This 200-901 practice question tests your understanding of network fundamentals. 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.
You are troubleshooting connectivity for a remote branch office. The branch router (BR) connects to the head office router (HQ) via a point-to-point T1 link. The HQ router is also connected to the internet via a separate interface. Users at the branch can access the internet but cannot reach servers at the head office (subnet 10.10.10.0/24). You run 'show ip route' on BR and see a default route pointing to HQ's IP address, but no specific route for 10.10.10.0/24. The HQ router has a connected route for that subnet. On HQ, you see that the interface towards BR is up/up, and you can ping the BR's interface IP. What is the most likely cause of the issue?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The HQ router does not have a route for the branch's local subnet.
The branch router (BR) has a default route pointing to the HQ router, which allows outbound traffic to the internet. However, for traffic from the branch to reach the HQ subnet (10.10.10.0/24), the HQ router must have a return route to the branch's local subnet. Without this specific route, the HQ router will drop packets destined for the branch because it does not know how to reach that network, even though the T1 link is up and the BR can ping the HQ interface. This is a classic asymmetric routing issue where the forward path works but the return path fails.
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.
✓
The HQ router does not have a route for the branch's local subnet.
Why this is correct
Without a return route, traffic from branch to HQ can leave but replies are dropped.
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.
✗
An ACL on the HQ router is blocking traffic from the branch subnet.
Why it's wrong here
There is no indication of ACL blocks; the issue is more fundamental.
✗
The default route on BR is not pointing to the correct next-hop.
Why it's wrong here
If internet works, the default route is correct for outgoing traffic.
✗
The T1 link is experiencing errors causing packet loss.
Why it's wrong here
Internet connectivity works, so the link is functional.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that a default route on the branch router is sufficient for outbound traffic, but candidates forget that the head office router also needs a route back to the branch's subnet for return traffic to succeed.
Detailed technical explanation
How to think about this question
In a point-to-point T1 link, routing is typically static or dynamic; here, the HQ router has a connected route for 10.10.10.0/24 but lacks a route back to the branch's subnet (e.g., 192.168.1.0/24). Without a return route, the HQ router will perform a route lookup for the branch's source IP and fail, sending an ICMP Destination Unreachable (Type 3, Code 0) back to the branch. In real-world scenarios, this is often resolved by adding a static route on HQ pointing to the BR's interface IP, or by using a dynamic routing protocol like EIGRP or OSPF to exchange subnet information.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The HQ router does not have a route for the branch's local subnet. — The branch router (BR) has a default route pointing to the HQ router, which allows outbound traffic to the internet. However, for traffic from the branch to reach the HQ subnet (10.10.10.0/24), the HQ router must have a return route to the branch's local subnet. Without this specific route, the HQ router will drop packets destined for the branch because it does not know how to reach that network, even though the T1 link is up and the BR can ping the HQ interface. This is a classic asymmetric routing issue where the forward path works but the return path fails.
What should I do if I get this 200-901 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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Question Discussion
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