- A
The hub router has 'next-hop-self' configured under BGP, causing spokes to send traffic to the hub instead of directly to the destination spoke.
In Phase 2, spokes need to know the next-hop is the remote spoke's tunnel IP. 'next-hop-self' on the hub overrides this, forcing traffic through the hub.
- B
The NHRP authentication is mismatched between spokes, preventing the establishment of spoke-to-spoke tunnels.
Why wrong: NHRP authentication mismatch would prevent NHRP registrations and mappings, but the problem states registrations are successful.
- C
The spoke routers have a lower MTU on the physical interface, causing fragmentation issues for the GRE/IPsec packets.
Why wrong: MTU issues can cause packet drops, but they would affect all traffic, not just spoke-to-spoke.
- D
The DMVPN phase is actually Phase 3, which requires additional configuration for spoke-to-spoke traffic.
Why wrong: Phase 3 also supports spoke-to-spoke traffic, but the configuration is different. The question states Phase 2.
Quick Answer
The answer is that the hub router has 'next-hop-self' configured under BGP, which causes spokes to send traffic to the hub instead of directly to the destination spoke. In a DMVPN Phase 2 network with VRF-Lite, spoke-to-spoke traffic relies on NHRP to dynamically build direct tunnels, but the spoke routers must also have a route pointing to the remote spoke’s subnet with the next-hop set to that spoke’s tunnel IP. When the hub applies 'next-hop-self' in BGP, it overwrites the original next-hop with its own tunnel address, so spokes forward inter-spoke traffic through the hub rather than using the direct spoke-to-spoke tunnel. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how BGP next-hop processing interacts with DMVPN Phase 2’s spoke-to-spoke design—a common trap is assuming NHRP registrations alone guarantee direct traffic flow. Remember: if DMVPN VRF-Lite spoke-to-spoke is not working despite correct NHRP mappings, check BGP for 'next-hop-self' on the hub; the mnemonic “NHRP builds the tunnel, but BGP points the way” can help you recall that routing must preserve the spoke’s next-hop.
300-410 VRF-Lite Practice Question
This 300-410 practice question tests your understanding of vrf-lite. 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.
A DMVPN Phase 2 network is configured with VRF-Lite. Spokes can communicate with the hub, but spoke-to-spoke traffic is not working. The engineer verifies that NHRP registrations are successful and that the spoke routers have the correct NHRP mappings for other spokes. Which 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 router has 'next-hop-self' configured under BGP, causing spokes to send traffic to the hub instead of directly to the destination spoke.
In DMVPN Phase 2, spoke-to-spoke tunnels are created dynamically using NHRP. However, for spoke-to-spoke traffic to work, the spoke routers must have a route to the destination subnet via the tunnel interface. Additionally, the next-hop for the spoke-to-spoke route must be the spoke router's tunnel IP, not the hub. If the hub is configured with 'next-hop-self' under BGP or if the IGP is not propagating the correct next-hop, spoke-to-spoke traffic may be forwarded through the hub instead of directly.
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 router has 'next-hop-self' configured under BGP, causing spokes to send traffic to the hub instead of directly to the destination spoke.
Why this is correct
In Phase 2, spokes need to know the next-hop is the remote spoke's tunnel IP. 'next-hop-self' on the hub overrides this, forcing traffic through the hub.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The NHRP authentication is mismatched between spokes, preventing the establishment of spoke-to-spoke tunnels.
Why it's wrong here
NHRP authentication mismatch would prevent NHRP registrations and mappings, but the problem states registrations are successful.
- ✗
The spoke routers have a lower MTU on the physical interface, causing fragmentation issues for the GRE/IPsec packets.
Why it's wrong here
MTU issues can cause packet drops, but they would affect all traffic, not just spoke-to-spoke.
- ✗
The DMVPN phase is actually Phase 3, which requires additional configuration for spoke-to-spoke traffic.
Why it's wrong here
Phase 3 also supports spoke-to-spoke traffic, but the configuration is different. The question states Phase 2.
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 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?
VRF-Lite — This question tests VRF-Lite — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The hub router has 'next-hop-self' configured under BGP, causing spokes to send traffic to the hub instead of directly to the destination spoke. — In DMVPN Phase 2, spoke-to-spoke tunnels are created dynamically using NHRP. However, for spoke-to-spoke traffic to work, the spoke routers must have a route to the destination subnet via the tunnel interface. Additionally, the next-hop for the spoke-to-spoke route must be the spoke router's tunnel IP, not the hub. If the hub is configured with 'next-hop-self' under BGP or if the IGP is not propagating the correct next-hop, spoke-to-spoke traffic may be forwarded through the hub instead of directly.
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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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. In a DMVPN network with VRF-Lite, Router R1 (hub) and R2 (spoke) are configured for VRF-A. The DMVPN tunnel is up, but spoke-to-spoke traffic between R2 and R3 (another spoke) fails. R1 has configuration: interface Tunnel0, ip vrf forwarding VRF-A, ip address 172.16.0.1 255.255.255.0, tunnel source Gig0/0, tunnel mode gre multipoint. R2 has similar configuration with tunnel destination dynamic. The NHRP map for R3 is missing on R2. What is the root cause?
hard- ✓ A.The NHRP mapping for R3 is missing on R2, preventing direct spoke-to-spoke tunnel establishment.
- B.The tunnel mode should be gre multipoint on all spokes.
- C.The VRF must be removed from the tunnel interface for DMVPN to work.
- D.The hub router must have a static route for each spoke.
Why A: In DMVPN, spoke-to-spoke tunnels require NHRP resolution. If R2 does not have an NHRP map for R3, it cannot establish a direct tunnel. The hub (R1) should facilitate NHRP resolution, but if the VRF configuration is not properly propagated, NHRP may fail. The root cause is that the NHRP mapping is missing, often due to VRF mismatch in NHRP configuration or because the hub is not properly forwarding NHRP requests.
Last reviewed: Jun 18, 2026
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