hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting IPv6 DMVPN phase 2…
A network engineer is troubleshooting IPv6 DMVPN phase 2 spoke-to-spoke tunnel failures. Spoke routers are able to communicate with the hub, but direct spoke-to-spoke traffic is not working. Router R1 (spoke) has the following relevant configuration:
interface Tunnel0
ipv6 address 2001:DB8:1::1/64 tunnel source GigabitEthernet0/0 tunnel mode gre multipoint ipv6 nhrp network-id 1 ipv6 nhrp nhs 2001:DB8:1::2 ipv6 nhrp map multicast dynamic !
Router R2 (hub) shows: show ipv6 nhrp brief output indicates that both spokes are registered. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between DMVPN phases, and the trap here is that candidates assume NHRP registration alone enables spoke-to-spoke communication, overlooking the mandatory redirect/shortcut commands for Phase 2.
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 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'.
In DMVPN Phase 2, spoke-to-spoke traffic requires the hub to send NHRP redirect messages and the spokes to use NHRP shortcuts. Without 'ipv6 nhrp redirect' on the hub and 'ipv6 nhrp shortcut' on the spokes, the spokes will forward all inter-spoke traffic through the hub instead of establishing a direct tunnel. The hub's NHRP brief shows both spokes are registered, confirming NHRP registration works, but the missing redirect/shortcut mechanism prevents direct spoke-to-spoke communication.
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 tunnel mode is multipoint, but the spokes need to be configured with 'tunnel mode gre ip' for direct communication.
Why it's wrong here
DMVPN phase 2 uses multipoint GRE tunnels on spokes as well.
- ✓
The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'.
Why this is correct
Without redirect and shortcut, spokes do not learn each other's NHRP mappings and send traffic through the hub.
- ✗
The spokes have different NHRP network IDs, preventing registration.
Why it's wrong here
The show output indicates both spokes are registered, so network IDs match.
- ✗
The IPv6 addresses on the tunnel interfaces are in different subnets.
Why it's wrong here
All tunnel interfaces should be in the same subnet for DMVPN to work.
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.
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Written by Johnson Ajibi, MSc IT Security
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