hardMultiple ChoiceObjective-mapped
300-410 Practice Question: A DMVPN network with hub R1 and spokes R2 and R3…
A DMVPN network with hub R1 and spokes R2 and R3 is configured with mGRE and NHRP. Spoke-to-spoke tunnels fail to form. R1 configuration: interface Tunnel0, ip address 10.0.0.1 255.255.255.0, tunnel source GigabitEthernet0/0, tunnel mode gre multipoint, ip nhrp network-id 1, ip nhrp map multicast dynamic. R2 shows: 'show dmvpn' shows no dynamic sessions. R3 shows: 'show ip nhrp' shows no entries for R2. What is the root cause?
⚠ Common exam trap
Cisco often tests the misconception that 'ip nhrp map multicast dynamic' alone is sufficient for spoke registration, but in reality, the 'ip nhrp nhs' command is mandatory for spokes to register their addresses with the hub.
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 are missing the 'ip nhrp nhs' command to point to the hub, preventing registration and resolution of spoke-to-spoke addresses.
In a DMVPN network, spokes must use the 'ip nhrp nhs' command to register their real (physical) IP addresses with the hub (NHS). Without this command, the hub never learns the spokes' addresses, so it cannot provide NHRP resolution replies to other spokes attempting to form spoke-to-spoke tunnels. The absence of dynamic sessions on R2 and no NHRP entries for R2 on R3 directly indicate that R2 never registered with the hub.
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 are missing the 'ip nhrp nhs' command to point to the hub, preventing registration and resolution of spoke-to-spoke addresses.
Why this is correct
Without NHS configuration, spokes do not register with the hub, so NHRP cannot provide the mapping for spoke-to-spoke tunnels.
- ✗
The tunnel mode is mGRE, but the spokes are configured with point-to-point GRE tunnels.
Why it's wrong here
Spokes typically use mGRE as well; if they used p2p GRE, they would only connect to the hub, but the scenario does not indicate this.
- ✗
The NHRP network-id is different on R2 and R3, preventing communication.
Why it's wrong here
The network-id must match, but the scenario shows it is 1 on the hub; if spokes had different IDs, they would not register.
- ✗
The tunnel source interface is not reachable between spokes.
Why it's wrong here
Spoke-to-spoke reachability is established via NHRP; if NHRP is not working, the tunnel source may be reachable but the tunnel itself fails.
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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