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300-410 Practice Question: A DMVPN Phase 3 network with EIGRP as the routing…
A DMVPN Phase 3 network with EIGRP as the routing protocol experiences intermittent connectivity between spokes. R1 (hub) has 'ip summary-address eigrp 100 10.0.0.0 255.255.0.0' configured on the tunnel interface. R2 (spoke) shows 'show ip eigrp topology all-links' listing 10.1.0.0/24 as reachable via R1, but traffic from R2 to 10.1.0.1 (another spoke) is forwarded to R1 instead of directly via the spoke-to-spoke tunnel. 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 on the hub creates a less specific route that is preferred over the more specific /24 routes learned from other spokes, causing suboptimal routing.
The hub's summary route 10.0.0.0/16 is advertised to spokes, causing them to prefer the summary route over the more specific /24 routes learned from other spokes via NHRP redirect. This prevents spoke-to-spoke direct tunnels from being established or used, as traffic is forwarded to the hub based on the summary route.
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 on the hub creates a less specific route that is preferred over the more specific /24 routes learned from other spokes, causing suboptimal routing.
Why this is correct
Correct. EIGRP summary routes have administrative distance 5 by default, which is lower than the distance of learned routes (90/170). This causes spokes to prefer the summary route and forward traffic to the hub, even though more specific routes exist.
- ✗
EIGRP stub configuration on spokes prevents them from learning routes from other spokes.
Why it's wrong here
Incorrect. EIGRP stub configuration would limit route advertisement, but the issue is about route preference, not learning.
- ✗
NHRP redirect is disabled on the hub, preventing spoke-to-spoke tunnel establishment.
Why it's wrong here
Incorrect. NHRP redirect is required for Phase 3, but even if enabled, the summary route would still be preferred.
- ✗
The spoke-to-spoke tunnel interface has a different MTU causing fragmentation.
Why it's wrong here
Incorrect. MTU mismatch would cause packet drops, not suboptimal routing.
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.
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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