Question 506 of 2,152

IPsec Site-to-Site VPN →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is a missing route on the local router for the remote LAN subnet. This is correct because the symptom reveals a classic split-horizon routing issue: the VPN tunnel is up and the remote router can reach the local LAN, but traffic from the remote LAN fails. When the remote router initiates a ping, it sources the packet from its own interface IP, which the local router can route back. However, traffic from a host on the remote LAN uses a different source subnet, and without a route to that remote LAN subnet in the local router’s routing table, the return traffic is dropped. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to differentiate between control plane and data plane failures—the tunnel being up does not guarantee end-to-end connectivity. A common trap is assuming the tunnel is fully functional because the router itself can ping. Memory tip: “Router can ping, LAN can’t? Check the route you want.”

300-410 IPsec Site-to-Site VPN Practice Question

This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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 is troubleshooting an IPsec site-to-site VPN where the tunnel is up but traffic from the remote LAN to the local LAN is not working. The engineer pings from the remote router to the local LAN IP and it succeeds. However, pings from a host on the remote LAN to a host on the local LAN fail. What 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.

Question 1hardmultiple choice

Read the full VPN explanation →

A
The crypto map access list on the remote router does not include the remote LAN subnet.
Why wrong: Incorrect because the tunnel is up and traffic from the remote router works, so the access list is likely correct.
B
The local router does not have a route to the remote LAN subnet in its routing table.
Correct because the local router must have a route to the remote LAN subnet to route the return traffic back through the tunnel. Without it, the return traffic is dropped.
C
The IPsec transform set is missing the esp-sha-hmac authentication.
Why wrong: Incorrect because the tunnel is up and working for router-initiated traffic, so the transform set is fine.
D
The pre-shared key is mismatched between the two routers.
Why wrong: Incorrect because a pre-shared key mismatch would prevent the tunnel from coming up at all.

Full breakdown with real-world context →

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

✓

The local router does not have a route to the remote LAN subnet in its routing table.

The symptom indicates that the VPN tunnel is working for traffic sourced from the router itself, but not for traffic from the remote LAN. This is typically caused by a missing route on the local router for the remote LAN subnet, or a missing route on the remote router for the local LAN subnet, preventing the return traffic from being routed correctly.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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 crypto map access list on the remote router does not include the remote LAN subnet.
Why it's wrong here
Incorrect because the tunnel is up and traffic from the remote router works, so the access list is likely correct.
✓
The local router does not have a route to the remote LAN subnet in its routing table.
Why this is correct
Correct because the local router must have a route to the remote LAN subnet to route the return traffic back through the tunnel. Without it, the return traffic is dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The IPsec transform set is missing the esp-sha-hmac authentication.
Why it's wrong here
Incorrect because the tunnel is up and working for router-initiated traffic, so the transform set is fine.
✗
The pre-shared key is mismatched between the two routers.
Why it's wrong here
Incorrect because a pre-shared key mismatch would prevent the tunnel from coming up at all.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Real-world example

How this comes up in practice

A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The local router does not have a route to the remote LAN subnet in its routing table. — The symptom indicates that the VPN tunnel is working for traffic sourced from the router itself, but not for traffic from the remote LAN. This is typically caused by a missing route on the local router for the remote LAN subnet, or a missing route on the remote router for the local LAN subnet, preventing the return traffic from being routed correctly.

What should I do if I get this 300-410 question wrong?

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?

CIDR notation defines the prefix length.

About these practice questions

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A network engineer is troubleshooting an IPsec site-to-site VPN that uses a GRE tunnel. The GRE tunnel is up/up, and EIGRP is forming an adjacency over it. However, traffic from the local LAN to the remote LAN is not working. The engineer pings the remote LAN IP from the local router and it succeeds. What is the most likely cause?

medium

✓ A.The local router does not have a route to the remote LAN subnet in its routing table.
B.The crypto map access list does not include the local LAN subnet.
C.The GRE tunnel keepalive is disabled.
D.The IPsec transform set is missing authentication.

Why A: The GRE tunnel and routing protocol are working, but traffic from the LAN is failing. This indicates that the routing table on the local router does not have a route to the remote LAN subnet, or the route points to the wrong next-hop. The ping from the router succeeds because the router uses its own IP as source, which is directly connected to the tunnel.

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