid WAN connectivity methods?（）

A.PPP
B.WAP
C.DSL
D.Ethernet
E.L2TPv3

单项选择题At which layer of the OSI model dose PPP perform?（）

A.Layer 3
B.Layer 4
C.Layer 2
D.Layer 5

单项选择题Which protocol is an open standard protocol framework that is commonly used in VPNs,to provid secure end-to-end communication?（）

A. L2TP
B. IPsec
C. PPTP
D.RSA