300-410 Layer 3 Technologies Practice Question
A network administrator is troubleshooting a DMVPN Phase 3 network using OSPF as the routing protocol. Spoke routers are not learning routes from other spokes. Which two actions should be taken to resolve this issue? (Choose two.)
⚠ Common exam trap
Watch out — candidates often confuse DMVPN Phase 3 with other phases or protocols, and assuming that OSPF network type changes or split horizon adjustments are needed, when the core issue is NHRP redirect and shortcut configuration.
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
✓
Configure the spoke routers with 'ip nhrp shortcut' on their tunnel interfaces.
In DMVPN Phase 3, spoke-to-spoke communication requires the hub to send NHRP redirect messages and the spokes to support NHRP shortcuts. The hub must have 'ip nhrp redirect' configured, and each spoke must have 'ip nhrp shortcut' configured. These commands enable the spokes to dynamically discover and establish direct tunnels to other spokes, bypassing the hub. Without them, spokes will not learn the specific routes to other spokes and will continue to route traffic through the hub, which is inefficient.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure the hub as a route reflector for BGP.
Why it's wrong here
BGP route reflection is not relevant in an OSPF-based DMVPN network. OSPF does not use route reflectors; that is a BGP concept. Implementing BGP would be a major change and is not required to solve the OSPF spoke-to-spoke reachability issue. This option is a distractor because it introduces an unrelated protocol. The scenario specifically mentions OSPF, so BGP solutions are incorrect.
- ✓
Configure the spoke routers with 'ip nhrp shortcut' on their tunnel interfaces.
Why this is correct
On spoke routers in DMVPN Phase 3, 'ip nhrp shortcut' allows them to install shortcut routes to other spokes based on NHRP redirect messages from the hub. Without this, spokes will not create direct tunnels and will continue to send traffic through the hub. This command is necessary for the spokes to dynamically learn the NBMA addresses of other spokes and establish direct connections. It is a key component of Phase 3 operation.
- ✓
Ensure that the hub router is configured with 'ip nhrp redirect' on its tunnel interface.
Why this is correct
In DMVPN Phase 3, the hub uses NHRP redirect to inform spokes of a more optimal path to other spokes. Without 'ip nhrp redirect' on the hub, spokes will continue to route traffic through the hub, and they may not learn the specific routes to other spokes. This command is essential for Phase 3 functionality, enabling spoke-to-spoke communication by allowing the hub to redirect traffic. It works in conjunction with 'ip nhrp shortcut' on the spokes.
- ✗
Disable split horizon on the hub's tunnel interface.
Why it's wrong here
Split horizon is a concept used in distance-vector protocols like RIP and EIGRP to prevent routing loops. OSPF is a link-state protocol and does not use split horizon. Therefore, disabling split horizon is irrelevant and will not affect OSPF route propagation in a DMVPN network. This option is a distractor based on confusion with other routing protocols. The issue is related to NHRP and DMVPN Phase 3 features, not split horizon.
- ✗
Change the OSPF network type to point-to-multipoint on all routers.
Why it's wrong here
Changing the OSPF network type to point-to-multipoint can affect neighbor relationships and route propagation, but it is not a specific requirement for DMVPN Phase 3 spoke-to-spoke communication. In fact, DMVPN typically uses point-to-multipoint or broadcast network types, but the key to Phase 3 is NHRP redirect and shortcut. Altering the network type might cause other issues and does not directly address the spoke-to-spoke route learning problem. This option is a distractor.
Visual reference
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
BGP Route Selection and Policy
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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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
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