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
单项选择题 In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?（）
单项选择题 Which command sets and automatically encrypts the privileged enable mode password?（）
单项选择题 Which statement is true?（）
