350-401 Question 69
Single answerA network engineer is troubleshooting a packet forwarding issue on a Cisco router. The engineer observes that packets are being dropped even though a route exists in the routing table (RIB). Upon further investigation, they notice that the Forwarding Information Base (FIB) is not updated with the route. Which of the following accurately describes why this issue might occur?
- A
CEF (Cisco Express Forwarding) is disabled on the router.
- B
The adjacency table does not contain the next-hop for the route.
- C
The CAM table is not populated with the MAC addresses of the devices.
- D
The TCAM has run out of space to store the route information.
Show answer and explanation
Correct answer: A
Explanation
CEF (Cisco Express Forwarding) is a critical mechanism for efficient packet forwarding in Cisco devices. It uses the RIB to build the FIB for forwarding decisions and relies on the adjacency table for next-hop information. If CEF is disabled, the FIB is not populated, causing packets to be dropped even if the RIB contains a valid route. This highlights the importance of enabling and maintaining CEF for proper hardware-based switching and routing.
- A. Correct.
CEF (Cisco Express Forwarding) must be enabled for the RIB to synchronize with the FIB and for efficient packet forwarding. If CEF is disabled, the FIB will not be populated, leading to packet drops.
- B. Incorrect.
While the adjacency table is used to store next-hop information, a missing adjacency would typically result in ARP resolution issues or incomplete adjacency entries, not necessarily a failure to populate the FIB.
- C. Incorrect.
The CAM table is used in Layer 2 switching to map MAC addresses to switch ports. This is unrelated to the FIB and RIB, which operate at Layer 3.
- D. Incorrect.
If the TCAM runs out of space, certain hardware-accelerated features may not work properly, but this does not directly cause the FIB to be empty. The issue described is more related to CEF and its interaction with the RIB and FIB.