单项选择题
A.DLCI 17描述了R2和R3之间的ISDN线路
B.DLCI 17描述了一条在R2上的PVC 不能用在R3和R1上
C.DLCI 17是R2使用的二层地址，用来描述一条到R3的PVC
D.DlDLCI 17描述了一条从R2 R3到服务提供商拨号网络
多项选择题 Why does the data communication industry use the layered OSI reference model (Choose two.)（）。
单项选择题 You and a co-worker have established wireless communication directly between your wireless laptops. What type of wireless topology has been created（）。
单项选择题 When upgrading the IOS image, the network administrator receives the exhibited error message. What could be the cause of this error（）。
