单项选择题
A. sessions
B. sa-validate
C. split-horizon
D. duplicate-protection
单项选择题 A metro Ethernet user is connected using a PPP over Ethernet service.How many Layer 2 headers are attached to each IP datagram?（）
单项选择题 Which set of commands correctly builds a PPP over Ethernet connection on a DSL connection?（）
多项选择题 The PPP over Ethernet Discovery Stage determines which two parameters? （）(Choose two.)