This N10-009 practice question tests your understanding of network implementation. 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 engineer has configured an IPsec site-to-site VPN between two offices. The tunnel is established and shows as active. However, users at the branch office (10.0.1.0/24) cannot reach servers at the main office (192.168.1.0/24). Both routers have the correct VPN policies and firewall rules permitting IPsec traffic. What should the engineer check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
B) That the routing tables on both routers include routes to the remote subnet
The tunnel being active means Phase 1 and Phase 2 of IPsec are established, but traffic still cannot flow because the routers lack routes to the remote subnets. Without a route for 10.0.1.0/24 on the main office router (or 192.168.1.0/24 on the branch router), packets will be dropped or sent out the wrong interface, even though the VPN policy and firewall rules are correct. The engineer must verify that static routes or a dynamic routing protocol (e.g., OSPF over the tunnel) are in place to direct traffic into the IPsec tunnel interface.
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.
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A) That the DNS server addresses are correctly configured
Why it's wrong here
DNS is not directly related to VPN traffic forwarding; the issue is about routing, not name resolution.
✓
B) That the routing tables on both routers include routes to the remote subnet
Why this is correct
Correct. Without routes for the remote network pointing to the tunnel interface, traffic will not be sent through the VPN.
Related concept
Read the scenario before looking for a memorised answer.
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C) That the MTU size is set to 1500 on both ends
Why it's wrong here
MTU mismatch can cause packet fragmentation issues but is not the primary cause when the tunnel is up but no traffic passes.
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D) That the SSID is correctly configured on the access points
Why it's wrong here
SSID is a wireless network identifier and has no bearing on VPN routing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a 'green light' tunnel status guarantees traffic flow, but candidates forget that routing is a separate layer that must explicitly direct traffic into the tunnel interface.
Detailed technical explanation
How to think about this question
IPsec VPNs create a virtual tunnel interface (e.g., crypto map or VTI) that requires explicit routing entries to direct traffic into the tunnel. Without a route pointing to the remote subnet via the tunnel interface, the router will use its default gateway or drop the packet, as it has no knowledge that the remote network is reachable through the VPN. In production, engineers often use dynamic routing over the tunnel (e.g., OSPF with 'ip ospf network point-to-point') to automatically propagate routes and avoid manual static route errors.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: B) That the routing tables on both routers include routes to the remote subnet — The tunnel being active means Phase 1 and Phase 2 of IPsec are established, but traffic still cannot flow because the routers lack routes to the remote subnets. Without a route for 10.0.1.0/24 on the main office router (or 192.168.1.0/24 on the branch router), packets will be dropped or sent out the wrong interface, even though the VPN policy and firewall rules are correct. The engineer must verify that static routes or a dynamic routing protocol (e.g., OSPF over the tunnel) are in place to direct traffic into the IPsec tunnel interface.
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.
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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