- A
The spokes have a default route pointing to the hub, which prevents them from using the direct spoke-to-spoke tunnel.
Correct. In Phase 2, spokes must have a route that allows them to use the tunnel for direct communication; a default route to the hub will force traffic through the hub.
- B
The 'ip nhrp authentication' string is mismatched between the spokes.
Why wrong: Incorrect. Authentication mismatch would prevent NHRP registration, but the issue is that spoke-to-spoke traffic is not established even though mappings exist.
- C
The tunnel mode is set to 'gre multipoint' on the spokes, which is not supported in Phase 2.
Why wrong: Incorrect. 'gre multipoint' is required on the hub, but spokes can use 'gre' or 'gre multipoint' in Phase 2.
- D
The 'ip nhrp map multicast' command is missing on the spokes, preventing multicast traffic.
Why wrong: Incorrect. Missing multicast mapping would affect routing protocol adjacencies, not spoke-to-spoke data traffic.
Quick Answer
The answer is that the spokes have a default route pointing to the hub, which prevents them from using the direct spoke-to-spoke tunnel. In DMVPN Phase 2, spoke-to-spoke traffic relies on NHRP redirect and shortcut mechanisms: the hub must send an NHRP redirect to the source spoke, which then sends an NHRP resolution request to the destination spoke. If the spokes have a default route or summary route forcing all traffic through the hub, the source spoke never attempts the direct tunnel because it forwards packets to the hub instead of triggering the NHRP shortcut. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the critical difference between Phase 1 (hub-centric routing) and Phase 2 (dynamic spoke-to-spoke tunnels). A common trap is assuming that having NHRP mappings alone guarantees traffic flow—you must also ensure the routing table on each spoke contains a specific route (or a tunnel interface route) for remote spoke subnets, not just a default toward the hub. Memory tip: “No default to the hub, or the shortcut gets stuck.”
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. 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 DMVPN Phase 2 on a hub-and-spoke network for an MPLS L3VPN customer. The spokes are configured with a single tunnel interface and use NHRP to register with the hub. The engineer notices that spoke-to-spoke traffic is not being established, even though the spokes have each other's NHRP mappings. The engineer verifies that the tunnel interfaces are up and that the hub can reach all spokes. 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 spokes have a default route pointing to the hub, which prevents them from using the direct spoke-to-spoke tunnel.
In DMVPN Phase 2, spoke-to-spoke tunnels are established dynamically using NHRP redirect and shortcut. However, a common edge case is that the spokes must have a route that points to the destination network via the tunnel interface. If the spokes have a default route or a summary route that points to the hub (Phase 1 behavior), they will not use the direct spoke-to-spoke tunnel. In Phase 2, the hub must send an NHRP redirect to the source spoke, and the source spoke must then send an NHRP resolution request to the destination spoke. If the spokes have a static route that forces traffic through the hub, the NHRP redirect will not work. Additionally, the 'ip nhrp redirect' command must be configured on the hub, and 'ip nhrp shortcut' on the spokes.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 spokes have a default route pointing to the hub, which prevents them from using the direct spoke-to-spoke tunnel.
Why this is correct
Correct. In Phase 2, spokes must have a route that allows them to use the tunnel for direct communication; a default route to the hub will force traffic through the hub.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The 'ip nhrp authentication' string is mismatched between the spokes.
Why it's wrong here
Incorrect. Authentication mismatch would prevent NHRP registration, but the issue is that spoke-to-spoke traffic is not established even though mappings exist.
- ✗
The tunnel mode is set to 'gre multipoint' on the spokes, which is not supported in Phase 2.
Why it's wrong here
Incorrect. 'gre multipoint' is required on the hub, but spokes can use 'gre' or 'gre multipoint' in Phase 2.
- ✗
The 'ip nhrp map multicast' command is missing on the spokes, preventing multicast traffic.
Why it's wrong here
Incorrect. Missing multicast mapping would affect routing protocol adjacencies, not spoke-to-spoke data traffic.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS L3VPN — This question tests MPLS L3VPN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The spokes have a default route pointing to the hub, which prevents them from using the direct spoke-to-spoke tunnel. — In DMVPN Phase 2, spoke-to-spoke tunnels are established dynamically using NHRP redirect and shortcut. However, a common edge case is that the spokes must have a route that points to the destination network via the tunnel interface. If the spokes have a default route or a summary route that points to the hub (Phase 1 behavior), they will not use the direct spoke-to-spoke tunnel. In Phase 2, the hub must send an NHRP redirect to the source spoke, and the source spoke must then send an NHRP resolution request to the destination spoke. If the spokes have a static route that forces traffic through the hub, the NHRP redirect will not work. Additionally, the 'ip nhrp redirect' command must be configured on the hub, and 'ip nhrp shortcut' on the spokes.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
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