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300-410 Practice Question: A DMVPN network with EIGRP as the routing…
A DMVPN network with EIGRP as the routing protocol experiences spoke-to-spoke tunnel failures. Hub router R1 has configuration: 'interface Tunnel0 ip nhrp network-id 1 ip nhrp map multicast dynamic ip nhrp redirect' and 'router eigrp 100 network 10.0.0.0'. Spoke router R2 shows 'show ip nhrp brief' output: '10.0.0.1/32 via 192.168.1.1, Tunnel0 created 00:10:00, dynamic' but 'show ip eigrp neighbors' shows only the hub. What is the root cause?
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
✓
Spoke routers are missing 'ip nhrp shortcut' interface configuration, preventing dynamic spoke-to-spoke tunnel establishment.
Spoke-to-spoke tunnels require NHRP redirect and shortcut features to trigger direct communication. The hub has 'ip nhrp redirect' but spokes must have 'ip nhrp shortcut' to install direct routes. Without this, spokes only communicate via hub, causing suboptimal routing and potential failure if hub is overloaded.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Spoke routers are missing 'ip nhrp shortcut' interface configuration, preventing dynamic spoke-to-spoke tunnel establishment.
Why this is correct
The 'ip nhrp shortcut' command on spokes enables them to install NHRP redirect routes for direct communication.
- ✗
EIGRP is not configured to advertise the tunnel network; need 'network 10.0.0.0' on spokes.
Why it's wrong here
EIGRP is already configured; neighbors are formed with hub.
- ✗
The hub router needs 'ip nhrp map multicast dynamic' to forward multicast for EIGRP; it is already present.
Why it's wrong here
Multicast forwarding works as hub sees spokes.
- ✗
NHRP authentication mismatch between hub and spokes causes NHRP registration failure.
Why it's wrong here
NHRP registration succeeds as shown in NHRP brief.
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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
DMVPN Phase 2
DMVPN Phase 2 is an advanced Cisco routing technology that allows spoke routers to communicate directly with one another without sending traffic through a central hub, using dynamic routing protocols and multipoint GRE tunnels.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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