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300-410 Practice Question: A DMVPN Phase 3 network with OSPF as the IGP…
A DMVPN Phase 3 network with OSPF as the IGP experiences routing loops between hubs. R1 and R2 are both hubs with OSPF adjacencies to each other and to spokes. R1 has a route to 10.0.0.0/8 via a spoke, and redistributes it into OSPF. R2 learns this route and also has a direct connection to the same spoke, causing a loop. 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
✓
R1 is redistributing a route learned from a spoke into OSPF, and R2 learns this route and forwards traffic back to R1, creating a loop due to lack of route tagging and filtering.
Redistribution of routes from one routing protocol into OSPF can cause routing loops if not properly filtered. In this case, R1 redistributes a route learned from a spoke into OSPF, and R2 learns it and may forward traffic back to R1, creating a loop. The issue is that OSPF does not have a mechanism to prevent loops from redistribution without proper route tagging and filtering.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
R1 is redistributing a route learned from a spoke into OSPF, and R2 learns this route and forwards traffic back to R1, creating a loop due to lack of route tagging and filtering.
Why this is correct
Correct. Redistribution without route tagging (e.g., using a route-map with tag) can cause loops. The route should be tagged to prevent re-redistribution.
- ✗
OSPF network type is set to broadcast on both hubs, causing DR/BDR election issues.
Why it's wrong here
Incorrect. DR/BDR election issues would cause adjacency problems, not routing loops.
- ✗
NHRP redirect is enabled on both hubs, causing conflicting redirect messages.
Why it's wrong here
Incorrect. NHRP redirect is for spoke-to-spoke tunnels, not for routing loops between hubs.
- ✗
R2 has a higher OSPF router ID, causing it to become DR and attract traffic.
Why it's wrong here
Incorrect. DR election affects OSPF adjacencies, not routing loops from redistribution.
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
DMVPN Phase 3
DMVPN Phase 3 is a Cisco networking technology that allows branch offices to connect directly to each other without always going through a central hub, but with smarter routing that lets the hub control the traffic paths more efficiently.
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Written by Johnson Ajibi, MSc IT Security
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