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300-410 Practice Question: In a DMVPN phase 3 network, Router R1 (hub) and…
In a DMVPN phase 3 network, Router R1 (hub) and R2 (spoke) have an mGRE tunnel. R1 has EIGRP configured over the tunnel with 'no ip split-horizon' and 'distance eigrp 90 100'. R2 has default EIGRP configuration. R2 learns a route to 192.168.1.0/24 via the tunnel with AD 90. Later, R2 also learns the same route via a physical interface from another spoke R3 using EIGRP with AD 90. R2's 'show ip route 192.168.1.0' shows the route via R3. 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 route via R3 has a better metric (lower composite) than the route via R1, so it is preferred.
EIGRP uses metric (composite) to choose between routes with equal AD. The route via R3 might have a better metric. However, if the distance is changed on R1 to 100 for external routes, the route via R1 might have AD 100, but the question says AD 90. The correct answer is that the route via R3 is an internal route (AD 90) with a lower metric than the route via R1 (also AD 90), so metric decides. But the trick is that the tunnel interface on R2 might have a higher delay, causing a worse metric.
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 route via R3 has a better metric (lower composite) than the route via R1, so it is preferred.
Why this is correct
Equal AD routes are compared by metric; the tunnel may have higher delay/bandwidth.
- ✗
R2's EIGRP process has 'variance 2' configured, causing load balancing.
Why it's wrong here
Variance does not cause preference; it allows unequal-cost load balancing.
- ✗
The route via R1 is an external route (AD 100) due to redistribution, so it is less preferred.
Why it's wrong here
The distance is set to 90 for internal and 100 for external; if the route is external, AD 100 > 90.
- ✗
R2 has a static route with AD 1 that overrides both.
Why it's wrong here
No static route.
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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