You manage a network with two routers connected over a serial link using EIGRP. Router A advertises the 10.1.0.0/16 network, while Router B advertises the 10.2.0.0/16 network. After configuring EIGRP on both routers, Router A can ping Router B’s serial interface, but cannot reach any hosts on the 10.2.0.0/16 network behind Router B. What could be the cause?
Correct Answer: B
Detailed Explanation: EIGRP routers advertise routes based on the network statements configured under the EIGRP process. The network command tells the router which interfaces to include in EIGRP and which routes to advertise to neighbors. In this scenario, Router B’s EIGRP configuration likely includes only the serial interface’s network, but not the LAN interfaces (10.2.0.0/16 subnet). As a result, Router B shares the serial link route but not the LAN subnet routes. Router A learns about Router B’s serial interface but has no routing information to the LAN hosts behind Router B. EIGRP uses the interfaces matching the network statement for sending hellos and advertising routes. If the LAN interfaces aren’t included, they will not be advertised. This is a common mistake, especially when summarizing or including network statements with too narrow a mask. Option A is incorrect because authentication issues typically prevent EIGRP neighbor adjacency altogether, not partial connectivity. Option C is ruled out because Router A can ping Router B’s serial interface, so the link is up. Option D is incorrect since a mismatched AS number would prevent the formation of the EIGRP neighbor relationship altogether. Proper EIGRP configuration requires the network statements to cover all interfaces that should participate in routing updates. For example, if Router B has an Ethernet interface with IP 10.2.0.1/16, the network command should be network 10.2.0.0 0.0.255.255 or equivalent to include the LAN subnet.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.