单项选择题 While troubleshooting a connectivity problem on the network, you issue the ping command from your PC command prompt, but the output shows "request times out."At which OSI layer is this problem associated with?（）

单项选择题 You are logged into a router and with to view the layer 3 information about your neighboring Cisco routers. What IOS command gives layer 3 information for of the directly connected router interfaces?（）

单项选择题 In the context of configuring dial-on-demand routing using ISDN, what is the purpose of the dialer-list command?（）