Question 1,155 of 2,152

DMVPN →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the hub router lacks a route to the spoke's LAN subnet. In DMVPN Phase 3, the hub must have a valid route to the spoke's destination LAN for the NHRP redirect mechanism to trigger; without this route, the hub forwards the traffic but cannot generate an NHRP redirect message because it has no way to inform the spoke of a better path. This is a critical distinction from Phase 2, where the hub only needs NHRP redirect enabled—Phase 3 requires the hub to also have reachability to the spoke subnet in its routing table. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the Phase 3 control-plane dependency: the hub’s routing table must include the spoke LAN, often via a dynamic routing protocol like EIGRP or OSPF over the DMVPN tunnel. A common trap is assuming that enabling `ip nhrp redirect` alone is sufficient, but the missing route silently prevents spoke-to-spoke tunnels. Memory tip: "No route, no redirect—the hub must see the spoke LAN to point the way."

300-410 DMVPN Practice Question

This 300-410 practice question tests your understanding of dmvpn. 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.

An engineer is troubleshooting a DMVPN phase 3 network where spoke-to-spoke tunnels are not being established dynamically. The hub router has NHRP redirect enabled, and spokes have NHRP shortcut enabled. The engineer notices that when a spoke sends traffic to another spoke, the hub forwards the traffic but does not send an NHRP redirect. The hub's NHRP configuration includes the command 'ip nhrp redirect'. 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.

Question 1hardmultiple choice

Read the full VPN explanation →

A
The spoke does not have 'ip nhrp shortcut' enabled.
Why wrong: Incorrect because the symptom is that the hub does not send the redirect; the spoke's shortcut setting does not affect the hub's redirect behavior.
B
The hub router does not have a route to the spoke's LAN subnet.
Correct because the hub must have a route to the spoke's subnet to generate an NHRP redirect; without it, the hub forwards traffic without sending a redirect.
C
The tunnel interface on the hub has 'no ip nhrp redirect' configured.
Why wrong: Incorrect because the engineer verified that 'ip nhrp redirect' is configured.
D
The spoke's NHRP registration does not include the LAN subnet.
Why wrong: Incorrect because NHRP registration includes the tunnel IP, not the LAN subnet; the hub learns the LAN subnet via routing protocols.

Full breakdown with real-world context →

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

✓

The hub router does not have a route to the spoke's LAN subnet.

In DMVPN phase 3, the hub must have 'ip nhrp redirect' enabled on the tunnel interface, and the spoke must have 'ip nhrp shortcut' enabled. Additionally, the hub must have a route to the spoke's subnet; otherwise, the hub will not send an NHRP redirect. The issue is that the hub does not have a route to the spoke's subnet.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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 spoke does not have 'ip nhrp shortcut' enabled.
Why it's wrong here
Incorrect because the symptom is that the hub does not send the redirect; the spoke's shortcut setting does not affect the hub's redirect behavior.
✓
The hub router does not have a route to the spoke's LAN subnet.
Why this is correct
Correct because the hub must have a route to the spoke's subnet to generate an NHRP redirect; without it, the hub forwards traffic without sending a redirect.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The tunnel interface on the hub has 'no ip nhrp redirect' configured.
Why it's wrong here
Incorrect because the engineer verified that 'ip nhrp redirect' is configured.
✗
The spoke's NHRP registration does not include the LAN subnet.
Why it's wrong here
Incorrect because NHRP registration includes the tunnel IP, not the LAN subnet; the hub learns the LAN subnet via routing protocols.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Real-world example

How this comes up in practice

A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

DMVPN — This question tests DMVPN — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The hub router does not have a route to the spoke's LAN subnet. — In DMVPN phase 3, the hub must have 'ip nhrp redirect' enabled on the tunnel interface, and the spoke must have 'ip nhrp shortcut' enabled. Additionally, the hub must have a route to the spoke's subnet; otherwise, the hub will not send an NHRP redirect. The issue is that the hub does not have a route to the spoke's subnet.

What should I do if I get this 300-410 question wrong?

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?

CIDR notation defines the prefix length.

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. An engineer is troubleshooting a DMVPN phase 3 network where spoke-to-spoke tunnels are established, but traffic between spokes is taking a suboptimal path through the hub. The engineer checks 'show ip nhrp shortcut' on the spoke and sees no shortcut entries. The hub has 'ip nhrp redirect' enabled, and the spoke has 'ip nhrp shortcut' enabled. The engineer also verifies that the spoke's routing table has a route to the remote spoke's LAN via the hub. What is the most likely cause?

hard

✓ A.The hub router does not have a route to the remote spoke's LAN subnet.
B.The spoke's 'ip nhrp shortcut' command is missing on the tunnel interface.
C.The spoke's routing table has a static route to the remote spoke's LAN via the hub.
D.The hub's tunnel interface has 'no ip nhrp redirect' configured.

Why A: In DMVPN phase 3, for spoke-to-spoke shortcuts to be installed, the spoke must receive an NHRP redirect from the hub. If the hub does not send a redirect, the spoke will not create a shortcut. The hub sends a redirect only if it has a route to the destination subnet. If the hub does not have a route to the remote spoke's LAN, it will not send a redirect.

Last reviewed: Jun 18, 2026

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