hardMultiple ChoiceObjective-mapped
300-410 Practice Question: DMVPN spoke-to-spoke tunnel failures are…
DMVPN spoke-to-spoke tunnel failures are occurring due to route summarization. Hub router R1 has:
interface Tunnel0 ip address 172.16.0.1 255.255.255.0 ip nhrp network-id 1 ip nhrp map multicast dynamic
tunnel source GigabitEthernet0/0 tunnel mode gre multipoint !
router eigrp 100 network 172.16.0.0
! Spoke R2 has:
interface Tunnel0 ip address 172.16.0.2 255.255.255.0 ip nhrp network-id 1 ip nhrp nhs 172.16.0.1
tunnel source GigabitEthernet0/0 tunnel mode gre multipoint ! R1 also has:
interface GigabitEthernet0/0 ip summary-address eigrp 100 10.0.0.0 255.255.252.0
! Spokes cannot establish direct tunnels to each other for subnets within 10.0.0.0/22. What is the root cause?
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 summary route causes spokes to forward traffic to the hub instead of establishing direct tunnels, as the summary is preferred over more specific routes.
The summary route 10.0.0.0/22 is advertised by R1 to all spokes. When a spoke wants to reach a subnet within that summary on another spoke, it sends traffic to R1 (the summary route) instead of using the NHRP redirect to establish a spoke-to-spoke tunnel. The summary overrides the more specific routes that would trigger NHRP redirects. The fix is to use a leak-map to advertise specific routes or disable summarization.
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 summary route causes spokes to forward traffic to the hub instead of establishing direct tunnels, as the summary is preferred over more specific routes.
Why this is correct
Spokes use the summary route to reach the hub, bypassing NHRP redirect.
- ✗
NHRP is not configured correctly on the spokes, so they cannot register.
Why it's wrong here
Spokes are registered with the NHS.
- ✗
EIGRP is not enabled on the tunnel interface, so routes are not exchanged.
Why it's wrong here
EIGRP is enabled via network command.
- ✗
The tunnel mode is not multipoint on the spokes.
Why it's wrong here
Spokes use multipoint GRE.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
BGP Route Aggregation
BGP Route Aggregation is a technique that combines multiple specific network routes into a single summary route to reduce the size of routing tables and improve network efficiency.
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
Senior Network & Security Engineer · founder of Courseiva
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