单项选择题
A. session
B. network
C. physical
D. data link
E. transport
单项选择题 Given an IP address 172.16.28.252 with a subnet mask of 255.255.240.0, what is the correct network addres?（）
单项选择题 Which command can be used to verify the DLCI destination address in a Frame Relay static configuration?（）
单项选择题 Which layer 2 protocol encapsulation type supports synchronous and asynchronous circuis and has built-in security mechanisms?（）