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300-410 Practice Question: An engineer is configuring a DMVPN Phase 3…
An engineer is configuring a DMVPN Phase 3 network with EIGRP as the routing protocol. The hub router has the following configuration snippet:
interface Tunnel0 ip address 10.0.0.1 255.255.255.0 ip nhrp network-id 1 ip nhrp redirect
tunnel source GigabitEthernet0/1 tunnel mode gre multipoint !
router eigrp 100 network 10.0.0.0 0.0.0.255
Which TWO additional configuration changes are required on the hub to ensure that spokes can establish direct spoke-to-spoke tunnels? (Choose TWO.)
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
✓
Add the command 'ip nhrp map multicast dynamic' under interface Tunnel0.
In DMVPN Phase 3, the hub must have 'ip nhrp redirect' to send redirect messages, and spokes need 'ip nhrp shortcut' to install the /32 route. However, the question asks about the hub. The hub already has 'ip nhrp redirect'. To allow spokes to learn the hub's NBMA address, the hub needs 'ip nhrp map multicast dynamic' to accept dynamic registrations. Also, the hub must have a valid tunnel source IP and the tunnel interface must be up. The hub does not need 'ip nhrp shortcut' (that's for spokes). The hub does not need 'ip nhrp nhs' (that's for spokes). The hub does not need 'no ip split-horizon' for EIGRP if using Phase 3 with BGP or if the hub is configured correctly; but in Phase 3, EIGRP split horizon can prevent spoke routes from being advertised to other spokes. Actually, to allow spokes to learn routes from other spokes, the hub must disable EIGRP split horizon on the tunnel interface. So the two correct changes are: add 'ip nhrp map multicast dynamic' and 'no ip split-horizon eigrp 100'.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Add the command 'ip nhrp shortcut' under interface Tunnel0.
Why it's wrong here
Incorrect. 'ip nhrp shortcut' is configured on spokes, not on the hub. The hub uses 'ip nhrp redirect'.
- ✗
Add the command 'ip nhrp nhs 10.0.0.1' under interface Tunnel0.
Why it's wrong here
Incorrect. 'ip nhrp nhs' is used on spokes to point to the hub. The hub does not need this command.
- ✓
Add the command 'ip nhrp map multicast dynamic' under interface Tunnel0.
Why this is correct
Correct. This command allows the hub to dynamically learn the NBMA addresses of spokes for multicast forwarding, which is required for routing protocol updates.
- ✓
Add the command 'no ip split-horizon eigrp 100' under interface Tunnel0.
Why this is correct
Correct. In DMVPN Phase 3, the hub must disable EIGRP split horizon so that routes learned from one spoke can be advertised to other spokes, enabling spoke-to-spoke communication.
- ✗
Add the command 'ip nhrp authentication cisco123' under interface Tunnel0.
Why it's wrong here
Incorrect. Authentication is optional and not required for spoke-to-spoke tunnels; it is used for security but not a mandatory change for the scenario.
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.
Key term
DMVPN Phase 3
DMVPN Phase 3 is a Cisco networking technology that allows branch offices to connect directly to each other without always going through a central hub, but with smarter routing that lets the hub control the traffic paths more efficiently.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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