- A
Named mode and classic mode are incompatible and cannot form an adjacency under any circumstances.
EIGRP named mode and classic mode use different packet structures and metric computation; they are not interoperable for adjacency formation.
- B
The K values must match exactly, but named mode defaults to different K values than classic mode.
Why wrong: Both modes default to the same K values (1,0,1,0,0), but the metric computation still differs.
- C
Named mode requires authentication, while classic mode does not.
Why wrong: Authentication is optional in both modes.
- D
The routers must be in the same autonomous system number, but named mode uses a different AS number format.
Why wrong: Both modes use the same 16-bit AS number.
Question 1,266 of 2,152
EIGRP Named Mode and Classic Mode Cannot Form Adjacency
This 300-410 practice question tests your understanding of device management. 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.
An engineer configures EIGRP named mode on two routers in the same AS. One router uses classic mode configuration. The routers fail to form an adjacency. 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.
Quick Answer
The answer is that named mode and classic mode are incompatible and cannot form an EIGRP adjacency under any circumstances. This occurs because the two modes use fundamentally different packet formats and metric computation methods, even when K values are manually matched. Named mode defaults to wide metrics with the K1 1 K2 0 K3 1 K4 0 K5 0 formula, while classic mode uses the same K values but computes metrics using a narrower, 32-bit format, causing the routers to reject each other’s hello packets. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of EIGRP mode interoperability, a common trap where candidates assume matching K values alone will suffice. Remember the memory tip: “Named and classic never kiss”—they cannot form a neighbor relationship regardless of AS number or metric settings.
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
✓
Named mode and classic mode are incompatible and cannot form an adjacency under any circumstances.
EIGRP named mode and classic mode use different packet formats and TLVs, making them incompatible at the protocol level. Even though both operate within the same autonomous system, the routers cannot form an adjacency because the hello packets are not recognized by the other mode. This incompatibility is fundamental and cannot be overcome by configuration adjustments.
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.
- ✓
Named mode and classic mode are incompatible and cannot form an adjacency under any circumstances.
Why this is correct
EIGRP named mode and classic mode use different packet structures and metric computation; they are not interoperable for adjacency formation.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The K values must match exactly, but named mode defaults to different K values than classic mode.
Why it's wrong here
Both modes default to the same K values (1,0,1,0,0), but the metric computation still differs.
- ✗
Named mode requires authentication, while classic mode does not.
Why it's wrong here
Authentication is optional in both modes.
- ✗
The routers must be in the same autonomous system number, but named mode uses a different AS number format.
Why it's wrong here
Both modes use the same 16-bit AS number.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume named mode and classic mode can interoperate because they share the same protocol number and AS number, but Cisco specifically designed them to be incompatible to enforce a clean migration path.
Detailed technical explanation
How to think about this question
EIGRP named mode uses a different packet structure with new TLV types (e.g., Wide Metrics, Multi-Topology) that classic mode cannot parse. When a classic mode router receives a hello packet from a named mode router, it sees an unrecognized TLV and discards the packet, preventing adjacency formation. This is a common issue in migration scenarios where both modes are inadvertently mixed on the same network segment.
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?
Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Named mode and classic mode are incompatible and cannot form an adjacency under any circumstances. — EIGRP named mode and classic mode use different packet formats and TLVs, making them incompatible at the protocol level. Even though both operate within the same autonomous system, the routers cannot form an adjacency because the hello packets are not recognized by the other mode. This incompatibility is fundamental and cannot be overcome by configuration adjustments.
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.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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