- A
The neighbor adjacency repeatedly goes up and down.
MTU mismatch causes packet loss, leading to hold time expiration and neighbor resets.
- B
The 'show ip eigrp neighbors' command shows a high retransmission count (Retrans) for the neighbor.
Retransmissions increase when EIGRP packets are dropped due to MTU issues.
- C
The 'show ip eigrp topology' command shows routes in active state.
Why wrong: Active state in topology indicates a query process, not directly an MTU problem.
- D
The neighbor may become stuck-in-active (SIA) during route convergence.
Packet loss from MTU mismatch can prevent query replies, leading to SIA.
- E
The 'show ip eigrp interfaces' command shows a hold time of zero.
Why wrong: Hold time is a configured timer, not a dynamic value that drops to zero due to MTU issues.
Quick Answer
The correct answer is that a neighbor may become stuck-in-active (SIA) during route convergence, along with increasing retransmission counts and repeated neighbor flapping. An MTU mismatch causes EIGRP packets to be fragmented or dropped at the interface level, forcing the router to retransmit packets until the hold timer expires, which destabilizes the neighbor relationship. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your ability to distinguish MTU mismatch symptoms from other neighbor issues like authentication or K-value mismatches. A common trap is confusing topology table output with neighbor state—remember that "show ip eigrp neighbors" reveals retransmission counts and flapping, while "show ip eigrp topology" shows routes, not neighbor health. For a memory tip, think "MTU = Must Tune Up"—if packets are too big, they get dropped, triggering retransmissions and SIA.
300-410 EIGRP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of eigrp troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Which THREE symptoms indicate that EIGRP is experiencing a neighbor relationship issue due to an MTU mismatch? (Choose THREE.)
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 neighbor adjacency repeatedly goes up and down.
An MTU mismatch causes EIGRP packets to be fragmented or dropped, leading to repeated retransmissions and neighbor flapping. The 'show ip eigrp neighbors' output shows increasing retransmission counts and possibly a stuck-in-active (SIA) state. The neighbor may repeatedly go up and down. Option C is false because the 'show ip eigrp topology' shows routes, not neighbor state. Option E is false because the hold time is a timer, not a direct symptom of MTU issues.
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 neighbor adjacency repeatedly goes up and down.
- ✓
The 'show ip eigrp neighbors' command shows a high retransmission count (Retrans) for the neighbor.
- ✗
The 'show ip eigrp topology' command shows routes in active state.
Why it's wrong here
Active state in topology indicates a query process, not directly an MTU problem.
- ✓
The neighbor may become stuck-in-active (SIA) during route convergence.
- ✗
The 'show ip eigrp interfaces' command shows a hold time of zero.
Why it's wrong here
Hold time is a configured timer, not a dynamic value that drops to zero due to MTU issues.
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 neighbor adjacency repeatedly goes up and down. — An MTU mismatch causes EIGRP packets to be fragmented or dropped, leading to repeated retransmissions and neighbor flapping. The 'show ip eigrp neighbors' output shows increasing retransmission counts and possibly a stuck-in-active (SIA) state. The neighbor may repeatedly go up and down. Option C is false because the 'show ip eigrp topology' shows routes, not neighbor state. Option E is false because the hold time is a timer, not a direct symptom of MTU issues.
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.
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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