300-410 Infrastructure Services Practice Question
A network administrator is deploying a DMVPN Phase 2 network with EIGRP as the routing protocol. The hub router is configured with a multipoint GRE interface and NHRP. Spokes are configured with tunnel interfaces and are registering with the hub. However, the administrator notices that spoke routers are not forming EIGRP neighbor adjacencies with the hub. Which command is most likely missing on the hub's mGRE interface?
⚠ Common exam trap
The trap here is assuming that NHRP registration alone is sufficient for routing protocol adjacencies, overlooking the need for multicast replication.
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
✓
ip nhrp map multicast dynamic
For EIGRP to form adjacencies over a DMVPN, the hub must be able to forward multicast hellos to all spokes. The command ip nhrp map multicast dynamic enables the hub to replicate multicast packets to all registered spokes. Without it, spokes do not receive EIGRP hellos, and neighbor relationships fail. This is a common oversight in DMVPN configurations. The other commands are either for Phase 3 or basic NHRP settings that are already working.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ip nhrp redirect
Why it's wrong here
ip nhrp redirect is a Phase 3 feature used to optimize spoke-to-spoke paths. It is not required for forming EIGRP adjacencies. In fact, in Phase 2, this command is not used. Its absence would not prevent EIGRP neighbors from forming; the issue is multicast replication, which is handled by a different command.
- ✓
ip nhrp map multicast dynamic
Why this is correct
Without ip nhrp map multicast dynamic, the hub cannot replicate multicast packets (such as EIGRP hellos) to all registered spokes. EIGRP uses multicast to discover and maintain neighbors. If the hub does not forward these multicasts, spokes will not receive hellos and adjacencies will not form. This command is essential for dynamic routing protocols over DMVPN.
- ✗
ip nhrp network-id 1
Why it's wrong here
ip nhrp network-id is a basic NHRP parameter that must match on all routers in the DMVPN. If it were missing or mismatched, NHRP registration would fail, and spokes would not appear in the hub's NHRP table. However, the scenario states that spokes are registering with the hub, implying that NHRP is working. Therefore, network-id is likely already configured correctly and is not the cause of the EIGRP adjacency issue.
- ✗
ip nhrp shortcut
Why it's wrong here
ip nhrp shortcut is configured on spokes in Phase 3 to allow them to use redirects. It is not used on the hub and is not related to EIGRP neighbor formation. Its absence on the hub would have no effect on EIGRP adjacencies. Therefore, it is not the missing command.
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
Device Management and Network Monitoring (SNMP, Syslog, NetFlow)
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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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.