The answer is that the hub is not configured to propagate spoke routes to other spokes. In DMVPN Phase 2, spoke-to-spoke routing relies on the hub to advertise the tunnel or LAN subnets of each spoke to all other spokes via a dynamic routing protocol like EIGRP or OSPF; without this route propagation, each spoke only knows its own directly connected networks and the hub’s tunnel IP, leaving it unable to reach a peer’s tunnel interface. On the ENCOR 350-401 exam, this question tests your understanding that Phase 2 separates NHRP’s mapping function from the routing protocol’s advertisement role—a common trap is confusing NHRP’s dynamic tunnel establishment with the need for explicit route injection at the hub. Remember the key distinction: NHRP resolves the next-hop address, but the routing protocol must carry the route; if the hub doesn’t redistribute or advertise spoke subnets, spokes remain isolated. A useful memory tip is “Routes first, then maps”—the hub must share the routes before NHRP can build the direct spoke-to-spoke tunnel.
CCNP VPN Technologies Practice Question
This 350-401 practice question tests your understanding of vpn technologies. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is tasked with deploying a DMVPN Phase 2 network for a company with multiple branch offices. The hub router is a Cisco 4451-X and the spoke routers are Cisco 4331s. After configuration, the spokes can ping the hub's tunnel IP, but cannot reach each other's tunnel IPs. The engineer checks the routing tables and sees that the hub has routes for both spoke subnets, but the spokes do not have routes to each other. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The hub is not configured to propagate spoke routes to other spokes.
In DMVPN Phase 2, spokes learn about other spoke networks via the hub using dynamic routing (e.g., EIGRP or OSPF). The hub must be configured to propagate spoke routes to other spokes. If the hub is not configured to redistribute or advertise the spoke subnets, the spokes will not have routes to each other. Option C is correct because the hub must have a routing configuration that allows spoke-to-spoke route propagation. Option A is incorrect because NHRP is used for mapping, not routing. Option B is incorrect because spoke-to-spoke tunnels are established dynamically via NHRP. Option D is incorrect because mGRE is the correct interface type for DMVPN.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The NHRP network ID is mismatched between the hub and spokes.
Why it's wrong here
If NHRP were mismatched, the spokes would not even register with the hub.
✗
The spokes are not configured with a crypto map for IPsec.
Why it's wrong here
IPsec is used for encryption, not for routing between spokes.
✓
The hub is not configured to propagate spoke routes to other spokes.
Why this is correct
Correct because without route propagation, spokes cannot learn each other's networks.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The tunnel mode is set to GRE instead of mGRE on the spokes.
Why it's wrong here
Spokes typically use GRE, not mGRE; mGRE is only on the hub.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
VPN Technologies — This question tests VPN Technologies — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The hub is not configured to propagate spoke routes to other spokes. — In DMVPN Phase 2, spokes learn about other spoke networks via the hub using dynamic routing (e.g., EIGRP or OSPF). The hub must be configured to propagate spoke routes to other spokes. If the hub is not configured to redistribute or advertise the spoke subnets, the spokes will not have routes to each other. Option C is correct because the hub must have a routing configuration that allows spoke-to-spoke route propagation. Option A is incorrect because NHRP is used for mapping, not routing. Option B is incorrect because spoke-to-spoke tunnels are established dynamically via NHRP. Option D is incorrect because mGRE is the correct interface type for DMVPN.
What should I do if I get this 350-401 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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