Question 1,418 of 2,152

DMVPN →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is the lack of route tagging and filtering during redistribution. When R1 redistributes a route learned from a spoke into OSPF, R2 learns that same route via OSPF and, because it also has a direct connection to the spoke, forwards traffic back toward R1 instead of directly to the spoke, creating a DMVPN OSPF redistribution routing loop. This occurs because OSPF has no inherent loop-prevention mechanism for redistributed routes; without a route tag or filter to distinguish the source, R2 cannot tell that the redistributed route is inferior to its own direct path. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of redistribution pitfalls in DMVPN Phase 3 designs, often appearing as a troubleshooting question where you must identify the missing distribute-list or route-map. A common trap is assuming OSPF’s SPF algorithm alone prevents loops, but redistribution bypasses that logic. Memory tip: “Tag it or bag it” — always tag redistributed routes to filter them back out.

300-410 DMVPN Practice Question

This 300-410 practice question tests your understanding of dmvpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

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?

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
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.
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.
B
OSPF network type is set to broadcast on both hubs, causing DR/BDR election issues.
Why wrong: Incorrect. DR/BDR election issues would cause adjacency problems, not routing loops.
C
NHRP redirect is enabled on both hubs, causing conflicting redirect messages.
Why wrong: Incorrect. NHRP redirect is for spoke-to-spoke tunnels, not for routing loops between hubs.
D
R2 has a higher OSPF router ID, causing it to become DR and attract traffic.
Why wrong: Incorrect. DR election affects OSPF adjacencies, not routing loops from redistribution.

Full breakdown with real-world context →

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.

Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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.
Related concept
OSPF neighbours must agree on key parameters.
✗
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.

Common exam traps

Common exam trap: OSPF can fail even when IP connectivity looks correct

OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.

Detailed technical explanation

How to think about this question

OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.

KKey Concepts to Remember

OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.

TExam Day Tips

Check area mismatch first when OSPF adjacency fails.
Review passive interfaces when a network is advertised but no neighbour forms.
Use show ip ospf neighbor and show ip route clues carefully.

Key takeaway

OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

Real-world example

How this comes up in practice

A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

What to study next

Got this wrong? Here's your next step.

Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

DMVPN — This question tests DMVPN — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: 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.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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