Which command is used to configure a default route on a Cisco router?
Correct Answer: B
Detailed Explanation: A default route is a critical component in routing configurations. It tells the router where to send packets that do not have a specific match in the routing table. On Cisco routers, the default route is configured using the “ip route” command with the destination network set to 0.0.0.0 and the subnet mask set to 0.0.0.0. The command format is: ip route 0.0.0.0 0.0.0.0 [next-hop IP address or exit interface] How It Works: This command creates a static route that matches all packets destined for networks that are not explicitly listed in the routing table. When a router receives a packet and finds no more specific route, it forwards the packet to the specified next-hop address (or out the designated exit interface). The default route is sometimes called the “gateway of last resort.” Use Cases: Edge Routers: In enterprise networks, default routes are commonly used on routers that connect to the Internet or external networks. The router forwards any traffic for unknown networks to an upstream provider or firewall. Stub Networks: In small or branch offices, a default route may be configured to send all traffic to a central router, reducing the complexity of the routing table on the local router. Comparison with Other Options: Option A (ip default-route): This is not a recognized Cisco IOS command. Option C (route default [next-hop]) and Option D (set ip route default [next-hop]): These commands do not follow the Cisco IOS syntax for configuring static routes. Option B is the only option that correctly matches the IOS command syntax for creating a default static route. Practical Example: Consider a scenario where an enterprise router connects to the Internet via a service provider. The administrator would configure a default route so that any traffic destined for the Internet is forwarded to the provider’s router: Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1 Here, 192.168.1.1 represents the next-hop IP address provided by the ISP. Troubleshooting Tips: If devices are unable to reach external networks, one of the first steps in troubleshooting should be to verify that a default route is configured. The command “show ip route” will display the default route (often indicated by a “S*” symbol) in the routing table. Misconfiguration or absence of a default route can lead to dropped packets for unknown destinations. Conclusion: The command “ip route 0.0.0.0 0.0.0.0 [next-hop]” (Option B) is used to configure a default route on a Cisco router. This default route ensures that traffic for any network not specifically listed in the routing table is forwarded to the designated next-hop, making it a critical part of routing configurations in many networks.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.