hardMultiple ChoiceObjective-mapped
300-410 Practice Question: DMVPN phase 2 network: spoke-to-spoke tunnels fail
DMVPN phase 2 network: spoke-to-spoke tunnels fail. Router Spoke1 has: tunnel source GigabitEthernet0/0, tunnel mode gre multipoint, ip nhrp network-id 1, ip nhrp nhs 10.0.0.1. Router Spoke2 shows: show dmvpn includes Spoke1 in NHRP cache but no spoke-to-spoke tunnel. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between NHRP cache entries (which indicate registration and mapping) and the actual tunnel establishment, which requires additional mechanisms like NHRP redirect in Phase 2 or NHRP shortcut switching in Phase 3.
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 configured with ip nhrp redirect, preventing shortcut initiation.
In a DMVPN Phase 2 network, spoke-to-spoke tunnels require the hub to send NHRP redirect messages to inform spokes of the shortcut path. Without the 'ip nhrp redirect' command on the hub, spokes will not attempt to build direct tunnels, even if they have each other's NHRP cache entries. This is the root cause because Spoke1 and Spoke2 can see each other in the NHRP cache but lack the trigger to initiate the spoke-to-spoke GRE tunnel.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Spoke1 and Spoke2 have different NHRP network IDs.
Why it's wrong here
Different network IDs prevent NHRP registration.
- ✓
The hub is not configured with ip nhrp redirect, preventing shortcut initiation.
Why this is correct
Without redirect, spokes do not learn each other's real IPs for direct tunnels.
- ✗
Spoke1 has a firewall blocking GRE traffic.
Why it's wrong here
Firewall would block all traffic, not just spoke-to-spoke.
- ✗
The tunnel mode should be point-to-point for spoke-to-spoke.
Why it's wrong here
DMVPN phase 2 uses multipoint GRE.
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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