- A
The IPsec tunnel uses transport mode, which does not encapsulate the original source IP, causing uRPF to see the remote router's physical IP as the source.
Why wrong: Transport mode still encapsulates the original packet; the source IP seen by uRPF is the remote router's physical IP after decryption.
- B
The return route for the remote site's physical IP points to the tunnel interface, but the packet arrives on the physical interface, so uRPF drops it because the source IP is not reachable via the incoming interface.
Strict uRPF requires that the source IP of the incoming packet has a route back through the same interface. If the route points to the tunnel, the check fails, and the packet is dropped.
- C
The uRPF configuration includes the 'allow-default' option, which allows packets with a default route, but the remote site's IP is not in the default route.
Why wrong: The 'allow-default' option allows packets if the only route is a default route; this would not cause the issue.
- D
The IPsec transform set uses ESP with authentication, which changes the source IP of the packet.
Why wrong: ESP does not change the source IP; it encapsulates the packet.
Quick Answer
The answer is that uRPF strict mode drops IPsec traffic because the return route for the remote site’s physical IP points to the tunnel interface, not the physical interface where the packet arrives. In strict mode, unicast Reverse Path Forwarding verifies that the source IP of an incoming packet is reachable via the exact interface on which it entered. For IPsec site-to-site VPN traffic, the encapsulated original packet hits the physical outside interface, but the routing table’s path to the remote tunnel endpoint leads to the virtual tunnel interface, causing a mismatch and a drop—even though the tunnel is up and routes are correct. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how uRPF interacts with overlay technologies; a common trap is assuming the tunnel’s “up” status guarantees traffic flow. Remember the mnemonic: “Strict checks the door, not the shortcut”—the packet must arrive on the same interface the router would use to send a reply.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on the outside interface of a router that terminates an IPsec site-to-site VPN. After the configuration, the VPN tunnel establishes, but traffic from the remote site is not forwarded correctly. The engineer verifies that the IPsec tunnel is up and that the routing table has the correct routes. What is the most likely explanation?
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
The return route for the remote site's physical IP points to the tunnel interface, but the packet arrives on the physical interface, so uRPF drops it because the source IP is not reachable via the incoming interface.
In strict mode, uRPF checks that the source IP address of an incoming packet is reachable via the exact interface on which the packet arrived. For IPsec site-to-site VPN traffic, the encapsulated (original) packet arrives on the physical outside interface, but the routing table's return route for the remote site's physical IP (the tunnel endpoint) points to the tunnel interface (e.g., a virtual tunnel interface or crypto map). Because the source IP is not reachable via the physical incoming interface, uRPF drops the packet, even though the IPsec tunnel is up and the routes are correct.
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 IPsec tunnel uses transport mode, which does not encapsulate the original source IP, causing uRPF to see the remote router's physical IP as the source.
Why it's wrong here
Transport mode still encapsulates the original packet; the source IP seen by uRPF is the remote router's physical IP after decryption.
- ✓
The return route for the remote site's physical IP points to the tunnel interface, but the packet arrives on the physical interface, so uRPF drops it because the source IP is not reachable via the incoming interface.
Why this is correct
Strict uRPF requires that the source IP of the incoming packet has a route back through the same interface. If the route points to the tunnel, the check fails, and the packet is 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.
- ✗
The uRPF configuration includes the 'allow-default' option, which allows packets with a default route, but the remote site's IP is not in the default route.
Why it's wrong here
The 'allow-default' option allows packets if the only route is a default route; this would not cause the issue.
- ✗
The IPsec transform set uses ESP with authentication, which changes the source IP of the packet.
Why it's wrong here
ESP does not change the source IP; it encapsulates the packet.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the subtle interaction between uRPF strict mode and IPsec VPNs, where candidates mistakenly think the tunnel mode or encryption causes the issue, rather than the interface-specific reverse path check.
Detailed technical explanation
How to think about this question
Under the hood, strict uRPF performs a Forwarding Information Base (FIB) lookup on the source IP and verifies that the best reverse path uses the same interface as the ingress interface. In IPsec VPNs, the routing table often has a static or dynamic route pointing to the tunnel interface (e.g., Tunnel0) for the remote subnet, but the physical interface (e.g., GigabitEthernet0/0) is the actual ingress. This mismatch causes uRPF to drop the traffic. A common real-world fix is to use loose mode uRPF or ensure the reverse route points to the physical interface (e.g., using a next-hop IP instead of the tunnel interface).
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.
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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 — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The return route for the remote site's physical IP points to the tunnel interface, but the packet arrives on the physical interface, so uRPF drops it because the source IP is not reachable via the incoming interface. — In strict mode, uRPF checks that the source IP address of an incoming packet is reachable via the exact interface on which the packet arrived. For IPsec site-to-site VPN traffic, the encapsulated (original) packet arrives on the physical outside interface, but the routing table's return route for the remote site's physical IP (the tunnel endpoint) points to the tunnel interface (e.g., a virtual tunnel interface or crypto map). Because the source IP is not reachable via the physical incoming interface, uRPF drops the packet, even though the IPsec tunnel is up and the routes are correct.
What should I do if I get this 300-410 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 24, 2026
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