- A
The crypto map is applied to the physical serial interface instead of the tunnel interface.
Applying crypto to the physical interface encrypts OSPF traffic on that link, causing OSPF to form adjacency over the physical interface and install routes via the serial link, bypassing the tunnel.
- B
OSPF network type on the serial interface is point-to-point, causing route preference over tunnel.
Why wrong: The network type does not affect the route source; the issue is which interface OSPF uses.
- C
The tunnel interface has an incorrect IP address.
Why wrong: An incorrect tunnel IP would prevent OSPF from forming adjacency over the tunnel, but the described symptom shows OSPF works over the physical link.
- D
The IPsec transform set does not include ESP authentication.
Why wrong: Missing authentication would cause IPsec to fail, but OSPF adjacency forms, so IPsec is working.
Quick Answer
The answer is that the crypto map applied to the physical serial interface causes OSPF to install the route via the physical interface instead of the tunnel. This occurs because when the crypto map is on the physical interface, IPsec encrypts OSPF packets at that layer, but the routing table still sees the next-hop via the serial link rather than the tunnel interface; OSPF learns the route through the physical adjacency and installs it with the serial interface as the outgoing interface, bypassing the tunnel entirely. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how crypto maps interact with routing protocols and the critical distinction between transport and overlay interfaces—a common trap is assuming OSPF will automatically prefer the tunnel. To fix this, the crypto map must be applied to the tunnel interface (e.g., GRE/IPsec) so OSPF runs over the tunnel and installs the route via that interface. Memory tip: “Map the crypto to the tunnel, or the route will stay on the serial funnel.”
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
R1 and R2 are connected via a point-to-point serial link running OSPF. R1 has an IPsec tunnel protecting traffic between loopback0 (10.1.1.1/32) and R2's loopback0 (10.2.2.2/32). The crypto map is applied to the physical serial interface. OSPF adjacencies form, but routes are not installed correctly. R1's show ip route ospf shows a route to 10.2.2.2/32 via the serial interface, not the tunnel. What is the root cause?
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 crypto map is applied to the physical serial interface instead of the tunnel interface.
When a crypto map is applied to the physical serial interface, IPsec encrypts the OSPF packets before they are sent, but the routing table still sees the next-hop via the physical interface (serial) rather than the tunnel interface. OSPF installs the route to 10.2.2.2/32 with the serial interface as the outgoing interface because the tunnel interface is not used for routing decisions; the route is learned via the physical interface adjacency. To have the route point to the tunnel interface, the crypto map must be applied to the tunnel interface (or a virtual tunnel interface like a GRE/IPsec tunnel) so that OSPF runs over the tunnel and installs the route via the 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.
- ✓
The crypto map is applied to the physical serial interface instead of the tunnel interface.
Why this is correct
Applying crypto to the physical interface encrypts OSPF traffic on that link, causing OSPF to form adjacency over the physical interface and install routes via the serial link, bypassing the tunnel.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
OSPF network type on the serial interface is point-to-point, causing route preference over tunnel.
Why it's wrong here
The network type does not affect the route source; the issue is which interface OSPF uses.
- ✗
The tunnel interface has an incorrect IP address.
Why it's wrong here
An incorrect tunnel IP would prevent OSPF from forming adjacency over the tunnel, but the described symptom shows OSPF works over the physical link.
- ✗
The IPsec transform set does not include ESP authentication.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that applying a crypto map to the physical interface automatically makes OSPF routes point to the tunnel interface, but in reality, the routing table uses the interface over which the adjacency was formed, which remains the physical interface unless a tunnel interface is used.
Trap categories for this question
Command / output trap
An incorrect tunnel IP would prevent OSPF from forming adjacency over the tunnel, but the described symptom shows OSPF works over the physical link.
Detailed technical explanation
How to think about this question
In a point-to-point serial link with IPsec, applying the crypto map to the physical interface encrypts the OSPF packets but does not change the routing table's next-hop interface; OSPF still sees the physical interface as the outgoing interface because the adjacency is formed over it. To have OSPF routes point to the tunnel interface, you must use a virtual tunnel interface (e.g., GRE over IPsec or VTI) where the crypto map is applied to the tunnel interface, allowing OSPF to run over the tunnel and install routes via the tunnel interface. This is a common design requirement for site-to-site VPNs where routing protocols must traverse the encrypted tunnel.
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
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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 crypto map is applied to the physical serial interface instead of the tunnel interface. — When a crypto map is applied to the physical serial interface, IPsec encrypts the OSPF packets before they are sent, but the routing table still sees the next-hop via the physical interface (serial) rather than the tunnel interface. OSPF installs the route to 10.2.2.2/32 with the serial interface as the outgoing interface because the tunnel interface is not used for routing decisions; the route is learned via the physical interface adjacency. To have the route point to the tunnel interface, the crypto map must be applied to the tunnel interface (or a virtual tunnel interface like a GRE/IPsec tunnel) so that OSPF runs over the tunnel and installs the route via the tunnel interface.
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.
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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