- A
The hub router has `ip next-hop-self eigrp 100` enabled by default, which sets the next-hop to the hub, preventing direct spoke-to-spoke routes.
Default EIGRP next-hop self on the hub causes spokes to see the hub as next-hop, not the originating spoke.
- B
The spoke routers need `ip split-horizon eigrp 100` enabled to advertise routes to other spokes.
Why wrong: Split horizon prevents routes from being advertised back out the same interface; disabling it is correct for DMVPN.
- C
EIGRP does not support DMVPN Phase 2; OSPF must be used for spoke-to-spoke routing.
Why wrong: EIGRP works with DMVPN Phase 2 when properly configured.
- D
The NHRP authentication is mismatched, so EIGRP packets are dropped.
Why wrong: NHRP authentication does not affect EIGRP packet forwarding.
Quick Answer
The answer is that the hub router has `ip next-hop-self eigrp 100` enabled by default, which sets the next-hop to the hub and prevents direct spoke-to-spoke routes. In DMVPN Phase 2, EIGRP relies on preserving the original next-hop address of the originating spoke so that when a remote spoke receives the route via the hub, it can initiate a direct tunnel to that next-hop. With next-hop-self active, the hub overwrites the next-hop with its own tunnel IP, causing the receiving spoke to see the hub as the destination—breaking the spoke-to-spoke shortcut. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how EIGRP interacts with DMVPN Phase 2’s on-demand tunneling, and it’s a common trap to forget that `no ip split-horizon` alone is insufficient. A helpful memory tip: “Split-horizon lets routes out, but next-hop-self blocks the shortcut.”
300-410 EIGRP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of eigrp troubleshooting. 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.
A DMVPN Phase 2 network is configured with EIGRP as the routing protocol over the tunnel interface. Spoke routers have `no ip split-horizon eigrp 100` configured on the tunnel interface. Unexpectedly, spoke-to-spoke tunnels are established, but EIGRP routes learned from one spoke are not installed in the routing table of another spoke, even though NHRP resolution works. Which 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 has `ip next-hop-self eigrp 100` enabled by default, which sets the next-hop to the hub, preventing direct spoke-to-spoke routes.
In DMVPN Phase 2, spoke-to-spoke tunnels require that EIGRP next-hop self behavior is disabled or that the next-hop is changed. By default, EIGRP advertises routes with the next-hop set to the advertising router's tunnel IP. When a spoke learns a route from another spoke via the hub, the next-hop is the hub's tunnel IP, not the originating spoke. The spoke must have `no ip next-hop-self eigrp 100` on the hub's tunnel interface to preserve the original next-hop, allowing direct spoke-to-spoke communication.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 has `ip next-hop-self eigrp 100` enabled by default, which sets the next-hop to the hub, preventing direct spoke-to-spoke routes.
- ✗
The spoke routers need `ip split-horizon eigrp 100` enabled to advertise routes to other spokes.
Why it's wrong here
Split horizon prevents routes from being advertised back out the same interface; disabling it is correct for DMVPN.
- ✗
EIGRP does not support DMVPN Phase 2; OSPF must be used for spoke-to-spoke routing.
Why it's wrong here
EIGRP works with DMVPN Phase 2 when properly configured.
- ✗
The NHRP authentication is mismatched, so EIGRP packets are dropped.
Why it's wrong here
NHRP authentication does not affect EIGRP packet forwarding.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The hub router has `ip next-hop-self eigrp 100` enabled by default, which sets the next-hop to the hub, preventing direct spoke-to-spoke routes. — In DMVPN Phase 2, spoke-to-spoke tunnels require that EIGRP next-hop self behavior is disabled or that the next-hop is changed. By default, EIGRP advertises routes with the next-hop set to the advertising router's tunnel IP. When a spoke learns a route from another spoke via the hub, the next-hop is the hub's tunnel IP, not the originating spoke. The spoke must have `no ip next-hop-self eigrp 100` on the hub's tunnel interface to preserve the original next-hop, allowing direct spoke-to-spoke communication.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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