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300-410 Practice Question: An engineer configures EIGRP named mode on two…
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?
⚠ Common exam trap
Test-takers frequently 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.
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.
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.
- ✗
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.
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.
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Last reviewed: Jul 4, 2026
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