hardMultiple ChoiceObjective-mapped
DMVPN Spoke-to-Spoke Tunnel Failure
A DMVPN network with NHRP is configured for spoke-to-spoke tunnels. Spoke routers R1 and R2 are both connected to a hub router H1. Spoke-to-spoke traffic is not working. R1's show dmvpn shows a dynamic NHRP mapping for R2's tunnel IP to R2's physical IP, but ping from R1's tunnel IP to R2's tunnel IP fails. R1's show ip nhrp shows the mapping as 'dynamic' with no flags. The hub has no special configuration. What is the root cause?
Quick Answer
The answer is that the hub is missing the `ip nhrp redirect` command and the spokes are missing `ip nhrp shortcut`, which prevents spoke-to-spoke direct communication. In a DMVPN Phase 3 network, when a spoke sends traffic to another spoke via the hub, the hub must send an NHRP redirect message to inform the source spoke of the destination spoke’s real physical address; the source spoke then uses the `ip nhrp shortcut` command to install a direct, on-demand tunnel. Without these commands, the hub forwards traffic between spokes without triggering the spoke-to-spoke tunnel, so even though R1 shows a dynamic NHRP mapping for R2, it lacks the shortcut flag and will not use that mapping for direct forwarding. This scenario tests your understanding of DMVPN Phase 3’s traffic flow optimization on the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming a dynamic NHRP mapping alone enables spoke-to-spoke traffic. Remember: “Redirect on the hub, Shortcut on the spoke—without both, the direct path is broke.”
⚠ Common exam trap
Cisco often tests the distinction between DMVPN phase 2 (where spokes automatically build direct tunnels without redirect) and phase 3 (which requires explicit redirect and shortcut commands), leading candidates to assume that a dynamic NHRP mapping alone is sufficient for spoke-to-spoke communication.
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 missing the ip nhrp redirect command, and the spokes are missing ip nhrp shortcut, preventing spoke-to-spoke direct communication.
In a DMVPN phase 3 network, spoke-to-spoke traffic requires the hub to send NHRP redirect messages and the spokes to process them via the `ip nhrp shortcut` command. Without `ip nhrp redirect` on the hub, the hub forwards traffic between spokes without signaling them to establish a direct tunnel. The dynamic NHRP mapping on R1 for R2's tunnel IP indicates that R1 has learned R2's physical address via NHRP registration, but without the shortcut flag, R1 will not use that mapping to send traffic directly; instead, it continues to send traffic through the hub, which fails if the hub does not have a route or if the spoke-to-spoke tunnel is not triggered.
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 missing the ip nhrp redirect command, and the spokes are missing ip nhrp shortcut, preventing spoke-to-spoke direct communication.
Why this is correct
Without these commands, the hub does not trigger NHRP resolution for spoke-to-spoke traffic, and spokes do not install the necessary routes.
- ✗
The spoke routers have incorrect NHRP authentication, causing the mapping to be invalid.
Why it's wrong here
Authentication mismatch would prevent NHRP registration, but the mapping exists.
- ✗
The tunnel interface on R1 is not in the correct VRF.
Why it's wrong here
VRF misconfiguration would cause other issues, but not specifically spoke-to-spoke failure with existing NHRP mapping.
- ✗
The IPsec transform set is mismatched between spokes.
Why it's wrong here
IPsec mismatch would prevent tunnel establishment, but the DMVPN session is up.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
DMVPN Phase 2
DMVPN Phase 2 is an advanced Cisco routing technology that allows spoke routers to communicate directly with one another without sending traffic through a central hub, using dynamic routing protocols and multipoint GRE tunnels.
Key term
DMVPN Phase 3
DMVPN Phase 3 is a Cisco networking technology that allows branch offices to connect directly to each other without always going through a central hub, but with smarter routing that lets the hub control the traffic paths more efficiently.
About these practice questions
One of 1,966 original 300-410 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way 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 configures DMVPN Phase 2 with spoke-to-spoke tunnels. Spokes can ping each other's physical interfaces, but cannot establish a direct tunnel. NHRP registration is successful. Which is the most likely explanation?
hard- ✓ A.The hub is not configured with 'ip nhrp redirect' and the spokes are not configured with 'ip nhrp shortcut'.
- B.The spokes have different NHRP authentication strings, causing NHRP resolution to fail.
- C.The tunnel interface on the spokes is configured with 'tunnel mode gre multipoint' but the hub uses 'tunnel mode gre ip'.
- D.The spokes are using different IPsec transform sets, causing the IPsec tunnel to fail.
Why A: In DMVPN Phase 2, spoke-to-spoke tunnels require NHRP redirect and shortcut mechanisms to dynamically build direct tunnels. The hub must be configured with 'ip nhrp redirect' to send redirect messages to spokes, and spokes must have 'ip nhrp shortcut' to install the NHRP-learned /32 host routes for direct traffic. Without these, spokes will forward traffic through the hub even though they can ping each other's physical interfaces, preventing the establishment of a direct tunnel.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.