A company has two routers, HQ-Router and Branch-Router, connected via a serial link. HQ-Router is connected to the internet, and both routers use OSPF for routing. Branch-Router users can access the HQ LAN but cannot reach the internet. What is the most likely cause of the internet connectivity issue at the Branch Office?
Correct Answer: A
Detailed Explanation: Why Option A is Correct: A default route (0.0.0.0/0) is required on the Branch-Router to forward traffic to unknown destinations (like the internet) to the HQ-Router. If the default route is missing, the Branch-Router won't know how to route internet-bound traffic, causing the connectivity issue. Why Other Options are Incorrect: Option B: If the OSPF neighbor relationship were down, Branch-Router users wouldn't be able to access the HQ LAN either. Option C: If the serial interface were shut down, there would be no connectivity between the HQ and Branch offices at all. Option D: If the HQ-Router's internet-facing interface were down, no one (including HQ users) would have internet access. Conclusion: The most likely cause is the missing default route on the Branch-Router. This is a common issue in multi-router setups where one router needs to forward traffic to another for internet access.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.