A.HDLCB.PPPC.帧中继D.X.25
A.Serial 0/0/0is down,line protocol is downB.Serial 0/0/0is up,line protocol is downC.Serial 0/0/0is up,line protocol is up(looped)D.Serial 0/0/0is up,line protocol is down(looped)
A.专用安全电路在每对通信节点之间建立B.通过服务提供商网络的连接在通信开始之前快速建立C.多对节点可通过统一网络信道进行通信D.通信开销较低