- A
The mutual redistribution creates a routing loop because the redistributed routes are re-injected into the original protocol without proper filtering, causing the hub to prefer the redistributed route with a lower AD.
Without route tagging, a route redistributed from EIGRP into OSPF (AD 110) and then back into EIGRP (AD 170) may be preferred over the original EIGRP internal route (AD 90) if the AD is misconfigured, but typically the original internal route has lower AD. However, if the route is external in EIGRP, the AD is 170, so the OSPF route (110) is preferred, causing a loop.
- B
The DMVPN tunnel interface is not included in the OSPF process, causing the redistributed routes to have an incorrect next-hop that points to the tunnel interface.
Why wrong: The tunnel interface being in EIGRP does not directly cause the oscillation; the issue is the redistribution loop.
- C
The EIGRP and OSPF processes are using different metric styles, causing the redistributed routes to have infinite metrics and be ignored.
Why wrong: If metrics are not set correctly, redistribution may fail, but the symptom is oscillation, not failure.
- D
The hub router's routing table is overloaded due to the DMVPN tunnel being a multipoint interface, causing route flapping.
Why wrong: Route flapping due to interface issues is possible, but the described symptom of oscillation between protocols indicates a redistribution loop.
Quick Answer
The answer is a mutual redistribution loop caused by the redistributed routes being re-injected into the original protocol without proper filtering. When EIGRP and OSPF are mutually redistributed on a DMVPN hub, a route learned via EIGRP (AD 90) is redistributed into OSPF (AD 110), then redistributed back into EIGRP as an external route (AD 170). Because the original internal EIGRP route has a lower AD (90) than the redistributed external route (170), the hub should prefer the original—but the corner case occurs when the redistributed route is learned via OSPF first, or when the original route is suppressed, causing the router to install the redistributed path and create a feedback loop that oscillates routes. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of administrative distance behavior in DMVPN and the need for route tagging or distribute-lists to prevent re-redistribution. A common trap is assuming mutual redistribution always works out of the box. Memory tip: “Redistribute once, filter twice—or your routes will dance in a loop.”
300-410 DMVPN Practice Question
This 300-410 practice question tests your understanding of dmvpn. 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 network engineer configures mutual redistribution between EIGRP and OSPF on a DMVPN hub router. The EIGRP domain includes the DMVPN tunnel network, and OSPF includes a corporate backbone. Unexpectedly, after a few minutes, the routing table on the hub shows oscillating routes between EIGRP and OSPF for the same prefix, causing intermittent connectivity. 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 mutual redistribution creates a routing loop because the redistributed routes are re-injected into the original protocol without proper filtering, causing the hub to prefer the redistributed route with a lower AD.
Mutual redistribution without route tagging or filtering can cause routing loops. When a route is redistributed from EIGRP into OSPF, it is then redistributed back into EIGRP with a different administrative distance (AD). Since OSPF has AD 110 and EIGRP has AD 90 (internal) or 170 (external), the redistributed route may be preferred over the original, causing a feedback loop. The corner case is that the default AD values cause the redistributed route to be installed, and then the router advertises it back, leading to instability. The fix is to use route tags or distribute-lists to prevent re-redistribution.
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 mutual redistribution creates a routing loop because the redistributed routes are re-injected into the original protocol without proper filtering, causing the hub to prefer the redistributed route with a lower AD.
Why this is correct
Without route tagging, a route redistributed from EIGRP into OSPF (AD 110) and then back into EIGRP (AD 170) may be preferred over the original EIGRP internal route (AD 90) if the AD is misconfigured, but typically the original internal route has lower AD. However, if the route is external in EIGRP, the AD is 170, so the OSPF route (110) is preferred, causing a loop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The DMVPN tunnel interface is not included in the OSPF process, causing the redistributed routes to have an incorrect next-hop that points to the tunnel interface.
Why it's wrong here
The tunnel interface being in EIGRP does not directly cause the oscillation; the issue is the redistribution loop.
- ✗
The EIGRP and OSPF processes are using different metric styles, causing the redistributed routes to have infinite metrics and be ignored.
Why it's wrong here
If metrics are not set correctly, redistribution may fail, but the symptom is oscillation, not failure.
- ✗
The hub router's routing table is overloaded due to the DMVPN tunnel being a multipoint interface, causing route flapping.
Why it's wrong here
Route flapping due to interface issues is possible, but the described symptom of oscillation between protocols indicates a redistribution loop.
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?
DMVPN — This question tests DMVPN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The mutual redistribution creates a routing loop because the redistributed routes are re-injected into the original protocol without proper filtering, causing the hub to prefer the redistributed route with a lower AD. — Mutual redistribution without route tagging or filtering can cause routing loops. When a route is redistributed from EIGRP into OSPF, it is then redistributed back into EIGRP with a different administrative distance (AD). Since OSPF has AD 110 and EIGRP has AD 90 (internal) or 170 (external), the redistributed route may be preferred over the original, causing a feedback loop. The corner case is that the default AD values cause the redistributed route to be installed, and then the router advertises it back, leading to instability. The fix is to use route tags or distribute-lists to prevent re-redistribution.
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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