- A
The hub router is not configured with the 'no ip split-horizon' command for EIGRP or the 'neighbor' command for OSPF, preventing spoke-to-spoke route propagation.
In DMVPN Phase 2, the hub must disable split horizon (EIGRP) or use a network type that allows route propagation (OSPF) so that spokes learn each other's subnets. Without this, spokes only have a default route via the hub, so traffic goes through the hub.
- B
The IPsec transform set on the spokes uses different encryption algorithms, preventing the spoke-to-spoke tunnel from passing traffic.
Why wrong: If the IPsec session is established, the transform sets are compatible; this would not cause traffic to go through the hub.
- C
The NHRP authentication string is mismatched between spokes, causing NHRP resolution to fail.
Why wrong: If NHRP resolution fails, the spoke-to-spoke tunnel would not form; the question states that IPsec sessions are established.
- D
The spoke routers have a static default route pointing to the hub, overriding the dynamic routes.
Why wrong: A static default route would not prevent spoke-to-spoke traffic if there is a more specific route; the issue is the lack of specific routes.
Quick Answer
The answer is that the hub router is missing the 'no ip split-horizon' command for EIGRP, which prevents spoke-to-spoke route propagation. In DMVPN Phase 2, spoke-to-spoke routing depends on each spoke learning the remote spoke’s prefix through the hub; without disabling split horizon, the hub will not advertise routes learned from one spoke out the same interface to other spokes, so the spokes never install the necessary routes for direct forwarding. This is a classic trap on the Cisco CCNP ENARSI 300-410 exam, where candidates often assume that working NHRP registrations and IPsec sessions guarantee direct traffic flow, but routing protocol behavior is the missing link. The key distinction is that Phase 2 requires the hub to act as a route reflector, and for EIGRP that means explicitly disabling split horizon. Memory tip: think “no split, no hub detour”—without that command, traffic always takes the long way through the hub.
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 a DMVPN Phase 2 network with IPsec protection. Spoke-to-spoke tunnels form, but traffic between spokes is not being forwarded directly; it still goes through the hub. The engineer verifies that NHRP registrations are successful and that the spoke-to-spoke IPsec sessions are established. 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 hub router is not configured with the 'no ip split-horizon' command for EIGRP or the 'neighbor' command for OSPF, preventing spoke-to-spoke route propagation.
In a DMVPN Phase 2 network, spoke-to-spoke traffic requires that each spoke learns the remote spoke's prefix via the hub. For EIGRP, the hub must disable split horizon with 'no ip split-horizon eigrp <as>' to propagate routes learned from one spoke to other spokes. Without this, the hub advertises only its own routes, so spokes lack the necessary routing information to forward traffic directly, causing it to be sent through the hub despite working NHRP and IPsec.
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 hub router is not configured with the 'no ip split-horizon' command for EIGRP or the 'neighbor' command for OSPF, preventing spoke-to-spoke route propagation.
Why this is correct
In DMVPN Phase 2, the hub must disable split horizon (EIGRP) or use a network type that allows route propagation (OSPF) so that spokes learn each other's subnets. Without this, spokes only have a default route via the hub, so traffic goes through the hub.
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 IPsec transform set on the spokes uses different encryption algorithms, preventing the spoke-to-spoke tunnel from passing traffic.
Why it's wrong here
If the IPsec session is established, the transform sets are compatible; this would not cause traffic to go through the hub.
- ✗
The NHRP authentication string is mismatched between spokes, causing NHRP resolution to fail.
Why it's wrong here
If NHRP resolution fails, the spoke-to-spoke tunnel would not form; the question states that IPsec sessions are established.
- ✗
The spoke routers have a static default route pointing to the hub, overriding the dynamic routes.
Why it's wrong here
A static default route would not prevent spoke-to-spoke traffic if there is a more specific route; the issue is the lack of specific routes.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the subtle distinction between NHRP resolution success and actual routing table propagation—candidates assume that if NHRP and IPsec are working, traffic must flow directly, but they overlook the hub's routing protocol configuration that prevents spoke-to-spoke route advertisement.
Detailed technical explanation
How to think about this question
In DMVPN Phase 2, NHRP resolves the tunnel destination IP (NBMA address) for a given spoke prefix, but the spoke must have a routing table entry for that prefix pointing to the tunnel interface. EIGRP split horizon on the hub prevents it from advertising routes learned from one spoke to another; the 'no ip split-horizon eigrp' command allows these routes to be propagated. For OSPF, the hub must use the 'neighbor' command under the tunnel interface to enable point-to-multipoint behavior, as OSPF by default does not forward Type 1 LSAs over non-broadcast networks without explicit neighbor configuration.
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
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 hub router is not configured with the 'no ip split-horizon' command for EIGRP or the 'neighbor' command for OSPF, preventing spoke-to-spoke route propagation. — In a DMVPN Phase 2 network, spoke-to-spoke traffic requires that each spoke learns the remote spoke's prefix via the hub. For EIGRP, the hub must disable split horizon with 'no ip split-horizon eigrp <as>' to propagate routes learned from one spoke to other spokes. Without this, the hub advertises only its own routes, so spokes lack the necessary routing information to forward traffic directly, causing it to be sent through the hub despite working NHRP and IPsec.
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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