Question 165 of 1,958
hardMultiple ChoiceObjective-mapped
350-401 Practice Question: An engineer configures EIGRP on a new router in a…
An engineer configures EIGRP on a new router in a DMVPN network. The router has a single physical interface with two subinterfaces: one for the DMVPN tunnel and one for a direct point-to-point link to a hub router. The engineer notices that EIGRP adjacencies form only on the point-to-point link, not on the DMVPN tunnel. The tunnel interface is configured with ip nhrp network-id 1 and ip nhrp nhs 10.1.1.1. What is the most likely reason?
⚠ Common exam trap
Cisco often tests the distinction between routing protocol configuration (e.g., 'ip eigrp') and tunnel-specific parameters (e.g., NHRP), leading candidates to incorrectly blame NHRP mapping or AS number mismatches when the real issue is the missing EIGRP enable command on the interface.
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 tunnel interface is not configured with the 'ip eigrp' command under the interface configuration.
The most common reason for EIGRP not forming an adjacency over a tunnel interface is the absence of the 'ip eigrp' command under that interface. Without this command, EIGRP is not enabled on the tunnel, so it will not send or process Hello packets, preventing adjacency formation even if NHRP and tunnel parameters are correctly configured.
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 tunnel interface is not configured with the 'ip nhrp map' command for the hub router.
Why it's wrong here
Incorrect. The 'ip nhrp map' command is used for static mapping, but NHRP dynamically learns mappings in a typical DMVPN. The adjacency issue is more likely due to EIGRP not being enabled on the tunnel.
- ✓
The tunnel interface is not configured with the 'ip eigrp' command under the interface configuration.
Why this is correct
Correct. For EIGRP to form an adjacency over the tunnel interface, the interface must be included in the EIGRP process, typically with 'ip eigrp <as-number>' under the interface. Without it, EIGRP will not send or receive hello packets on that interface.
- ✗
The DMVPN tunnel is using a different autonomous system number than the point-to-point link.
Why it's wrong here
Incorrect. If the autonomous system numbers differ, EIGRP adjacencies would not form on any interface. The scenario states adjacencies form on the point-to-point link, so AS numbers match.
- ✗
The tunnel interface is in a different VRF than the point-to-point link.
Why it's wrong here
Incorrect. While VRF mismatch can prevent adjacencies, the scenario does not mention VRF configuration. The most common cause is missing EIGRP configuration on the tunnel interface.
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: Jun 24, 2026
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