- 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.
EIGRP MTU Mismatch Symptoms
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.)
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.
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.
Option A is correct because an MTU mismatch causes EIGRP packets to be fragmented or dropped, leading to repeated adjacency flaps as the neighbor relationship is re-established each time the hold timer expires. This instability is a classic symptom of MTU mismatch in EIGRP.
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 neighbor adjacency repeatedly goes up and down.
Why this is correct
MTU mismatch causes packet loss, leading to hold time expiration and neighbor resets.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
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.
Why this is correct
Packet loss from MTU mismatch can prevent query replies, leading to SIA.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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: answer the scenario, not the keyword
Cisco often tests the distinction between symptoms of MTU mismatch (flapping, high retransmits, SIA) and symptoms of other EIGRP issues like active routes (DUAL query process) or hold time zero (hello failure), so candidates must associate MTU mismatch specifically with packet delivery failures rather than routing table states.
Detailed technical explanation
How to think about this question
EIGRP uses the MTU of the interface to determine the maximum packet size for reliable transport. When MTU mismatches occur, EIGRP packets larger than the path MTU are fragmented or dropped, causing retransmissions and eventual hold timer expiry. This triggers the neighbor to be reset, and the cycle repeats, leading to the flapping behavior seen in option A and the high retransmission count in option B. The stuck-in-active (SIA) condition in option D can occur during convergence if queries are lost due to MTU issues, preventing the DUAL algorithm from completing.
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.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The neighbor adjacency repeatedly goes up and down. — Option A is correct because an MTU mismatch causes EIGRP packets to be fragmented or dropped, leading to repeated adjacency flaps as the neighbor relationship is re-established each time the hold timer expires. This instability is a classic symptom of MTU mismatch in EIGRP.
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: Jul 4, 2026
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