The answer is that the hub’s EIGRP is not configured to advertise the LAN subnet. In a DMVPN Phase 3 network, the hub uses NHRP redirect and shortcut to dynamically direct spoke-to-spoke traffic, but the hub’s own LAN subnet must still be explicitly injected into EIGRP for spokes to learn a valid route to it. Without a network statement or a redistribute command covering the hub’s LAN interface, EIGRP will not advertise that subnet across the tunnel, leaving spokes with no path to reach it even though the hub may have routes for spoke LANs. This scenario directly tests your understanding of EIGRP route advertisement boundaries in DMVPN Phase 3 on the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming that NHRP redirect alone handles all routing. A quick memory tip: “If the hub’s LAN is not in EIGRP, the spokes can’t reach it—NHRP shortcuts only redirect traffic, they don’t originate routes.”
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 spokes are unable to reach the hub's LAN subnet. The hub router is running EIGRP over the DMVPN tunnel interface, and the spokes are learning the hub's LAN route. However, pings from a spoke to the hub's LAN IP fail. The engineer checks the hub's routing table and sees the spoke's LAN route. The hub's tunnel interface has 'ip nhrp redirect' and 'ip nhrp shortcut' enabled. 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's EIGRP is not configured to advertise the LAN subnet.
In DMVPN phase 3, the hub's NHRP redirect and shortcut features can cause the hub to forward traffic to the spoke's LAN via the spoke's tunnel IP, but if the spoke's LAN subnet is not advertised via EIGRP, the hub may not have a route. However, the issue is that the hub's EIGRP is not advertising the hub's LAN subnet to the spokes, or the spokes are not receiving the route. The most common cause is that the hub's EIGRP network statement does not include the LAN 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 hub's EIGRP is not configured to advertise the LAN subnet.
Why this is correct
Correct because if the hub's LAN subnet is not advertised via EIGRP, the spokes will not have a route to it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The spoke's tunnel interface has 'ip nhrp shortcut' disabled.
Why it's wrong here
Incorrect because shortcut is for spoke-to-spoke, not for hub-to-spoke traffic.
✗
The hub's tunnel interface has 'no ip nhrp redirect' configured.
Why it's wrong here
Incorrect because redirect is for spoke-to-spoke traffic, not hub-to-spoke.
✗
The spoke's NHRP registration is not reaching the hub.
Why it's wrong here
Incorrect because if registration failed, the spoke would not have a tunnel IP or routes.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
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's EIGRP is not configured to advertise the LAN subnet. — In DMVPN phase 3, the hub's NHRP redirect and shortcut features can cause the hub to forward traffic to the spoke's LAN via the spoke's tunnel IP, but if the spoke's LAN subnet is not advertised via EIGRP, the hub may not have a route. However, the issue is that the hub's EIGRP is not advertising the hub's LAN subnet to the spokes, or the spokes are not receiving the route. The most common cause is that the hub's EIGRP network statement does not include the LAN subnet.
What should I do if I get this 300-410 question wrong?
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.
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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