- A
The hub is not sending NHRP redirect messages to the spokes.
In DMVPN Phase 2, the hub must send NHRP redirect messages to inform spokes of the existence of a direct path. Without these redirects, spokes do not know to send traffic directly to each other.
- B
The spoke routers have 'ip nhrp shortcut' disabled.
Why wrong: The 'ip nhrp shortcut' command is used in Phase 3 to allow spoke-to-spoke tunnels; in Phase 2, it is not required as spokes can learn from NHRP redirects.
- C
The hub is configured with 'no ip split-horizon' on the tunnel interface.
Why wrong: In DMVPN Phase 2, split horizon must be disabled on the hub to allow spoke-to-spoke traffic; if it is enabled, the hub will not advertise routes from one spoke to another, preventing direct communication.
- D
The spoke routers have mismatched IPsec transform sets.
Why wrong: Mismatched IPsec transform sets would prevent the spoke-to-spoke tunnel from forming, but the question states spoke-to-spoke tunnels are established.
300-410 MPLS Operations Practice Question
This 300-410 practice question tests your understanding of mpls operations. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configures a DMVPN Phase 2 network. Spoke-to-spoke tunnels are established, but traffic between spokes is not being forwarded directly; it is still going through the hub. The engineer verifies that NHRP is working and spoke-to-spoke registrations are successful. What is the most likely explanation?
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 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 is not sending NHRP redirect messages to the spokes.
In DMVPN Phase 2, spoke-to-spoke traffic requires that the spoke routers have a route to the destination prefix via the tunnel interface with the next-hop being the remote spoke's tunnel IP. If the hub is not configured to send an NHRP redirect or if the spoke does not have a route for the remote subnet via the tunnel, traffic will continue to go through the hub. The edge case is that the hub may be using a different next-hop in its routing table, or the spoke's routing table does not have a more specific route for the remote spoke's subnet via the tunnel.
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 is not sending NHRP redirect messages to the spokes.
Why this is correct
In DMVPN Phase 2, the hub must send NHRP redirect messages to inform spokes of the existence of a direct path. Without these redirects, spokes do not know to send traffic directly to each other.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The spoke routers have 'ip nhrp shortcut' disabled.
Why it's wrong here
The 'ip nhrp shortcut' command is used in Phase 3 to allow spoke-to-spoke tunnels; in Phase 2, it is not required as spokes can learn from NHRP redirects.
- ✗
The hub is configured with 'no ip split-horizon' on the tunnel interface.
Why it's wrong here
In DMVPN Phase 2, split horizon must be disabled on the hub to allow spoke-to-spoke traffic; if it is enabled, the hub will not advertise routes from one spoke to another, preventing direct communication.
- ✗
The spoke routers have mismatched IPsec transform sets.
Why it's wrong here
Mismatched IPsec transform sets would prevent the spoke-to-spoke tunnel from forming, but the question states spoke-to-spoke tunnels are established.
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.
Trap categories for this question
Command / output trap
The 'ip nhrp shortcut' command is used in Phase 3 to allow spoke-to-spoke tunnels; in Phase 2, it is not required as spokes can learn from NHRP redirects.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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?
MPLS Operations — This question tests MPLS Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The hub is not sending NHRP redirect messages to the spokes. — In DMVPN Phase 2, spoke-to-spoke traffic requires that the spoke routers have a route to the destination prefix via the tunnel interface with the next-hop being the remote spoke's tunnel IP. If the hub is not configured to send an NHRP redirect or if the spoke does not have a route for the remote subnet via the tunnel, traffic will continue to go through the hub. The edge case is that the hub may be using a different next-hop in its routing table, or the spoke's routing table does not have a more specific route for the remote spoke's subnet via the tunnel.
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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Last reviewed: Jun 18, 2026
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