- A
CEF (Cisco Express Forwarding) is disabled or has a corruption
CEF creates a forwarding table based on the routing table. If CEF is disabled or the FIB is corrupt, the router will not forward packets even though the route appears in the routing table.
- B
The OSPF neighbor relationship is in the EXSTART state
Why wrong: If the OSPF neighbor is in EXSTART, the adjacency is still forming. However, the routing table already has the route, so the neighbor state should be FULL.
- C
The outbound interface has been administratively shut down
Why wrong: If the outbound interface were shutdown, the next-hop would not be reachable. The technician can ping the next-hop, so the interface is up.
- D
There is an ACL blocking the return traffic
Why wrong: An ACL blocking return traffic would cause user reports of no connectivity, but the technician is testing forwarding from the router itself. The problem is with the forwarding path, not return traffic.
Quick Answer
The answer is that CEF (Cisco Express Forwarding) being disabled or corrupted is the most likely cause when a router has a valid OSPF route but is not forwarding packets. This is because CEF is the hardware-based switching mechanism that populates the Forwarding Information Base (FIB), which the router actually uses to forward packets at line rate; the routing table (RIB) only stores the control plane information. When CEF is disabled, the router falls back to process switching, which can fail to handle traffic correctly even if the next hop is reachable. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of the separation between the control plane and the data plane—a common trap is to blame OSPF or a misconfigured ACL when the routing table looks fine. Remember the memory tip: "RIB knows the road, FIB drives the car; if CEF is off, the car won't go far."
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network technician is troubleshooting a router that is not forwarding packets to a remote destination network. The routing table shows a valid route learned via OSPF. The technician can successfully ping the next-hop IP address from the router. However, packets to the destination network are not being forwarded. Which of the following is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
CEF (Cisco Express Forwarding) is disabled or has a corruption
When a router has a valid OSPF-learned route and can ping the next-hop IP but still fails to forward packets, the issue is often with the forwarding plane rather than the control plane. CEF (Cisco Express Forwarding) is the default hardware-based switching mechanism that handles packet forwarding; if it is disabled or its Forwarding Information Base (FIB) becomes corrupted, the router will not forward packets even though the routing table (RIB) is correct. Disabling CEF forces the router to use process switching, which can also cause forwarding failures if the process is not properly handling the traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
CEF (Cisco Express Forwarding) is disabled or has a corruption
Why this is correct
CEF creates a forwarding table based on the routing table. If CEF is disabled or the FIB is corrupt, the router will not forward packets even though the route appears in the routing table.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The OSPF neighbor relationship is in the EXSTART state
Why it's wrong here
If the OSPF neighbor is in EXSTART, the adjacency is still forming. However, the routing table already has the route, so the neighbor state should be FULL.
- ✗
The outbound interface has been administratively shut down
Why it's wrong here
If the outbound interface were shutdown, the next-hop would not be reachable. The technician can ping the next-hop, so the interface is up.
- ✗
There is an ACL blocking the return traffic
Why it's wrong here
An ACL blocking return traffic would cause user reports of no connectivity, but the technician is testing forwarding from the router itself. The problem is with the forwarding path, not return traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a valid route in the routing table and successful ping to the next-hop guarantee packet forwarding, but Cisco tests the distinction between the control plane (routing table) and the data plane (CEF forwarding).
Detailed technical explanation
How to think about this question
CEF maintains two main data structures: the FIB (Forwarding Information Base), which mirrors the routing table but is optimized for fast lookups, and the adjacency table, which contains Layer 2 next-hop information. A common real-world scenario is that after a routing table change (e.g., a route flap), the FIB may become out of sync with the RIB, causing packets to be dropped silently; the command 'show ip cef' and 'clear ip cef' can be used to diagnose and repair this. Additionally, disabling CEF with 'no ip cef' globally or per-interface forces the router to use process switching, which is CPU-intensive and can lead to packet loss under load.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: CEF (Cisco Express Forwarding) is disabled or has a corruption — When a router has a valid OSPF-learned route and can ping the next-hop IP but still fails to forward packets, the issue is often with the forwarding plane rather than the control plane. CEF (Cisco Express Forwarding) is the default hardware-based switching mechanism that handles packet forwarding; if it is disabled or its Forwarding Information Base (FIB) becomes corrupted, the router will not forward packets even though the routing table (RIB) is correct. Disabling CEF forces the router to use process switching, which can also cause forwarding failures if the process is not properly handling the traffic.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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