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350-401 Practice Question: Consider the following DMVPN configuration on a…
Consider the following DMVPN configuration on a hub router:
interface Tunnel0 ip address 10.0.0.1 255.255.255.0 no ip redirects ip nhrp map multicast dynamic ip nhrp network-id 100
tunnel source GigabitEthernet0/0/0 tunnel mode gre multipoint
What is the effect of the command 'ip nhrp map multicast dynamic'?
⚠ Common exam trap
Cisco often tests the distinction between static and dynamic NHRP multicast mapping, where candidates mistakenly think 'dynamic' refers to the hub dynamically registering with a multicast group (like IGMP) rather than dynamically learning spoke NBMA addresses 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
✓
It enables the hub to dynamically add spoke NBMA addresses to the multicast NHRP map for forwarding multicast traffic to all spokes.
The command 'ip nhrp map multicast dynamic' on a DMVPN hub router instructs the router to dynamically add the NBMA addresses of spoke routers to the multicast NHRP mapping table as they register. This allows the hub to replicate multicast traffic (e.g., routing protocol hellos) to all spokes by using the dynamically learned NBMA addresses, enabling efficient dynamic spoke-to-spoke communication via the hub.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It statically maps the hub's own NBMA address to the multicast group.
Why it's wrong here
This option mischaracterizes the command as a static operation. The command 'ip nhrp map multicast dynamic' on the hub is inherently dynamic; it automatically learns spoke NBMA addresses as they register. Static mapping is configured with 'ip nhrp map' without the 'dynamic' keyword, and the hub's own NBMA address is never mapped to the multicast group because the hub does not need to send multicast traffic to itself.
- ✓
It enables the hub to dynamically add spoke NBMA addresses to the multicast NHRP map for forwarding multicast traffic to all spokes.
Why this is correct
This is the correct function. When a spoke sends an NHRP Registration Request, the hub records the spoke's NBMA (tunnel source) address and dynamically adds it to the multicast NHRP map for the tunnel interface. The hub then uses this map to replicate broadcast and multicast traffic (e.g., routing protocol hellos) to every registered spoke by sending a separate IPsec-encapsulated copy to each spoke's NBMA address, enabling pseudo-broadcast over the point-to-multipoint DMVPN network.
- ✗
It configures the hub to send NHRP registration requests to the multicast address 224.0.0.1.
Why it's wrong here
NHRP Registration Requests flow from spokes to the hub, not from the hub to a multicast address. The hub receives these unicast registrations and uses them to build its dynamic multicast map; it never emits registration requests. Furthermore, the command does not define or use 224.0.0.1 specifically — the multicast mapping is keyed to the multicast keyword in the command, and the actual destination used for replication is each spoke's individual NBMA address.
- ✗
It disables multicast forwarding over the tunnel interface.
Why it's wrong here
The command 'ip nhrp map multicast dynamic' does not disable multicast forwarding; it is the exact opposite. It enables the hub to build a dynamic multicast NHRP map so that multicast and broadcast traffic can be forwarded to all spokes over the point-to-point DMVPN tunnels. Disabling multicast forwarding would require a 'no ip nhrp map multicast' statement or removal of the dynamic mapping, not this 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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.