Which command is used to configure a loopback interface on a Cisco router?
Correct Answer: A
Detailed Explanation: A loopback interface is a virtual interface on a Cisco router that is always up and reachable as long as the router is operational. It is commonly used for testing, management, and as a stable endpoint for routing protocols. Configuring a loopback interface is a simple task that is performed in global configuration mode. The correct syntax to configure a loopback interface is to enter the interface configuration mode with the command: interface Loopback0 This command creates (or enters) the configuration for a loopback interface numbered 0. Once in this mode, you can assign an IP address to the loopback interface: Router(config)# interface Loopback0 Router(config-if)# ip address 10.0.0.1 255.255.255.255 The IP address assigned to a loopback interface is not tied to any physical port, which means it is not affected by physical connectivity issues. This makes it a reliable address for management, routing protocol identification, or as a stable endpoint for various network functions. Benefits of Loopback Interfaces: Stability: Because loopback interfaces are virtual, they remain operational regardless of the status of physical interfaces. This stability is crucial when using a loopback address as a router ID for routing protocols such as OSPF and BGP. Testing and Troubleshooting: Loopbacks can be used to test routing and connectivity within the router itself or between routers in a network. Management and Monitoring: A loopback address is often used for management purposes (e.g., SNMP, Telnet/SSH sessions) because it provides a consistent point of contact for a device. Comparison with Other Options: Option B (“ip loopback 0”), Option C (“loopback interface 0”), and Option D (“create loopback 0”): These commands are not part of the standard Cisco IOS command set. The recognized command to create a loopback interface is simply to enter the interface configuration mode with “interface Loopback0”. Practical Example: When configuring a router as part of a routing protocol such as OSPF, you often use a loopback interface for the router ID. This provides a reliable and unique identifier that does not change even if the physical interfaces go down. Conclusion: To configure a loopback interface on a Cisco router, you use the command “interface Loopback0” (Option A). This command is fundamental for creating virtual interfaces that are widely used in network management, testing, and routing protocol configurations.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.