单项选择题 In a lab, two routers are connected directly together using serial interfaces in a back-to-back configuration. No external DCE devices are being used. What additional command is needed to bring this link up?（）

单项选择题 You have just obtained a modular Cisco router with 2 serial connections and a BRI/U interface. What other physical piece of hardware is required in order to establish an ISDN connection?（）

多项选择题 Which of the following can LCP successfully negotiate during the establishment of a PPP connection?（）