单项选择题
A. Ensure that an interface has been configured with an IP address
B. Ensure that IP classless is enabled
C. Ensure that the interfaces can ping their directly connected neighbors
D. Ensure that an interface has been configured with an IP address and is up
多项选择题 Open Shortes t Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks by the Interior Gateway Protocol (IGP) working group of the Internet Engineering Task Force (IETF). Which two statements best describe the OSPF link - state routing pro tocol?（）
单项选择题 OSPF is configured over a Frame Relay network as shown in the exhibit. All PVCs are active.However, R1 and R3 fail to see all OSPF routes in their routing tables. The show ip ospf neighbor command executed on R2 shows the state of t he neighbors. What should be done to fix the problem? （）
单项选择题 Refer to the exhibit. During the process of configuring a virtual link to connect area 2 with the backbone area, the network administrator received this console message on R3: *Mar 1 00:2 5:01.084: %OSPF -4- ERRRCV: Received invalid packet: mismatch area ID, from backbone area must be virtual link but not found from 20.20.20.1, Serial 0 .How should the virtual link be configured on the OSPF routers to establish full connectivity between the areas?（）