- A
One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'.
The authentication mode must match on both sides; MD5 and SHA-256 are incompatible, causing the neighbor relationship to fail.
- B
Both routers use the same key chain name but one router has key ID 1 with string 'cisco' and the other has key ID 2 with string 'cisco'.
The key ID must match on both sides; even if the key string is the same, different key IDs cause authentication failure.
- C
One router has 'ip authentication key-chain eigrp 100 MYCHAIN' configured, while the other router has no authentication configuration at all.
Why wrong: If only one side has authentication configured, the neighbor relationship will still form, but a warning message is generated; authentication mismatch does not prevent adjacency in this case.
- D
Both routers use the same key chain and key ID, but the key string on one router is 'cisco123' and on the other is 'Cisco123' (case-sensitive).
EIGRP authentication keys are case-sensitive; 'cisco123' and 'Cisco123' are different strings, causing authentication failure.
- E
The 'accept-lifetime' on one router is set to 00:00:00 Jan 1 2020 to 23:59:59 Dec 31 2020, and the current time is in 2023.
Why wrong: The 'accept-lifetime' controls when the key is accepted for received packets; if the current time is outside the lifetime, authentication fails, so this would prevent the neighbor relationship.
Quick Answer
The answer is that an EIGRP authentication mismatch prevents neighbor formation when both routers use the same key chain and key ID but the key strings differ in case, such as 'cisco123' versus 'Cisco123', because EIGRP authentication is case-sensitive. This occurs because MD5 or SHA-256 authentication requires the key string to be byte-for-byte identical on both sides; any difference, including capitalization, causes the hash calculation to fail, breaking the neighbor relationship. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how EIGRP authentication parameters must match exactly, and a common trap is assuming case-insensitivity or that mismatched key IDs still allow adjacency. Remember that authentication type (MD5 vs. SHA-256), key ID, key string, and time-based lifetimes must all align—a single mismatch in any of these can silently drop hello packets. Memory tip: think "CASE and KEY must AGREE" — Case-sensitive strings, Authentication type, Same key ID, and Enabled lifetimes must all be Equal for a neighbor to form.
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 TWO actions will prevent an EIGRP neighbor relationship from forming due to authentication mismatch? (Choose TWO.)
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
One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'.
EIGRP authentication can be configured using MD5 or SHA-256. The key chain must match in name and key ID, and the key string must be identical. If one side uses MD5 and the other uses SHA-256, the authentication type differs. If the key ID does not exist on one side, authentication fails. The 'accept-lifetime' and 'send-lifetime' can cause failure if the current time is outside the valid range. If only one side has authentication configured, the neighbor relationship will form but with a warning, not a failure.
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.
- ✓
One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'.
Why this is correct
The authentication mode must match on both sides; MD5 and SHA-256 are incompatible, causing the neighbor relationship to fail.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
Both routers use the same key chain name but one router has key ID 1 with string 'cisco' and the other has key ID 2 with string 'cisco'.
- ✗
One router has 'ip authentication key-chain eigrp 100 MYCHAIN' configured, while the other router has no authentication configuration at all.
Why it's wrong here
If only one side has authentication configured, the neighbor relationship will still form, but a warning message is generated; authentication mismatch does not prevent adjacency in this case.
- ✓
Both routers use the same key chain and key ID, but the key string on one router is 'cisco123' and on the other is 'Cisco123' (case-sensitive).
- ✗
The 'accept-lifetime' on one router is set to 00:00:00 Jan 1 2020 to 23:59:59 Dec 31 2020, and the current time is in 2023.
Why it's wrong here
The 'accept-lifetime' controls when the key is accepted for received packets; if the current time is outside the lifetime, authentication fails, so this would prevent the neighbor relationship.
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: One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'. — EIGRP authentication can be configured using MD5 or SHA-256. The key chain must match in name and key ID, and the key string must be identical. If one side uses MD5 and the other uses SHA-256, the authentication type differs. If the key ID does not exist on one side, authentication fails. The 'accept-lifetime' and 'send-lifetime' can cause failure if the current time is outside the valid range. If only one side has authentication configured, the neighbor relationship will form but with a warning, not a failure.
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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