- A
The interfaces are configured with IP addresses from different subnets.
Correct because EIGRP will not form an adjacency if the interfaces are not in the same subnet, as the hello packets will be dropped.
- B
The EIGRP process is shut down on one of the routers.
Why wrong: Incorrect because if the process is shut down, 'show ip eigrp neighbors' would still show the local process, but no neighbors. However, the more common cause is mismatched subnets.
- C
The passive-interface default command is configured under the EIGRP process.
Why wrong: Incorrect because passive-interface default would suppress hello packets on all interfaces, but the engineer would likely see a different symptom, such as no neighbors and the interface being listed as passive.
- D
The EIGRP router ID is the same on both routers.
Why wrong: Incorrect because a duplicate router ID does not prevent adjacency formation; it only causes issues during route exchange and may cause instability.
Quick Answer
The answer is a subnet mismatch between the directly connected interfaces. EIGRP requires that the primary IP addresses on the same link belong to the exact same subnet; if the subnet masks differ, each router considers the other to be on a different network, and the hello packets are silently discarded, preventing adjacency formation. On the Cisco CCNP ENARSI 300-410 exam, this is a classic troubleshooting scenario where you must verify that both interface IPs share an identical prefix length, as a common trap is assuming a /24 mask on one side and a /30 on the other will still work. A quick memory tip: “Same subnet, same mask—EIGRP won’t ask.”
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.
A network engineer is troubleshooting an EIGRP adjacency issue between two directly connected routers, R1 and R2. Both routers are configured with the same autonomous system number, but the adjacency fails to come up. The engineer checks the interfaces and verifies that they are up/up. On R1, the output of 'show ip eigrp neighbors' shows nothing. What is the most likely cause of this problem?
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 interfaces are configured with IP addresses from different subnets.
EIGRP requires that the primary IP addresses of the interfaces on the same link belong to the same subnet. If the subnet masks do not match, the routers will not form an adjacency because they will consider the other router to be on a different network.
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 interfaces are configured with IP addresses from different subnets.
- ✗
The EIGRP process is shut down on one of the routers.
Why it's wrong here
Incorrect because if the process is shut down, 'show ip eigrp neighbors' would still show the local process, but no neighbors. However, the more common cause is mismatched subnets.
- ✗
The passive-interface default command is configured under the EIGRP process.
Why it's wrong here
Incorrect because passive-interface default would suppress hello packets on all interfaces, but the engineer would likely see a different symptom, such as no neighbors and the interface being listed as passive.
- ✗
The EIGRP router ID is the same on both routers.
Why it's wrong here
Incorrect because a duplicate router ID does not prevent adjacency formation; it only causes issues during route exchange and may cause instability.
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.
Trap categories for this question
Command / output trap
Incorrect because if the process is shut down, 'show ip eigrp neighbors' would still show the local process, but no neighbors. However, the more common cause is mismatched subnets.
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 interfaces are configured with IP addresses from different subnets. — EIGRP requires that the primary IP addresses of the interfaces on the same link belong to the same subnet. If the subnet masks do not match, the routers will not form an adjacency because they will consider the other router to be on a different network.
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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